A&P Module 4

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3/20/2012

Biology 141 – Human Anatomy and Physiology
Text: Marieb and Hoehn, 8th edn – 2010 Instructor: J i H i i Mi h ll I Jamie Heisig-Mitchell Lecture Outline: Chapters 10 - 14

Skeletal Muscles: Functional Groups
1.

Prime movers


Provide the major force for producing a specific movement

2.

Antagonists g


Oppose or reverse a particular movement

Skeletal Muscles: Functional Groups
3.

Synergists
 

Add force to a movement Reduce undesirable or unnecessary movement

4. 4

Fixators


Synergists that immobilize a bone or muscle’s origin

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Muscle Mechanics: Lever Systems


Components of a lever system
 Lever—rigid

bar (bone) that moves on a fixed point or fulcrum (joint)  Effort—force (supplied by muscle contraction) applied to a lever to move a resistance (load)  Load—resistance (bone + tissues + any added weight) moved by the effort

Effort x length of effort arm = load x length of load arm (force x distance) = (resistance x distance)
Effort 10 kg

0.25 cm Effort 25 cm Fulcrum Load

10 x 25 = 1000 x 0.25 250 = 250

1000 kg Load

Fulcrum

(a) Mechanical advantage with a power lever

Figure 10.2a

Effort 100 kg Effort Load 25 cm 50 cm Fulcrum Fulcrum 50 kg Load

100 x 25 = 50 x 50 2500 = 2500

(b) Mechanical disadvantage with a speed lever

Figure 10.2b

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Classes of Lever Systems


First class
 Fulcrum

between load and effort

(a) First-class lever Arrangement of the elements is load-fulcrum-effort Load Effort

Fulcrum Load

Fulcrum Example: scissors

Effort

Figure 10.3a (1 of 2)

(a) First-class lever Arrangement of the elements is load-fulcrum-effort

Fulcrum

Load

Effort

In the body: A first-class lever system raises your head off your chest. The posterior neck muscles provide the effort, the atlanto-occipital joint is the fulcrum, and the weight to be lifted is the facial skeleton.
Figure 10.3a (2 of 2)

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Classes of Lever Systems


Second class
 Load

between fulcrum and effort

(b) Second-class lever Arrangement of the elements is fulcrum-load-effort Load

Fulcrum Load

Effort

Effort Example: wheelbarrow

Fulcrum

Figure 10.3b (1 of 2)

(b) Second-class lever Arrangement of the elements is fulcrum-load-effort

Effort

Load Fulcrum In the body: Second-class leverage is exerted when you stand on tip-toe. The effort is exerted by the calf muscles pulling upward on the heel; the joints of the ball of the foot are the fulcrum; and the weight of the body is the load.
Figure 10.3b (2 of 2)

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Classes of Lever Systems


Third class
 Effort

applied between fulcrum and load

(c) Third-class lever Arrangement of the elements is load-effort-fulcrum Load Effort

Fulcrum Load

Fulcrum Effort Example: tweezers or forceps
Figure 10.3c (1 of 2)

(c) Third-class lever Arrangement of the elements is load-effort-fulcrum

Effort

Load Fulcrum In the body: Flexing the forearm by the biceps brachii muscle exemplifies third-class leverage. The effort is exerted on the proximal radius of the forearm, the fulcrum is the elbow joint, and the load is the hand and distal end of the forearm.
Figure 10.3c (2 of 2)

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Functions of the Nervous System
1.

Sensory input


Information gathered by sensory receptors about internal and external changes

2.

Integration g


Interpretation of sensory input

3.

Motor output


Activation of effector organs (muscles and glands) produces a response

Sensory input

Integration Motor output

Figure 11.1

Divisions of the Nervous System


Central nervous system (CNS)
 Brain  Integration

and spinal cord and command center



Peripheral nervous system (PNS)
 Paired

spinal and cranial nerves carry messages to and from the CNS

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Peripheral Nervous System (PNS)


Two functional divisions
1.

Sensory (afferent) division
 

Somatic afferent fibers—convey impulses from skin, skeletal muscles, and joints Visceral afferent fibers—convey impulses from visceral fibers convey organs Transmits impulses from the CNS to effector organs

2.

Motor (efferent) division


Motor Division of PNS
1.

Somatic (voluntary) nervous system


Conscious control of skeletal muscles

Motor Division of PNS
2.

Autonomic (involuntary) nervous system (ANS)
  

Visceral motor nerve fibers Regulates smooth muscle, cardiac muscle, and glands Two functional subdivisions v
 

Sympathetic – “Fight or Flight” Parasympathetic - digestion, urination, defecation, etc

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Central nervous system (CNS) Brain and spinal cord Integrative and control centers

Peripheral nervous system (PNS) Cranial nerves and spinal nerves Communication lines between the CNS and the rest of the body

Sensory (afferent) division Somatic and visceral sensory nerve fibers Conducts impulses from receptors to the CNS

Motor (efferent) division Motor nerve fibers Conducts impulses from the CNS to effectors (muscles and glands)

Somatic sensory fiber

Skin

Somatic nervous system Somatic motor (voluntary) Conducts impulses from the CNS to skeletal muscles

Visceral sensory fiber Stomach Skeletal muscle Motor fiber of somatic nervous system Sympathetic division Mobilizes body systems during activity

Autonomic nervous system (ANS) Visceral motor (involuntary) Conducts impulses from the CNS to cardiac muscles, smooth muscles, and glands

Parasympathetic division Conserves energy Promotes housekeeping functions during rest

Sympathetic motor fiber of ANS Structure Function Sensory (afferent) division of PNS Motor (efferent) division of PNS

Heart

Parasympathetic motor fiber of ANS

Bladder

Figure 11.2

Histology of Nervous Tissue


Two principal cell types
1.

Neurons—excitable cells that transmit electrical signals

Histology of Nervous Tissue
2.

Neuroglia (glial cells)—supporting cells:
     

Astrocytes (CNS) Microglia (CNS) Ependymal cells (CNS) Oligodendrocytes (CNS) Satellite cells (PNS) Schwann cells (PNS)

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Astrocytes


 

Most abundant, versatile, and highly branched glial cells Cling to neurons, synaptic endings, and capillaries Support and brace neurons

Astrocytes
   

Help determine capillary permeability Guide migration of young neurons Control the chemical environment Participate in information processing i th b i P ti i t i i f ti i in the brain

Capillary

Neuron

Astrocyte

(a) Astrocytes are the most abundant CNS neuroglia.
Figure 11.3a

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Microglia
  

Small, ovoid cells with thorny processes Migrate toward injured neurons Phagocytize microorganisms and neuronal debris

Neuron Microglial cell

(b) Microglial cells are defensive cells in the CNS.
Figure 11.3b

Ependymal Cells
 

Range in shape from squamous to columnar May be ciliated
 Line  Separate

the central cavities of the brain and spinal column the CNS interstitial fluid from the cerebrospinal fluid in the cavities

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Fluid-filled cavity Ependymal cells Brain or spinal cord tissue (c) Ependymal cells line cerebrospinal fluid-filled cavities.

Figure 11.3c

Oligodendrocytes
 

Branched cells Processes wrap CNS nerve fibers, forming insulating myelin sheaths

Myelin sheath Process of oligodendrocyte Nerve N fibers (d) Oligodendrocytes have processes that form myelin sheaths around CNS nerve fibers.

Figure 11.3d

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Satellite Cells and Schwann Cells


Satellite cells
 Surround

neuron cell bodies in the PNS



Schwann cells (neurolemmocytes)
 Surround

peripheral nerve fibers and form myelin sheaths  Vital to regeneration of damaged peripheral nerve fibers

Satellite cells

Cell body of neuron Schwann cells (forming myelin sheath) Nerve fib N fiber

(e) Satellite cells and Schwann cells (which form myelin) surround neurons in the PNS.

Figure 11.3e

Neurons (Nerve Cells)


Special characteristics:
 Long-lived

( 100 years or more) few exceptions  High metabolic rate—depends on continuous supply of oxygen and glucose d l  Plasma membrane functions in:
 Amitotic—with
 Electrical  Cell-to-cell

signaling interactions during development

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Cell Body (Perikaryon or Soma)
   

Biosynthetic center of a neuron Spherical nucleus with nucleolus Well-developed Golgi apparatus Rough R h ER called Nissl bodies (chromatophilic ll d Ni l b di ( h t hili substance)

Cell Body (Perikaryon or Soma)
 



Network of neurofibrils (neurofilaments) Axon hillock—cone-shaped area from which axon arises Clusters of cell bodies are called nuclei in the CNS CNS, ganglia in the PNS

Dendrites (receptive regions)

Cell body (biosynthetic center and receptive region)

Nucleolus Axon (impulse generating and conducting region) Nucleus Nissl bodies Axon hillock (b) Impulse direction Node of Ranvier Axon terminals (secretory region)
Figure 11.4b

Schwann cell Neurilemma (one interTerminal node) branches

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Processes
 

Dendrites and axons Bundles of processes are called
 Tracts  Nerves

in the CNS in the PNS

Dendrites
  

Short, tapering, and diffusely branched Receptive (input) region of a neuron Convey electrical signals toward the cell body as graded potentials

The Axon
  

One axon per cell arising from the axon hillock Long axons (nerve fibers) Occasional branches (axon collaterals)

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The Axon
 

Numerous terminal branches (telodendria) Knoblike axon terminals (synaptic knobs or boutons)
 Secretory  Release

region of neuron neurotransmitters to excite or inhibit other cells

Axons: Function
 

Conducting region of a neuron Generates and transmits nerve impulses (action potentials) away from the cell body

Axons: Function


Molecules and organelles are moved along axons by motor molecules in two directions:
 Anterograde—toward
 Examples:

axonal terminal

mitochondria, membrane components, enzymes

 Retrograde—toward
 Examples:

the cell body

organelles to be degraded, signal molecules, viruses, and bacterial toxins

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Myelin Sheath




Segmented protein-lipoid sheath around most long or large-diameter axons It functions to:
 Protect  Increase

and electrically insulate the axon speed of nerve impulse transmission

Myelin Sheaths in the PNS


Schwann cells wraps many times around the axon
 Myelin

sheath—concentric layers of Schwann cell membrane



Neurilemma—peripheral bulge of Schwann cell p p g cytoplasm

Myelin Sheaths in the PNS


Nodes of Ranvier
 Myelin  Sites

sheath gaps between adjacent Schwann cells where axon collaterals can emerge

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Schwann cell plasma membrane Schwann cell cytoplasm Axon

1 A Schwann cell envelopes an axon.

Schwann cell nucleus

2 The Schwann cell then rotates around the axon, wrapping its plasma membrane loosely around it in successive layers.

Neurilemma Myelin sheath

(a) Myelination of a nerve fiber (axon)

3 The Schwann cell cytoplasm is forced from between the membranes. The tight membrane wrappings surrounding the axon form the myelin sheath.
Figure 11.5a

Unmyelinated Axons
 

Thin nerve fibers are unmyelinated One Schwann cell may incompletely enclose 15 or more unmyelinated axons

Myelin Sheaths in the CNS


  

Formed by processes of oligodendrocytes, not the whole cells Nodes of Ranvier are present No neurilemma Thinnest fibers are unmyelinated

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Myelin sheath Process of oligodendrocyte Nerve N fibers (d) Oligodendrocytes have processes that form myelin sheaths around CNS nerve fibers.

Figure 11.3d

White Matter and Gray Matter


White matter
 Dense

collections of myelinated fibers neuron cell bodies and unmyelinated fibers



Gray matter
 Mostly

Structural Classification of Neurons


Three types:
1.

Multipolar—1 axon and several dendrites
 

Most abundant Motor neurons and interneurons Rare, e.g., retinal neurons

2.

Bipolar—1 axon and 1 dendrite


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Structural Classification of Neurons
3.

Unipolar (pseudounipolar)—single, short process that has two branches:
 

Peripheral process—more distal branch, often associated with a sensory receptor Central process branch entering the CNS process—branch

Table 11.1 (1 of 3)

Table 11.1 (2 of 3)

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Functional Classification of Neurons


Three types:
1.

Sensory (afferent)


Transmit impulses from sensory receptors toward the CNS Carry impulses from the CNS to effectors

2.

Motor (efferent)


Functional Classification of Neurons
3.

Interneurons (association neurons)


Shuttle signals through CNS pathways; most are entirely within the CNS

Table 11.1 (3 of 3)

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Neuron Function
 



Neurons are highly irritable Respond to adequate stimulus by generating an action potential (nerve impulse) Impulse is always the same regardless of stimulus

Principles of Electricity
 





Opposite charges attract each other Energy is required to separate opposite charges across a membrane Energy is liberated when the charges move toward one another If opposite charges are separated, the system has potential energy

Definitions






Voltage (V): measure of potential energy generated by separated charge Potential difference: voltage measured between two points p Current (I): the flow of electrical charge (ions) between two points

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Definitions


 

Resistance (R): hindrance to charge flow (provided by the plasma membrane) Insulator: substance with high electrical resistance Conductor: substance with low electrical resistance

Role of Membrane Ion Channels
 

Proteins serve as membrane ion channels Two main types of ion channels
1.

Leakage (nongated) channels—always open

Role of Membrane Ion Channels
2.

Gated channels (three types):
  

Chemically gated (ligand-gated) channels—open with binding of a specific neurotransmitter Voltage-gated channels—open and close in response to changes in membrane potential Mechanically gated channels—open and close in response to channels open physical deformation of receptors

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Receptor Na+

Neurotransmitter chemical attached to receptor Na+ Na+ Na+

Chemical binds

Membrane voltage changes h K+ Open Closed Open

K+ Closed

(a) Chemically (ligand) gated ion channels open when the appropriate neurotransmitter binds to the receptor, allowing (in this case) simultaneous movement of Na+ and K+.

(b) Voltage-gated ion channels open and close in response to changes in membrane voltage.

Figure 11.6

Gated Channels


When gated channels are open:
 Ions

diffuse quickly across the membrane along their electrochemical gradients
 Along

chemical concentration gradients from higher concentration to lower concentration  Along electrical gradients toward opposite electrical charge
 Ion

flow creates an electrical current and voltage changes across the membrane

Resting Membrane Potential (Vr)


Potential difference across the membrane of a resting cell
 Approximately

–70 mV in neurons (cytoplasmic side of membrane is negatively charged relative to outside) in ionic makeup of ICF and ECF permeability of the plasma membrane



Generated by: G db
 Differences  Differential

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Resting Membrane Potential


Differences in ionic makeup
has lower concentration of Na+ and Cl– than ECF  ICF has higher concentration of K+ and negatively charged proteins (A–) than ECF
 ICF

Resting Membrane Potential


Differential permeability of membrane
to A–  Slightly permeable to Na+ (through leakage channels)  75 times more permeable to K+ (more leakage p ( g channels)  Freely permeable to Cl–
 Impermeable

Resting Membrane Potential




Negative interior of the cell is due to much greater diffusion of K+ out of the cell than Na+ diffusion into the cell Sodium-potassium pump stabilizes the resting p p p g membrane potential by maintaining the concentration gradients for Na+ and K+

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The concentrations of Na+ and K+ on each side of the membrane are different.
Outside cell

The Na+ concentration is higher outside the cell.

K+ (5 mM )

Na+ (140 mM )

The K+ concentration is higher inside the cell.

K+ (140 mM )
Inside cell

Na+ (15 mM )

The permeabilities of Na+ and K+ across the membrane are different.
K+ leakage channels K+ K+

Na+-K+ ATPases (pumps) maintain the concentration gradients of Na+ and K+ across the membrane.

Suppose a cell has only K+ channels... K+ loss through abundant leakage channels establishes a negative membrane potential.

K+
K+

K+
K+

Cell interior –90 mV

Na+

Now, let’s add some Na+ channels to our cell... Na+ entry through leakage channels reduces the negative membrane potential slightly.

K

K+

Na+

Cell interior –70 mV

Na+-K+ pump K+ K+

Na+

Finally, let’s add a pump to compensate for leaking ions. Na+-K+ ATPases (pumps) maintain the concentration gradients, resulting in the resting membrane potential.

K+

K+

Na+

Cell interior –70 mV

Figure 11.8

Membrane Potentials That Act as Signals


Membrane potential changes when:
 Concentrations  Permeability

of ions across the membrane change of membrane to ions changes



Changes in membrane potential are signals used to receive, integrate and send information

Membrane Potentials That Act as Signals


Two types of signals
 Graded  Action

potentials
short-distance signals signals of axons

 Incoming

potentials

 Long-distance

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Changes in Membrane Potential


Depolarization
A

reduction in membrane potential (toward zero) of the membrane becomes less negative than the resting potential  Increases the probability of producing a nerve impulse
 Inside

Depolarizing stimulus

Inside positive Inside negative Depolarization

Resting potential Time (ms) (a) Depolarization: The membrane potential moves toward 0 mV, the inside becoming less negative (more positive).
Figure 11.9a

Changes in Membrane Potential


Hyperpolarization
 An

increase in membrane potential (away from zero) of the membrane becomes more negative than the resting potential  Reduces the probability of producing a nerve impulse
 Inside

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Hyperpolarizing stimulus

Resting potential

Hyperpolarization Time (ms) (b) Hyperpolarization: The membrane potential increases, the inside becoming more negative.
Figure 11.9b

Graded Potentials


 

Short-lived, localized changes in membrane potential Depolarizations or hyperpolarizations Graded potential spreads as local currents change the membrane potential of adjacent regions

Stimulus Depolarized region

Plasma membrane (a) Depolarization: A small patch of the membrane (red area) has become depolarized.

Figure 11.10a

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(b) Spread of depolarization: The local currents (black arrows) that are created depolarize adjacent membrane areas and allow the wave of depolarization to spread.
Figure 11.10b

Graded Potentials


Occur when a stimulus causes gated ion channels to open
 E.g.,

receptor potentials, generator potentials, postsynaptic potentials







Magnitude varies di l ( d d) with stimulus M i d i directly (graded) i h i l strength Decrease in magnitude with distance as ions flow and diffuse through leakage channels Short-distance signals

Memb brane potential (mV)

Active area (site of initial depolarization)

–70 Resting potential

Distance (a few mm) (c) Decay of membrane potential with distance: Because current is lost through the “leaky” plasma membrane, the voltage declines with distance from the stimulus (the voltage is decremental). Consequently, graded potentials are short-distance signals.
Figure 11.10c

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Action Potential (AP)


  

Brief reversal of membrane potential with a total amplitude of ~100 mV Occurs in muscle cells and axons of neurons Does not decrease in magnitude over distance Principal means of long-distance neural communication

The big picture
1
Resting state

2

Depolarization

3

Repolarization

Membrane pote ential (mV)

3 4 Hyperpolarization 2
Action potential

Threshold

1

4

1

Time (ms)

Figure 11.11 (1 of 5)

Generation of an Action Potential


Resting state
 Only  All

leakage channels for Na+ and K+ are open gated Na+ and K+ channels are closed

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Properties of Gated Channels


Properties of gated channels
 Each


Na+ channel has two voltage-sensitive gates
gates gates
Closed at rest; open with depolarization Open at rest; block channel once it is open

 Activation

 Inactivation


Properties of Gated Channels
  

Each K+ channel has one voltage-sensitive gate Closed at rest Opens slowly with depolarization

Depolarizing Phase


 

Depolarizing local currents open voltage-gated Na+ channels Na+ influx causes more depolarization At threshold (–55 to –50 mV) positive feedback ( 55 50 leads to opening of all Na+ channels, and a reversal of membrane polarity to +30mV (spike of action potential)

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Repolarizing Phase


Repolarizing phase
 Na+

channel slow inactivation gates close permeability to Na+ declines to resting levels  Slow voltage-sensitive K+ gates open  K+ exits the cell and internal negativity is restored
 Membrane

Hyperpolarization


Hyperpolarization
K+ channels remain open, allowing excessive K+ efflux  This causes after-hyperpolarization of the membrane (undershoot)
 Some

The AP is caused by permeability changes in the plasma membrane
Relative membran permeability ne
Figure 11.11 (2 of 5)

Membrane po otential (mV)

3

Action potential Na+ permeability K+ permeability

2

1

4
Time (ms)

1

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Role of the Sodium-Potassium Pump


Repolarization
 Restores  Does

the resting electrical conditions of the neuron not restore the resting ionic conditions



Ionic redistribution back to resting conditions is restored by the thousands of sodium-potassium pumps

Propagation of an Action Potential


 

Na+ influx causes a patch of the axonal membrane to depolarize Local currents occur Na+ channels toward the point of origin are inactivated and not affected by the local currents

Propagation of an Action Potential






Local currents affect adjacent areas in the forward direction Depolarization opens voltage-gated channels and triggers an AP gg Repolarization wave follows the depolarization wave

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Voltage at 0 ms

Recording electrode

(a) Time = 0 ms. Action potential has not yet reached the recording electrode. Resting potential Peak of action potential Hyperpolarization
Figure 11.12a

Voltage at 2 ms

(b) Time = 2 ms. Action potential peak is at the recording electrode.

Figure 11.12b

Voltage at 4 ms

(c) Time = 4 ms. Action potential peak is past the recording electrode. Membrane at the recording electrode is still hyperpolarized.

Figure 11.12c

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Threshold


At threshold:
 Membrane  Na+

is depolarized by 15 to 20 mV permeability increases  N influx exceeds K+ efflux Na  The positive feedback cycle begins

Threshold






Subthreshold stimulus - weak local depolarization that does not reach threshold Threshold stimulus - strong enough to push the membrane potential toward and beyond threshold p y AP is an all-or-none phenomenon - action potentials either happen completely, or not at all

Coding for Stimulus Intensity


All action potentials are alike and are independent of stimulus intensity
 How

does the CNS tell the difference between a weak stimulus and a strong one?





Strong stimuli can generate action potentials more often than weaker stimuli The CNS determines stimulus intensity by the frequency of impulses

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Action potentials

Threshold

Stimulus

Time (ms)
Figure 11.13

Absolute Refractory Period


 

Time from the opening of the Na+ channels until the resetting of the channels Ensures that each AP is an all-or-none event Enforces one-way transmission of nerve impulses

Absolute refractory period Depolarization (Na+ enters)

Relative refractory period

Repolarization (K+ leaves)

After-hyperpolarization Stimulus Time (ms)
Figure 11.14

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Relative Refractory Period


Follows the absolute refractory period
Na+ channels have returned to their resting state K+ channels are still open  Repolarization is occurring
 Most  Some

 

Threshold for AP generation is elevated Exceptionally strong stimulus may generate an AP

Conduction Velocity
 

Conduction velocities of neurons vary widely Effect of axon diameter
 Larger

diameter fibers have less resistance to local current flow and have faster impulse conduction conduction in unmyelinated axons is slower than saltatory conduction in myelinated axons



Effect of myelination
 Continuous

Conduction Velocity


Effects of myelination
 Myelin  Saltatory

sheaths insulate and prevent leakage of charge conduction in myelinated axons is about 30 times faster
 V lt Voltage-gated t d  APs

Na+ channels are l t d at the nodes N h l located t th d appear to jump rapidly from node to node

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Stimulus

Size of voltage

(a) In a bare plasma membrane (without voltage-gated channels), as on a dendrite, voltage decays because current leaks across the membrane. Voltage-gated Stimulus ion channel

(b) In an unmyelinated axon, voltage-gated Na+ and K+ channels regenerate the action potential at each point along the axon, so voltage does not decay. Conduction is slow because movements of ions and of the gates of channel proteins take time and must occur before voltage regeneration occurs. Stimulus Myelin sheath

Node of Ranvier 1 mm

(c) In a myelinated axon, myelin keeps current in axons (voltage doesn’t decay much). APs are generated only in the nodes of Ranvier and appear to jump rapidly from node to node.

Myelin sheath

Figure 11.15

Multiple Sclerosis (MS)
 

  

An autoimmune disease that mainly affects young adults Symptoms: visual disturbances, weakness, loss of muscular control, speech disturbances, and urinary incontinence Myelin sheaths in the CNS become nonfunctional scleroses Shunting d h Sh i and short-circuiting of nerve i i ii f impulses occurs l Impulse conduction slows and eventually ceases

Multiple Sclerosis: Treatment


Some immune system–modifying drugs, including interferons and Copazone:
 Hold

symptoms at bay complications p  Reduce disability
 Reduce

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Nerve Fiber Classification


Nerve fibers are classified according to:
 Diameter  Degree  Speed Sp

of myelination of conduction

Nerve Fiber Classification


Group A fibers
 Large

diameter, myelinated somatic sensory and motor

fibers


Group B fibers p
 Intermediate

diameter, lightly myelinated ANS fibers



Group C fibers
 Smallest

diameter, unmyelinated ANS fibers

The Synapse


A junction that mediates information transfer from one neuron:
 To  To

another neuron, or an effector cell

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The Synapse




Presynaptic neuron—conducts impulses toward the synapse Postsynaptic neuron—transmits impulses away from the synapse y p

Types of Synapses






Axodendritic—between the axon of one neuron and the dendrite of another Axosomatic—between the axon of one neuron and the soma of another Less common types:
 Axoaxonic

(axon to axon) (dendrite to dendrite)  Dendrosomatic (dendrite to soma)
 Dendrodendritic

Axodendritic synapses Axosomatic synapses Dendrites Cell body Axoaxonic synapses (a) Axon Axon

Axosomatic synapses

(b)

Cell body (soma) of postsynaptic neuron
Figure 11.16

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Electrical Synapses


Less common than chemical synapses
 Neurons

are electrically coupled (joined by gap junctions)  Communication is very rapid, and may be unidirectional or bidirectional  Are important in:
 Embryonic  Some

nervous tissue brain regions

Chemical Synapses




Specialized for the release and reception of neurotransmitters Typically composed of two parts
 Axon

terminal of the presynaptic neuron, which contains synaptic vesicles  Receptor region on the postsynaptic neuron

Synaptic Cleft




Fluid-filled space separating the presynaptic and postsynaptic neurons Prevents nerve impulses from directly passing from one neuron to the next

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Synaptic Cleft


Transmission across the synaptic cleft:
 Is

a chemical event (as opposed to an electrical one) release, diffusion, and binding of neurotransmitters  Ensures unidirectional communication between neurons
 Involves

Information Transfer






AP arrives at axon terminal of the presynaptic neuron and opens voltage-gated Ca2+ channels Synaptotagmin protein binds Ca2+ and promotes fusion of synaptic vesicles with axon membrane y p Exocytosis of neurotransmitter occurs

Information Transfer




Neurotransmitter diffuses and binds to receptors (often chemically gated ion channels) on the postsynaptic neuron Ion channels are opened, causing an excitatory or p , g y inhibitory event (graded potential)

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Chemical synapses transmit signals from one neuron to another using neurotransmitters.
Presynaptic neuron Presynaptic neuron

Postsynaptic neuron 1 Action potential arrives at axon terminal. 2 Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal.

Mitochondrion Ca2+ Ca2+ Ca2+ Ca2+ Synaptic cleft Synaptic vesicles

3 Ca2+ entry causes neurotransmittercontaining synaptic vesicles to release their contents by exocytosis. 4 Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane.

Axon terminal

Postsynaptic neuron

Ion movement Graded potential Reuptake

Enzymatic degradation

Diffusion away from synapse

5 Binding of neurotransmitter opens ion channels, resulting in graded potentials. 6 Neurotransmitter effects are terminated by reuptake through transport proteins, enzymatic degradation, or diffusion away from the synapse.

Figure 11.17

Termination of Neurotransmitter Effects


Within a few milliseconds, the neurotransmitter effect is terminated
 Degradation  Reuptake p

by enzymes by astrocytes or axon terminal y y  Diffusion away from the synaptic cleft

Synaptic Delay






Neurotransmitter must be released, diffuse across the synapse, and bind to receptors Synaptic delay—time needed to do this (0.3–5.0 ms) ) Synaptic delay is the rate-limiting step of neural transmission

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Postsynaptic Potentials
 

Graded potentials Strength determined by:
 

Amount of neurotransmitter released Time the neurotransmitter is in the area



Types of postsynaptic potentials
1. 2.

EPSP—excitatory postsynaptic potentials IPSP—inhibitory postsynaptic potentials

Table 11.2 (1 of 4)

Table 11.2 (2 of 4)

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Table 11.2 (3 of 4)

Table 11.2 (4 of 4)

Excitatory Synapses and EPSPs






Neurotransmitter binds to and opens chemically gated channels that allow simultaneous flow of Na+ and K+ in opposite directions Na+ influx is greater that K+ efflux, causing a net g , g depolarization EPSP helps trigger AP at axon hillock if EPSP is of threshold strength and opens the voltage-gated channels

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Membra ane potential (mV)

Threshold

An EPSP is a local depolarization of the postsynaptic membrane that brings the neuron closer to AP threshold. Neurotransmitter binding opens chemically gated ion channels, allowing the simultaneous passage of Na+ and K+.

Stimulus Time (ms)

(a) Excitatory postsynaptic potential (EPSP)

Figure 11.18a

Inhibitory Synapses and IPSPs






Neurotransmitter binds to and opens channels for K+ or Cl– Causes a hyperpolarization (the inner surface of membrane becomes more negative) g ) Reduces the postsynaptic neuron’s ability to produce an action potential

Membra ane potential (mV)

Threshold

An IPSP is a local hyperpolarization of the postsynaptic membrane and drives the neuron away from AP threshold. Neurotransmitter binding opens K+ or Cl– channels.

Stimulus Time (ms) (b) Inhibitory postsynaptic potential (IPSP)
Figure 11.18b

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Integration: Summation
  

A single EPSP cannot induce an action potential EPSPs can summate to reach threshold IPSPs can also summate with EPSPs, canceling each other out

Integration: Summation


Temporal summation
 One

or more presynaptic neurons transmit impulses in rapid-fire order neuron is stimulated by a large number of terminals at the same time



Spatial summation p
 Postsynaptic

E1

E1

Threshold of axon of postsynaptic neuron Resting potential

E1

E1 Time

E1 E1 Time (b) Temporal summation: 2 excitatory stimuli close in time cause EPSPs that add together.

(a) No summation: 2 stimuli separated in time cause EPSPs that do not add together.
Excitatory synapse 1 (E1) Excitatory synapse 2 (E2) Inhibitory synapse (I1)

Figure 11.19a, b

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E1

E1

E2

I1

E1 + E2 Time (c) Spatial summation: 2 simultaneous stimuli at different locations cause EPSPs that add together.

I1 Time

E1 + I1

(d) Spatial summation of EPSPs and IPSPs: Changes in membane potential can cancel each other out.

Figure 11.19c, d

Integration: Synaptic Potentiation
 



Repeated use increases the efficiency of neurotransmission Ca2+ concentration increases in presynaptic terminal and ostsynaptic neuron Brief high-frequency stimulation partially depolarizes the postsynaptic neuron
Chemically gated channels (NMDA receptors) allow Ca2+ entry  Ca2+ activates kinase enzymes that promote more effective responses to subsequent stimuli


Integration: Presynaptic Inhibition




Release of excitatory neurotransmitter by one neuron may be inhibited by the activity of another neuron via an axoaxonic synapse Less neurotransmitter is released and smaller EPSPs are formed

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Neurotransmitters


 

Most neurons make two or more neurotransmitters, which are released at different stimulation frequencies 50 or more neurotransmitters have been identified Classified by chemical structure and by function

Chemical Classes of Neurotransmitters


Acetylcholine (Ach)
 Released

at neuromuscular junctions and some ANS neurons  Synthesized by enzyme choline acetyltransferase  Degraded by the enzyme acetylcholinesterase (AChE)

Chemical Classes of Neurotransmitters


Biogenic amines include:
 Catecholamines


Dopamine, norepinephrine (NE), and epinephrine Serotonin and histamine

 Indolamines


 

Broadly distributed in the brain Play roles in emotional behaviors and the biological clock

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Chemical Classes of Neurotransmitters


Amino acids include:
 GABA—Gamma  Glycine  Aspartate  Glutamate

()-aminobutyric acid

Chemical Classes of Neurotransmitters


Peptides (neuropeptides) include:
 Substance


P

Mediator of pain signals Act as natural opiates; reduce pain perception

 Endorphins


 Gut-brain


peptides

Somatostatin and cholecystokinin

Chemical Classes of Neurotransmitters


Purines such as ATP:
 Act

in both the CNS and PNS fast or slow responses  Induce Ca2+ influx in astrocytes  Provoke pain sensation
 Produce

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Chemical Classes of Neurotransmitters


Gases and lipids
 Nitric

oxide (NO)
on demand the intracellular receptor guanylyl cyclase to cyclic

 Synthesized  Activates

GMP
 Involved

in learning and memory

 Carbon

monoxide (CO) is a regulator of cGMP in the

brain

Chemical Classes of Neurotransmitters


Gases and lipids
 Endocannabinoids
 Lipid  Bind

soluble; synthesized on demand from membrane lipids with G protein–coupled receptors in the brain  Involved in learning and memory

Functional Classification of Neurotransmitters


Neurotransmitter effects may be excitatory (depolarizing) and/or inhibitory (hyperpolarizing)
Determined by the receptor type of the postsynaptic neuron GABA and glycine are usually inhibitory  Glutamate is usually excitatory  Acetylcholine  Excitatory at neuromuscular junctions in skeletal muscle  Inhibitory in cardiac muscle
 

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Neurotransmitter Actions


Direct action
 Neurotransmitter

binds to channel-linked receptor and opens ion channels  Promotes rapid responses  Examples: ACh and amino acids

Neurotransmitter Actions


Indirect action
 Neurotransmitter

binds to a G protein-linked receptor and acts through an intracellular second messenger  Promotes long-lasting effects  Examples: biogenic amines, neuropeptides, and dissolved gases

Neurotransmitter Receptors


Types
1. 2.

Channel-linked receptors G protein-linked receptors

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Channel-Linked (Ionotropic) Receptors
    

Ligand-gated ion channels Action is immediate and brief Excitatory receptors are channels for small cations Na influx N + i fl contributes most t d t ib t t to depolarization l i ti Inhibitory receptors allow Cl– influx or K+ efflux that causes hyperpolarization

Ion flow blocked Ligand

Ions flow

Closed ion channel

Open ion channel

(a) Channel-linked receptors open in response to binding Figure 11.20a of ligand (ACh in this case).

G Protein-Linked (Metabotropic) Receptors
 



Transmembrane protein complexes Responses are indirect, slow, complex, and often prolonged and widespread Examples: muscarinic ACh receptors and those that bind biogenic amines and neuropeptides

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G Protein-Linked Receptors: Mechanism
  

Neurotransmitter binds to G protein–linked receptor G protein is activated Activated G protein controls production of second messengers, e.g., cyclic AMP, cyclic GMP, g , g, y , y , diacylglycerol or Ca2+

G Protein-Linked Receptors: Mechanism


Second messengers
 Open

or close ion channels kinase enzymes  Phosphorylate channel proteins p y p  Activate genes and induce protein synthesis
 Activate

1 Neurotransmitter (1st messenger) binds and activates receptor.

Adenylate cyclase

Closed ion channel

Open ion channel

Receptor G protein
5a cAMP changes membrane permeability y p g g by opening or closing ion channels.

5c cAMP activates specific genes.

GDP
2 Receptor 3 G protein 4 Adenylate activates G activates cyclase converts protein. adenylate ATP to cAMP cyclase. (2nd messenger).

cAMP activates enzymes.

5b

Nucleus Active enzyme (b) G-protein linked receptors cause formation of an intracellular second messenger (cyclic AMP in this case) that brings about the cell’s response.
Figure 11.17b

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Developmental Aspects of Neurons




The nervous system originates from the neural tube and neural crest formed from ectoderm The neural tube becomes the CNS
Neuroepithelial cells of the neural tube undergo differentiation to form cells needed for development  Cells (neuroblasts) become amitotic and migrate  Neuroblasts sprout axons to connect with targets and become neurons


Axonal Growth


Growth cone at tip of axon interacts with its environment via:
Cell surface adhesion proteins (laminin, integrin, and nerve cell adhesion molecules or N-CAMs) N Neurotropins that attract or repel the growth cone h l h h  Nerve growth factor (NGF), which keeps the neuroblast alive




Astrocytes provide physical support and cholesterol essential for construction of synapses

Cell Death


About 2/3 of neurons die before birth
 Death

results in cells that fail to make functional synaptic contacts  Many cells also die due to apoptosis (programmed cell death) during development

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Central Nervous System (CNS)
 

CNS consists of the brain and spinal cord Cephalization
 Evolutionary

development of the rostral (anterior) p portion of the CNS  Increased number of neurons in the head  Highest level is reached in the human brain

Embryonic Development
 

Neural plate forms from ectoderm Neural plate invaginates to form a neural groove and neural folds

Embryonic Development




Neural groove fuses dorsally to form the neural tube Neural tube gives rise to the brain and spinal cord

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Embryonic Development


Anterior end of the neural tube gives rise to three primary brain vesicles
 Prosencephalon—forebrain  Mesencephalon—midbrain p  Rhombencephalon—hindbrain

Embryonic Development


Primary vesicles give rise to five secondary brain vesicles
 Telencephalon

and diencephalon arise from the forebrain  Mesencephalon remains undivided  Metencephalon and myelencephalon arise from the hindbrain

Embryonic Development




Telencephalon  cerebrum (two hemispheres with cortex, white matter, and basal nuclei) Diencephalon  thalamus, hypothalamus, epithalamus, and retina p ,

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Embryonic Development
 

 

Mesencephalon  brain stem (midbrain) Metencephalon  brain stem (pons) and cerebellum Myelencephalon  brain stem (medulla oblongata) Central canal of the neural tube enlarges to form fluid-filled ventricles

(c) Secondary brain vesicles

(d) Adult brain structures

(e) Adult neural canal regions

Telencephalon

Cerebrum: cerebral hemispheres (cortex, white matter, basal nuclei) Diencephalon (thalamus, hypothalamus, epithalamus), retina Brain stem: midbrain

Lateral ventricles

Diencephalon

Third ventricle

Mesencephalon Metencephalon

Cerebral aqueduct

Brain stem: pons Cerebellum Fourth ventricle

Myelencephalon

Brain stem: medulla oblongata Spinal cord

Central canal
Figure 12.2c-e

Effect of Space Restriction on Brain Development
 



Midbrain flexure and cervical flexure cause forebrain to move toward the brain stem Cerebral hemispheres grow posteriorly and laterally y Cerebral hemisphere surfaces crease and fold into convolutions

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Regions and Organization of the CNS


Adult brain regions
1. 2. 3. 4.

Cerebral hemispheres Diencephalon Brain stem (midbrain, p , and medulla) ( , pons, ) Cerebellum

Regions and Organization of the CNS


Spinal cord
 Central  External

cavity surrounded by a gray matter core white matter composed of myelinated fiber

tracts

Regions and Organization of the CNS


Brain
 Similar  Nuclei

pattern with additional areas of gray matter in cerebellum and cerebrum C Cortex of cerebellum and cerebrum

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Central cavity Migratory pattern of neurons Cerebrum Cerebellum Region of cerebellum

Cortex of gray matter Inner gray matter Outer white matter Gray matter Central cavity Inner gray matter Outer white matter Brain stem Gray matter Central cavity Outer white matter Spinal cord Inner gray matter
Figure 12.4

Ventricles of the Brain




Connected to one another and to the central canal of the spinal cord Lined by ependymal cells

Ventricles of the Brain


Contain cerebrospinal fluid
 Two

C-shaped lateral ventricles in the cerebral hemispheres  Third ventricle in the diencephalon  Fourth ventricle in the hindbrain, dorsal to the pons, develops from the lumen of the neural tube

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Cerebral Hemispheres


Surface markings
Ridges (gyri), shallow grooves (sulci), and deep grooves (fissures)  Five lobes  Frontal  Parietal  Temporal  Occipital  Insula


Cerebral Hemispheres


Surface markings
Central sulcus  Separates the precentral gyrus of the frontal lobe and the postcentral gyrus of the parietal lobe  Longitudinal fissure  Separates the two hemispheres  Transverse cerebral fissure  Separates the cerebrum and the cerebellum


Cerebral Cortex
  





Thin (2–4 mm) superficial layer of gray matter 40% of the mass of the brain Site of conscious mind: awareness, sensory perception, voluntary motor initiation, communication, memory storage, understanding Each hemisphere connects to contralateral side of the body There is lateralization of cortical function in the hemispheres

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Functional Areas of the Cerebral Cortex


The three types of functional areas are:
 Motor

areas—control voluntary movement areas—conscious awareness of sensation  Association areas—integrate diverse information g v
 Sensory


Conscious behavior involves the entire cortex

Motor Areas
   

Primary (somatic) motor cortex Premotor cortex Broca’s area Frontal F t l eye fi ld field

Primary Motor Cortex
  



Large pyramidal cells of the precentral gyri Long axons  pyramidal (corticospinal) tracts Allows conscious control of precise, skilled, voluntary movements Motor homunculi: upside-down caricatures representing the motor innervation of body regions

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Posterior

Motor map in precentral gyrus

Motor Anterior

Toes

Jaw Tongue Swallowing Primary motor cortex (precentral gyrus) Figure 12.9

Premotor Cortex
 

 

Anterior to the precentral gyrus Controls learned, repetitious, or patterned motor skills Coordinates simultaneous or sequential actions Involved in the planning of movements that depend on sensory feedback

Broca’s Area
  



Anterior to the inferior region of the premotor area Present in one hemisphere (usually the left) A motor speech area that directs muscles of the tongue Is active as one prepares to speak

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Frontal Eye Field




Anterior to the premotor cortex and superior to Broca’s area Controls voluntary eye movements

Sensory Areas




 

Primary somatosensory cortex Somatosensory association cortex Visual areas Auditory areas

   

Olfactory cortex Gustatory cortex Visceral sensory area Vestibular cortex

Primary Somatosensory Cortex
 



In the postcentral gyri Receives sensory information from the skin, skeletal muscles, and joints Capable of spatial discrimination: identification of body region being stimulated

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Posterior

Sensory Anterior

Sensory map in postcentral gyrus

Genitals

Primary somatosensory cortex (postcentral gyrus)

Intraabdominal

Figure 12.9

Somatosensory Association Cortex
 



Posterior to the primary somatosensory cortex Integrates sensory input from primary somatosensory cortex Determines size texture and relationship of parts size, texture, of objects being felt

Visual Areas


Primary visual (striate) cortex
 Extreme  Most

posterior tip of the occipital lobe of it is buried in the calcarine sulcus  Receives visual information from the retinas v v

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Visual Areas


Visual association area
 Surrounds  Uses

the primary visual cortex past visual experiences to interpret visual stimuli (e.g., color, form, and movement)  Complex processing involves entire posterior half of the hemispheres

Auditory Areas


Primary auditory cortex
 Superior  Interprets

margin of the temporal lobes information from inner ear as pitch, loudness, and location posterior to the primary auditory cortex memories of sounds and permits perception of



Auditory association area A d
 Located  Stores

sounds

OIfactory Cortex
 

Medial aspect of temporal lobes (in piriform lobes) Part of the primitive rhinencephalon, along with the olfactory bulbs and tracts
 (Remainder

of the rhinencephalon in humans is part of the limbic system)



Region of conscious awareness of odors

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Gustatory Cortex
 

In the insula Involved in the perception of taste

Visceral Sensory Area
 

Posterior to gustatory cortex Conscious perception of visceral sensations, e.g., upset stomach or full bladder

Vestibular Cortex




Posterior part of the insula and adjacent parietal cortex Responsible for conscious awareness of balance (p (position of the head in space) p )

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Multimodal Association Areas
 



Receive inputs from multiple sensory areas Send outputs to multiple areas, including the premotor cortex Allow us to give meaning to information received received, store it as memory, compare it to previous experience, and decide on action to take

Multimodal Association Areas


Three parts
 Anterior

association area (prefrontal cortex) association area  Limbic association area
 Posterior

Anterior Association Area (Prefrontal Cortex)
 





Most complicated cortical region Involved with intellect, cognition, recall, and personality Contains working memory needed for judgment judgment, reasoning, persistence, and conscience Development depends on feedback from social environment

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Posterior Association Area






Large region in temporal, parietal, and occipital lobes Plays a role in recognizing patterns and faces and localizing us in space g p Involved in understanding written and spoken language (Wernicke’s area)

Limbic Association Area
 

Part of the limbic system Provides emotional impact that helps establish memories

Lateralization of Cortical Function


Lateralization
 Division

of labor between hemispheres



Cerebral dominance
 Designates

the hemisphere dominant for language (left hemisphere in 90% of people)

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Lateralization of Cortical Function


Left hemisphere
 Controls

language, math, and logic



Right hemisphere
 Insight Insight,

visual-spatial skills intuition and artistic skills skills, intuition,



Left and right hemispheres communicate via fiber tracts in the cerebral white matter

Cerebral White Matter
 

Myelinated fibers and their tracts Responsible for communication
 Commissures

(in corpus callosum)—connect gray matter of the two hemispheres p  Association fibers—connect different parts of the same hemisphere  Projection fibers—(corona radiata) connect the hemispheres with lower brain or spinal cord

Basal Nuclei (Ganglia)
 

Subcortical nuclei Consists of the corpus striatum
 Caudate  Lentiform

nucleus nucleus (putamen + globus pallidus)



Functionally associated with the subthalamic nuclei (diencephalon) and the substantia nigra (midbrain)

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Functions of Basal Nuclei


Though somewhat elusive, the following are thought to be functions of basal nuclei
 Influence  Help p

muscular control regulate attention and cognition g g  Regulate intensity of slow or stereotyped movements  Inhibit antagonistic and unnecessary movements

Diencephalon


Three paired structures
 Thalamus  Hypothalamus  Epithalamus p



Encloses the third ventricle

Thalamus
  

 

80% of diencephalon Superolateral walls of the third ventricle Connected by the interthalamic adhesion (intermediate mass) Contains several nuclei, named for their location Nuclei project and receive fibers from the cerebral cortex

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Thalamic Function
 

Gateway to the cerebral cortex Sorts, edits, and relays information
Afferent impulses from all senses and all parts of the body Impulses from the hypothalamus for regulation of emotion and visceral function  Impulses from the cerebellum and basal nuclei to help direct the motor cortices
 



Mediates sensation, motor activities, cortical arousal, learning, and memory

Hypothalamus
 

Forms the inferolateral walls of the third ventricle Contains many nuclei
 Example:
 Paired

mammillary bodies

anterior nuclei  Olfactory relay stations


Infundibulum—stalk that connects to the pituitary gland

Hypothalamic Function




Autonomic control center for many visceral functions (e.g., blood pressure, rate and force of heartbeat, digestive tract motility) Center for emotional response: Involved in p perception of pleasure, fear, and rage and in biological rhythms and drives

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Hypothalamic Function


 



Regulates body temperature, food intake, water balance, and thirst Regulates sleep and the sleep cycle Controls release of hormones by the anterior pituitary Produces posterior pituitary hormones

Epithalamus




Most dorsal portion of the diencephalon; forms roof of the third ventricle Pineal gland—extends from the posterior border and secretes melatonin
 Melatonin—helps

regulate sleep-wake cycles

Brain Stem


Three regions
 Midbrain  Pons  Medulla M

oblongata g

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Brain Stem


 



Similar structure to spinal cord but contains embedded nuclei Controls automatic behaviors necessary for survival Contains fiber tracts connecting higher and lower neural centers Associated with 10 of the 12 pairs of cranial nerves

Midbrain
 

Located between the diencephalon and the pons Cerebral peduncles
 Contain

pyramidal motor tracts between third and fourth ventricles



Cerebral aqueduct
 Channel

Midbrain Nuclei




Nuclei that control cranial nerves III (oculomotor) and IV (trochlear) Corpora quadrigemina—domelike dorsal protrusions
 

Superior colliculi—visual reflex centers Inferior colliculi—auditory relay centers

 

Substantia nigra—functionally linked to basal nuclei Red nucleus—relay nuclei for some descending motor pathways and part of reticular formation

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Tectum Periaqueductal gray matter Oculomotor nucleus (III) Medial lemniscus Red nucleus Substantia nigra Fibers of pyramidal tract (a) Midbrain

Dorsal

Superior colliculus Cerebral aqueduct Reticular formation

Ventral

Crus cerebri of cerebral peduncle
Figure 12.16a

Pons
 

Forms part of the anterior wall of the fourth ventricle Fibers of the pons
 

Connect higher brain centers and the spinal cord Relay impulses between the motor cortex and the cerebellum



 

Origin of cranial nerves V (trigeminal), VI (abducens), and VII (facial) Some nuclei of the reticular formation Nuclei that help maintain normal rhythm of breathing

Fourth ventricle Superior cerebellar peduncle Trigeminal main sensory nucleus Trigeminal motor nucleus Middle cerebellar peduncle Trigeminal nerve (V) Medial lemniscus (b) Pons

Reticular formation

Pontine nuclei Fibers of pyramidal tract
Figure 12.16b

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Medulla Oblongata
   



Joins spinal cord at foramen magnum Forms part of the ventral wall of the fourth ventricle Contains a choroid plexus of the fourth ventricle Pyramids—two ventral longitudinal ridges formed y g g by pyramidal tracts Decussation of the pyramids—crossover of the corticospinal tracts

Medulla Oblongata








Inferior olivary nuclei—relay sensory information from muscles and joints to cerebellum Cranial nerves VIII, X, and XII are associated with the medulla Vestibular nuclear complex—mediates responses that maintain equilibrium Several nuclei (e.g., nucleus cuneatus and nucleus gracilis) relay sensory information

Medulla Oblongata
 

Autonomic reflex centers Cardiovascular center
 Cardiac

center adjusts force and rate of heart contraction  Vasomotor center adjusts blood vessel diameter for blood pressure regulation

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Medulla Oblongata


Respiratory centers
 Generate  Control

respiratory rhythm rate and depth of breathing, with pontine

centers

Medulla Oblongata


Additional centers regulate
 Vomiting  Hiccuping  Swallowing S g  Coughing  Sneezing

Hypoglossal nucleus (XII) Dorsal motor nucleus of vagus (X) Inferior cerebellar peduncle
Reticular formation

Fourth ventricle Choroid plexus

Solitary nucleus Vestibular nuclear complex (VIII) Cochlear nuclei (VIII) Nucleus ambiguus Inferior olivary nucleus Pyramid
Figure 12.16c

Lateral nuclear group Medial nuclear group Raphe nucleus Medial lemniscus (c) Medulla oblongata

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The Cerebellum
  

11% of brain mass Dorsal to the pons and medulla Subconsciously provides precise timing and appropriate patterns of skeletal muscle contraction

Anatomy of the Cerebellum
 

Two hemispheres connected by vermis Each hemisphere has three lobes
 Anterior,

posterior, and flocculonodular

 

Folia—transversely oriented gyri Arbor vitae—distinctive treelike pattern of the cerebellar white matter

Anterior lobe Cerebellar cortex Arbor vitae

Cerebellar peduncles • Superior • Middle • Inferior Medulla oblongata (b)

Flocculonodular lobe

Posterior lobe Choroid plexus of fourth ventricle
Figure 12.17b

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Anterior lobe Posterior lobe

(d)

Vermis
Figure 12.17d

Cerebellar Peduncles
 

All fibers in the cerebellum are ipsilateral Three paired fiber tracts connect the cerebellum to the brain stem
 Superior

peduncles connect the cerebellum to the midbrain db  Middle peduncles connect the pons to the cerebellum  Inferior peduncles connect the medulla to the cerebellum

Cerebellar Processing for Motor Activity
 





Cerebellum receives impulses from the cerebral cortex of the intent to initiate voluntary muscle contraction Signals from proprioceptors and visual and equilibrium pathways continuously “inform” the cerebellum of the body’s b d ’ position and momentum d Cerebellar cortex calculates the best way to smoothly coordinate a muscle contraction A “blueprint” of coordinated movement is sent to the cerebral motor cortex and to brain stem nuclei

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Cognitive Function of the Cerebellum
 

Recognizes and predicts sequences of events during complex movements Plays a role in nonmotor functions such as word association and puzzle solving p g

Functional Brain Systems


Networks of neurons that work together and span wide areas of the brain
 Limbic  Reticular

system formation

Limbic System




Structures on the medial aspects of cerebral hemispheres and diencephalon Includes parts of the diencephalon and some cerebral structures that encircle the brain stem

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Septum pellucidum Diencephalic structures of the limbic system •Anterior thalamic nuclei (flanking 3rd ventricle) •Hypothalamus •Mammillary body Corpus callosum

Fiber tracts connecting limbic system structures •Fornix •Anterior commissure Cerebral structures of the y limbic system •Cingulate gyrus •Septal nuclei •Amygdala •Hippocampus •Dentate gyrus •Parahippocampal gyrus

Olfactory bulb
Figure 12.18

Limbic System


Emotional or affective brain
 Amygdala—recognizes

angry or fearful facial expressions, assesses danger, and elicits the fear response  Cingulate gyrus plays a role in expressing emotions gyrus—plays via gestures, and resolves mental conflict


Puts emotional responses to odors
 Example:

skunks smell bad

Limbic System: Emotion and Cognition


The limbic system interacts with the prefrontal lobes, therefore:
 We

can react emotionally to things we consciously understand to be happening  We are consciously aware of emotional richness in our lives


Hippocampus and amygdala—play a role in memory

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Reticular Formation


Three broad columns along the length of the brain stem
 Raphe

nuclei ( g (large cell) group of nuclei )g p  Lateral (small cell) group of nuclei
 Medial


Has far-flung axonal connections with hypothalamus, thalamus, cerebral cortex, cerebellum, and spinal cord

Reticular Formation: RAS and Motor Function


RAS (reticular activating system)
 Sends

impulses to the cerebral cortex to keep it conscious and alert  Filters out repetitive and weak stimuli (~99% of all stimuli!)  Severe injury results in permanent unconsciousness (coma)

Reticular Formation: RAS and Motor Function


Motor function
 Helps  Reticular

control coarse limb movements autonomic centers regulate visceral motor functions
 Vasomotor  Cardiac  Respiratory

centers

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Radiations to cerebral cortex

Visual impulses Reticular formation Ascending general sensory tracts (touch, pain, temperature)

Auditory impulses Descending motor projections to spinal cord
Figure 12.19

Electroencephalogram (EEG)




Records electrical activity that accompanies brain function Measures electrical potential differences between various cortical areas

Brain Waves
   

Patterns of neuronal electrical activity Generated by synaptic activity in the cortex Each person’s brain waves are unique Can be C b grouped into four classes based on di t f l b d frequency measured as Hertz (Hz)

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Types of Brain Waves








Alpha waves (8–13 Hz)—regular and rhythmic, lowamplitude, synchronous waves indicating an “idling” brain Beta waves (14–30 Hz)—rhythmic, less regular waves occurring when mentally alert Theta waves (4–7 Hz)—more irregular; common in children and uncommon in adults Delta waves (4 Hz or less)—high-amplitude waves seen in deep sleep and when reticular activating system is damped, or during anesthesia; may indicate brain damage

1-second interval

Alpha waves—awake but relaxed

Beta waves—awake, alert

Theta waves—common in children

Delta waves—deep sleep (b) Brain waves shown in EEGs fall into four general classes.

Figure 12.20b

Brain Waves: State of the Brain






Change with age, sensory stimuli, brain disease, and the chemical state of the body EEGs used to diagnose and localize brain lesions, tumors, infarcts, infections, abscesses, and epileptic , , , , p p lesions A flat EEG (no electrical activity) is clinical evidence of death

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Epilepsy






A victim of epilepsy may lose consciousness, fall stiffly, and have uncontrollable jerking Epilepsy is not associated with intellectual impairments p Epilepsy occurs in 1% of the population

Epileptic Seizures


Absence seizures, or petit mal
 Mild

seizures seen in young children where the expression goes blank loses consciousness, bones are often broken due to intense contractions, may experience loss of bowel and bladder control, and severe biting of the tongue



Tonic-clonic (grand mal) seizures (g )
 Victim

Control of Epilepsy
 

Anticonvulsive drugs Vagus nerve stimulators implanted under the skin of the chest can keep electrical activity of the brain from becoming chaotic g

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Consciousness
  



Conscious perception of sensation Voluntary initiation and control of movement Capabilities associated with higher mental processing (memory logic judgment etc.) (memory, logic, judgment, etc ) Loss of consciousness (e.g., fainting or syncopy) is a signal that brain function is impaired

Consciousness


Clinically defined on a continuum that grades behavior in response to stimuli
 Alertness  Drowsiness  Stupor  Coma

( (lethargy) gy)

Sleep




State of partial unconsciousness from which a person can be aroused by stimulation Two major types of sleep (defined by EEG patterns)
 Nonrapid  Rapid

eye movement (NREM) eye movement (REM)

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Sleep




First two stages of NREM occur during the first 30– 45 minutes of sleep Fourth stage is achieved in about 90 minutes, and then REM sleep begins abruptly p g p y

Awake

REM: Skeletal muscles (except ocular muscles and diaphragm) are actively inhibited; most dreaming occurs. NREM stage 1: Relaxation begins; EEG shows alpha waves, arousal is easy. NREM stage 2: Irregular EEG with sleep spindles (short high- amplitude bursts); arousal is more difficult. NREM stage 3: Sleep deepens; theta and delta waves appear; vital signs decline. NREM stage 4: EEG is dominated by delta waves; arousal is difficult; bed-wetting, night terrors, and sleepwalking may occur. Figure 12.21a

(a) Typical EEG patterns

Sleep Patterns








Alternating cycles of sleep and wakefulness reflect a natural circadian (24-hour) rhythm RAS activity is inhibited during, but RAS also mediates, dreaming sleep , g p The suprachiasmatic and preoptic nuclei of the hypothalamus time the sleep cycle A typical sleep pattern alternates between REM and NREM sleep

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Awake REM Stage 1 Stage 2 Non Stage 3 REM Stage 4 Time (hrs) (b) Typical progression of an adult through one night’s sleep stages
Figure 12.21b

Importance of Sleep






 

Slow-wave sleep (NREM stages 3 and 4) is presumed to be the restorative stage People deprived of REM sleep become moody and depressed REM sleep may be a reverse learning process where superfluous information is purged from the brain Daily sleep requirements decline with age Stage 4 sleep declines steadily and may disappear after age 60

Sleep Disorders


Narcolepsy
 Lapsing

abruptly into sleep from the awake state



Insomnia
 Chronic

inability to obtain the amount or quality of sleep needed cessation of breathing during sleep



Sleep apnea
 Temporary

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Language


Language implementation system
 Basal

nuclei area and Wernicke’s area (in the association cortex on the left side) A l Analyzes i incoming word sounds i d d  Produces outgoing word sounds and grammatical structures
 Broca’s


Corresponding areas on the right side are involved with nonverbal language components

Memory
 

Storage and retrieval of information Two stages of storage
 Short-term

memory (STM, or working memory)— temporary holding of information; limited to seven or p y g ; eight pieces of information  Long-term memory (LTM) has limitless capacity

Outside stimuli

General and special sensory receptors Afferent inputs Temporary storage (buffer) in cerebral cortex Automatic memory Data selected for transfer Data permanently lost

Forget F t

Short-term memory (STM)

Forget

Data transfer influenced by: Retrieval Excitement Rehearsal Association of old and new data Long-term memory (LTM)

Figure 12.22

Data unretrievable

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Transfer from STM to LTM


Factors that affect transfer from STM to LTM
 Emotional

state—best if alert, motivated, surprised, and aroused  Rehearsal—repetition and practice  Association—tying new information with old memories  Automatic memory—subconscious information stored in LTM

Categories of Memory
1.

Declarative memory (factual knowledge)
 



Explicit information Related to our conscious thoughts and our language ability Stored in LTM with context in which it was learned

Categories of Memory
2.

Nondeclarative memory
   

Less conscious or unconscious Acquired through experience and repetition Best remembered by doing; hard to unlearn y g; Includes procedural (skills) memory, motor memory, and emotional memory

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Brain Structures Involved in Declarative Memory
 

Hippocampus and surrounding temporal lobes function in consolidation and access to memory ACh from basal forebrain is necessary for memory formation and retrieval

Thalamus Touch Hearing Vision Taste Smell Basal forebrain Prefrontal cortex

Hippocampus
Sensory input Thalamus

(a) Declarative memory circuits

Association cortex

Medial temporal lobe (hippocampus, etc.) ACh Basal forebrain

Prefrontal cortex ACh

Figure 12.23a

Brain Structures Involved in Nondeclarative Memory


Procedural memory
 Basal

nuclei relay sensory and motor inputs to the thalamus and premotor cortex  Dopamine from substantia nigra is necessary
 

Motor memory—cerebellum Emotional memory—amygdala

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Sensory and motor inputs

Association cortex

Basal nuclei

Thalamus

Premotor cortex

Dopamine Substantia nigra

Premotor cortex

Thalamus

Basal nuclei Substantia nigra

(b) Procedural (skills) memory circuits
Figure 12.23b

Molecular Basis of Memory


During learning:
Altered mRNA is synthesized and moved to axons and dendrites  Dendritic spines change shape  Extracellular proteins are deposited at synapses involved in LTM  Number and size of presynaptic terminals may increase  More neurotransmitter is released by presynaptic neurons


Molecular Basis of Memory




Increase in synaptic strength (long-term potentiation, or LTP) is crucial Neurotransmitter (glutamate) binds to NMDA receptors, opening calcium channels in postsynaptic p , p g p y p terminal

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Molecular Basis of Memory




Calcium influx triggers enzymes that modify proteins of the postsynaptic terminal and presynaptic terminal (via release of retrograde messengers) Enzymes trigger postsynaptic gene activation for synthesis of synaptic proteins, in presence of CREB (cAMP response-element binding protein) and BDNF (brain-derived neurotrophic factor)

Protection of the Brain
   

Bone (skull) Membranes (meninges) Watery cushion (cerebrospinal fluid) Blood-brain b i Bl d b i barrier

Meninges
   

Cover and protect the CNS Protect blood vessels and enclose venous sinuses Contain cerebrospinal fluid (CSF) Form partitions i th skull F titi in the k ll

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Meninges


Three layers
 Dura

mater mater  Pia mater
 Arachnoid

Superior sagittal sinus itt l i Subdural space Subarachnoid space

Skin of scalp Periosteum Bone of skull Periosteal Dura Meningeal mater Arachnoid mater Pia mater Arachnoid villus Blood vessel Falx cerebri (in longitudinal fissure only)
Figure 12.24

Dura Mater
 

Strongest meninx Two layers of fibrous connective tissue (around the brain) separate to form dural sinuses

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Dura Mater


Dural septa limit excessive movement of the brain
 Falx

cerebri—in the longitudinal fissure; attached to crista galli  Falx cerebelli—along the vermis of the cerebellum  Tentorium cerebelli—horizontal dural fold over cerebellum and in the transverse fissure

Superior sagittal sinus Straight sinus Crista galli of the ethmoid bone Pituitary gland Falx cerebri

Tentorium cerebelli Falx cerebelli

(a) Dural septa

Figure 12.25a

Arachnoid Mater
 

 

Middle layer with weblike extensions Separated from the dura mater by the subdural space Subarachnoid space contains CSF and blood vessels Arachnoid villi protrude into the superior sagittal sinus and permit CSF reabsorption

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Pia Mater


Layer of delicate vascularized connective tissue that clings tightly to the brain

Cerebrospinal Fluid (CSF)


Composition
 Watery  Less

solution protein and different ion concentrations than plasma  Constant volume

Cerebrospinal Fluid (CSF)


Functions
 Gives

buoyancy to the CNS organs the CNS from blows and other trauma N Nourishes the brain and carries chemical signals g
 Protects

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Choroid Plexuses
  



Produce CSF at a constant rate Hang from the roof of each ventricle Clusters of capillaries enclosed by pia mater and a layer of ependymal cells Ependymal cells use ion pumps to control the composition of the CSF and help cleanse CSF by removing wastes

Ependymal cells Capillary Connective tissue of pia mater Section of choroid plexus

Wastes and unnecessary solutes absorbed

CSF forms as a filtrate containing glucose, oxygen, vitamins, and ions (Na+, Cl–, Mg2+, etc.) (b) CSF formation by choroid plexuses Cavity of ventricle
Figure 12.26b

Superior sagittal sinus Choroid plexus Interventricular foramen

4

Arachnoid villus Subarachnoid space Arachnoid mater Meningeal dura mater Periosteal dura mater
1

3

Right lateral ventricle (deep to cut) Choroid plexus of fourth ventricle
1 CSF is produced by the choroid plexus of each ventricle. 2 CSF flows through the ventricles and into the subarachnoid space via the median and lateral apertures. Some CSF flows through the central canal of the spinal cord. 3 CSF flows through the subarachnoid space. 4 CSF is absorbed into the dural venous sinuses via the arachnoid villi. Figure 12.26a

Third ventricle Cerebral aqueduct Lateral aperture Fourth ventricle Median aperture Central canal of spinal cord (a) CSF circulation

2

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Blood-Brain Barrier
 

Helps maintain a stable environment for the brain Separates neurons from some bloodborne substances

Blood-Brain Barrier


Composition
 Continuous  Basal

endothelium of capillary walls lamina  Feet of astrocytes y
 Provide

signal to endothelium for the formation of tight

junctions

Capillary

Neuron

Astrocyte

(a) Astrocytes are the most abundant CNS neuroglia. Figure 11.3a

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Blood-Brain Barrier: Functions


Selective barrier
 Allows  Allows

nutrients to move by facilitated diffusion any fat-soluble substances to pass, including alcohol, nicotine, and anesthetics



Absent in some areas, e.g., vomiting center and the hypothalamus, where it is necessary to monitor the chemical composition of the blood

Homeostatic Imbalances of the Brain


Traumatic brain injuries
 Concussion—temporary  Contusion—permanent

alteration in function damage S Subdural or subarachnoid hemorrhage—may force g y brain stem through the foramen magnum, resulting in death  Cerebral edema—swelling of the brain associated with traumatic head injury

Homeostatic Imbalances of the Brain


Cerebrovascular accidents (CVAs)(strokes)
Blood circulation is blocked and brain tissue dies, e.g., blockage of a cerebral artery by a blood clot  Typically leads to hemiplegia, or sensory and speed deficits  Transient ischemic attacks (TIAs)—temporary episodes of reversible cerebral ischemia  Tissue plasminogen activator (TPA) is the only approved treatment for stroke


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Homeostatic Imbalances of the Brain


Degenerative brain disorders
Alzheimer’s disease (AD): a progressive degenerative disease of the brain that results in dementia  Parkinson’s disease: degeneration of the dopaminereleasing neurons of th substantia nigra l i f the b t ti i  Huntington’s disease: a fatal hereditary disorder caused by accumulation of the protein huntingtin that leads to degeneration of the basal nuclei and cerebral cortex


The Spinal Cord: Embryonic Development


By week 6, there are two clusters of neuroblasts
 Alar

plate—will become interneurons; axons form white matter of cord  Basal plate—will become motor neurons; axons will grow to effectors


Neural crest cells form the dorsal root ganglia sensory neurons; axons grow into the dorsal aspect of the cord

Dorsal root ganglion: sensory neurons from neural crest

Alar plate: interneurons White matter Basal plate: motor neurons

Neural tube cells

Central cavity

Figure 12.28

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Spinal Cord


Location
 Begins  Ends

at the foramen magnum as conus medullaris at L1 vertebra two-way communication to and from the brain spinal reflex centers



Functions
 Provides  Contains

Spinal Cord: Protection
 



Bone, meninges, and CSF Cushion of fat and a network of veins in the epidural space between the vertebrae and spinal dura mater CSF in subarachnoid space

Spinal Cord: Protection




Denticulate ligaments: extensions of pia mater that secure cord to dura mater Filum terminale: fibrous extension from conus medullaris; anchors the spinal cord to the coccyx ; p y

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Cervical enlargement Dura and arachnoid mater Lumbar enlargement Conus medullaris Cauda equina Filum terminale

Cervical spinal nerves

Thoracic spinal nerves

Lumbar spinal nerves Sacral spinal nerves

(a) The spinal cord and its nerve roots, with the bony vertebral arches removed. The dura mater and arachnoid mater are cut open and reflected laterally.

Figure 12.29a

Spinal Cord


Spinal nerves
 31

pairs



Cervical and lumbar enlargements
 The

nerves serving the upper and lower limbs emerge here collection of nerve roots at the inferior end of the vertebral canal



Cauda equina
 The

Cross-Sectional Anatomy


Two lengthwise grooves divide cord into right and left halves
 Ventral  Dorsal

(anterior) median fissure (p (posterior) median sulcus )



Gray commissure—connects masses of gray matter; encloses central canal

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Epidural space (contains fat) Subdural space Subarachnoid space (contains CSF)

Pia mater Arachnoid mater Dura mater

Spinal meninges Bone of vertebra Dorsal root ganglion Body of vertebra

(a) Cross section of spinal cord and vertebra

Figure 12.31a

Dorsal median sulcus Dorsal funiculus White Ventral funiculus columns Lateral funiculus Dorsal root ganglion Spinal nerve Dorsal root (fans out into dorsal rootlets) Ventral root (derived from several ventral rootlets) Gray commissure Dorsal horn Gray Ventral horn matter Lateral horn

Central canal

Ventral median fissure Pia mater Arachnoid mater Spinal dura mater

(b) The spinal cord and its meningeal coverings
Figure 12.31b

Gray Matter








Dorsal horns—interneurons that receive somatic and visceral sensory input Ventral horns—somatic motor neurons whose axons exit the cord via ventral roots Lateral horns (only in thoracic and lumbar regions) – sympathetic neurons Dorsal root (spinal) gangia—contain cell bodies of sensory neurons

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Dorsal root (sensory) Dorsal root ganglion Somatic sensory neuron Visceral sensory neuron Visceral motor neuron Somatic motor neuron Spinal nerve Ventral root (motor) Ventral horn (motor neurons) Dorsal horn (interneurons)

Interneurons receiving input from somatic sensory neurons Interneurons receiving input from visceral sensory neurons Visceral motor (autonomic) neurons Somatic motor neurons
Figure 12.32

White Matter








Consists mostly of ascending (sensory) and descending (motor) tracts Transverse tracts (commissural fibers) cross from one side to the other Tracts are located in three white columns (funiculi on each side—dorsal (posterior), lateral, and ventral (anterior) Each spinal tract is composed of axons with similar functions

Pathway Generalizations
  



Pathways decussate (cross over) Most consist of two or three neurons (a relay) Most exhibit somatotopy (precise spatial relationships) Pathways are paired symmetrically (one on each side of the spinal cord or brain)

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Ascending tracts Fasciculus gracilis Dorsal white Fasciculus cuneatus column Dorsal spinocerebellar tract Ventral spinocerebellar tract Lateral spinothalamic tract Ventral spinothalamic tract

Descending tracts Ventral white commissure Lateral reticulospinal tract Lateral corticospinal tract Rubrospinal tract Medial reticulospinal tract Ventral corticospinal tract Vestibulospinal tract Tectospinal tract
Figure 12.33

Ascending Pathways
 

Consist of three neurons First-order neuron
 Conducts

impulses from cutaneous receptors and p p proprioceptors p  Branches diffusely as it enters the spinal cord or medulla  Synapses with second-order neuron

Ascending Pathways


Second-order neuron
 Interneuron  Cell

body in dorsal horn of spinal cord or medullary nuclei  Axons extend to thalamus or cerebellum

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Ascending Pathways


Third-order neuron
 Interneuron  Cell

 Axon

body in thalamus extends to somatosensory cortex y

Ascending Pathways


Two pathways transmit somatosensory information to the sensory cortex via the thalamus
 Dorsal  Spinothalamic p

column-medial lemniscal pathways pathways p y



Spinocerebellar tracts terminate in the cerebellum

Dorsal Column-Medial Lemniscal Pathways
 

Transmit input to the somatosensory cortex for discriminative touch and vibrations Composed of the paired fasciculus cuneatus and fasciculus gracilis in the spinal cord and the medial g p lemniscus in the brain (medulla to thalamus)

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Primary somatosensory cortex Axons of third-order neurons Thalamus
Cerebrum Midbrain

Cerebellum Dorsal spinocerebellar tract (axons of second-order second order neurons) Pons Medial l M di l lemniscus (t i (tract) t) (axons of second-order neurons) Nucleus gracilis Nucleus cuneatus Medulla oblongata Fasciculus cuneatus (axon of first-order sensory neuron) Joint stretch receptor (proprioceptor) Cervical spinal cord Fasciculus gracilis (axon of first-order sensory neuron) Lumbar spinal cord Touch receptor Dorsal column–medial lemniscal pathway Figure 12.34a

Axon of first-order neuron Muscle spindle (proprioceptor)

(a)

Spinocerebellar pathway

Anterolateral Pathways
 

Lateral and ventral spinothalamic tracts Transmit pain, temperature, and coarse touch impulses within the lateral spinothalamic tract

Primary somatosensory cortex Axons of third-order neurons

Thalamus
Cerebrum Midbrain

Cerebellum Pons

Lateral spinothalamic t L t l i th l i tract t (axons of second-order neurons)
Medulla oblongata

Pain receptors
Cervical spinal cord Lumbar spinal cord (b)

Axons of first-order neurons Temperature receptors

Spinothalamic pathway
Figure 12.34b

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Spinocerebellar Tracts
 

Ventral and dorsal tracts Convey information about muscle or tendon stretch to the cerebellum

Descending Pathways and Tracts


Deliver efferent impulses from the brain to the spinal cord
 Direct  Indirect

pathways—pyramidal tracts p pathways—all others y

Descending Pathways and Tracts


Involve two neurons:
1.

Upper motor neurons


Pyramidal cells in primary motor cortex Ventral horn motor neurons Innervate skeletal muscles

2.

Lower motor neurons
 

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The Direct (Pyramidal) System






Impulses from pyramidal neurons in the precentral gyri pass through the pyramidal (corticospinal)l tracts Axons synapse with interneurons or ventral horn y p motor neurons The direct pathway regulates fast and fine (skilled) movements

Pyramidal cells (upper motor neurons)

Primary motor cortex Internal capsule
Cerebrum Midbrain Cerebellum Pons

Cerebral peduncle Ventral corticospinal tract Pyramids Decussation of pyramid Lateral corticospinal tract Skeletal muscle

Medulla oblongata

Cervical spinal cord

Lumbar spinal cord (a) Pyramidal (lateral and ventral corticospinal) pathways

Somatic motor neurons (lower motor neurons)
Figure 12.35a

Indirect (Extrapyramidal) System




Includes the brain stem motor nuclei, and all motor pathways except pyramidal pathways Also called the multineuronal pathways

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Indirect (Extrapyramidal) System


These pathways are complex and multisynaptic, and regulate:
 Axial

muscles that maintain balance and posture controlling coarse movements g  Head, neck, and eye movements that follow objects
 Muscles

Indirect (Extrapyramidal) System


 

Reticulospinal and vestibulospinal tracts—maintain balance Rubrospinal tracts—control flexor muscles Superior colliculi and tectospinal tracts mediate head movements in response to visual stimuli

Cerebrum

Red nucleus
Midbrain Cerebellum Pons

Rubrospinal tract
Medulla oblongata

Cervical spinal cord

(b)

Rubrospinal tract
Figure 12.35b

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Spinal Cord Trauma


Functional losses
 Parasthesias
 Sensory

loss

 Paralysis
 Loss

of motor function

Spinal Cord Trauma


Flaccid paralysis—severe damage to the ventral root or ventral horn cells
 Impulses

do not reach muscles; there is no voluntary or involuntary control of muscles  Muscles atrophy

Spinal Cord Trauma


Spastic paralysis—damage to upper motor neurons of the primary motor cortex
 Spinal

neurons remain intact; muscles are stimulated by reflex activity  No voluntary control of muscles

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Spinal Cord Trauma


Transection
 Cross

sectioning of the spinal cord at any level in total motor and sensory loss in regions inferior to the cut  Paraplegia—transection between T1 and L1  Quadriplegia—transection in the cervical region
 Results

Poliomyelitis


 



Destruction of the ventral horn motor neurons by the poliovirus Muscles atrophy Death may occur due to paralysis of respiratory muscles or cardiac arrest Survivors often develop postpolio syndrome many years later, as neurons are lost

Amyotrophic Lateral Sclerosis (ALS)
 



 

Also called Lou Gehrig’s disease Involves progressive destruction of ventral horn motor neurons and fibers of the pyramidal tract Symptoms—loss of the ability to speak, swallow, and breathe Death typically occurs within five years Linked to glutamate excitotoxicity, attack by the immune system, or both

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Developmental Aspects of the CNS
 





CNS is established during the first month of development Gender-specific areas appear in both brain and spinal cord, depending on presence or absence of fetal testosterone Maternal exposure to radiation, drugs (e.g., alcohol and opiates), or infection can harm the developing CNS Smoking decreases oxygen in the blood, which can lead to neuron death and fetal brain damage

Developmental Aspects of the CNS






The hypothalamus is one of the last areas of the CNS to develop Visual cortex develops slowly over the first 11 weeks Neuromuscular coordination progresses in superiorto-inferior and proximal-to-distal directions along with myelination

Developmental Aspects of the CNS


 

Age brings some cognitive declines, but these are not significant in healthy individuals until they reach their 80s Shrinkage of brain accelerates in old age g g Excessive use of alcohol causes signs of senility unrelated to the aging process

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Peripheral Nervous System (PNS)


All neural structures outside the brain
 Sensory

receptors nerves and associated ganglia M Motor endings g
 Peripheral

Central nervous system (CNS)

Peripheral nervous system (PNS)

Sensory (afferent) division

Motor (efferent) division

Somatic nervous system

Autonomic nervous system (ANS)

Sympathetic division

Parasympathetic division

Figure 13.1

Sensory Receptors






Specialized to respond to changes in their environment (stimuli) Activation results in graded potentials that trigger nerve impulses p Sensation (awareness of stimulus) and perception (interpretation of the meaning of the stimulus) occur in the brain

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Classification of Receptors


Based on:
 Stimulus  Location  Structural S

type complexity p y

Classification by Stimulus Type


  



Mechanoreceptors—respond to touch, pressure, vibration, stretch, and itch Thermoreceptors—sensitive to changes in temperature Photoreceptors—respond to light energy (e.g., retina) Chemoreceptors—respond to chemicals ( Ch d h i l (e.g., smell, taste, ll changes in blood chemistry) Nociceptors—sensitive to pain-causing stimuli (e.g. extreme heat or cold, excessive pressure, inflammatory chemicals)

Classification by Location
1.

Exteroceptors
 



Respond to stimuli arising outside the body Receptors in the skin for touch, pressure, pain, and temperature Most special sense organs

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Classification by Location
2.

Interoceptors (visceroceptors)




Respond to stimuli arising in internal viscera and blood vessels Sensitive to chemical changes, tissue stretch, and temperature changes

Classification by Location
3.

Proprioceptors




Respond to stretch in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles Inform the brain of one’s movements

Classification by Structural Complexity
1.

Complex receptors (special sense organs)


Vision, hearing, equilibrium, smell, and taste

2.

Simple receptors for general senses:
 

Tactile sensations (touch, pressure, stretch, vibration), temperature, pain, and muscle sense Unencapsulated (free) or encapsulated dendritic endings

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Unencapsulated Dendritic Endings


Thermoreceptors
 Cold  Heat

receptors (10–40ºC); in superficial dermis receptors (32–48ºC); in deeper dermis

Unencapsulated Dendritic Endings


Nociceptors
 Respond

to:
from damaged tissue outside the range of thermoreceptors

 Pinching  Chemicals  Capsaicin  Temperatures

Unencapsulated Dendritic Endings


Light touch receptors
 Tactile  Hair

(Merkel) discs follicle receptors

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Table 13.1

Encapsulated Dendritic Endings


All are mechanoreceptors
Meissner’s (tactile) corpuscles—discriminative touch Pacinian (lamellated) corpuscles—deep pressure and vibration  Ruffini endings—deep continuous pressure  Muscle spindles—muscle stretch  Golgi tendon organs—stretch in tendons  Joint kinesthetic receptors—stretch in articular capsules
 

Table 13.1

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From Sensation to Perception
 



Survival depends upon sensation and perception Sensation: the awareness of changes in the internal and external environment Perception: the conscious interpretation of those stimuli

Sensory Integration




Input comes from exteroceptors, proprioceptors, and interoceptors Input is relayed toward the head, but is processed along the way g y

Sensory Integration


Levels of neural integration in sensory systems:
1. 2. 3.

Receptor level—the sensor receptors Circuit level—ascending pathways Perceptual level—neuronal circuits in the cerebral p v cortex

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3

Perceptual level (processing in cortical sensory centers) Motor cortex Somatosensory cortex Thalamus

2

Reticular formation Pons Circuit level Ci it l l (processing in Spinal ascending pathways) cord

Cerebellum Medulla

Free nerve endings (pain, cold, warmth) Muscle spindle
1

Receptor level (sensory reception Joint and transmission kinesthetic to CNS) receptor

Figure 13.2

Processing at the Receptor Level
  

Receptors have specificity for stimulus energy Stimulus must be applied in a receptive field Transduction occurs
 Stimulus

energy is converted into a graded potential called a receptor potential

Processing at the Receptor Level


In general sense receptors, the receptor potential and generator potential are the same thing stimulus  receptor/generator potential in afferent neuron  action potential at first node of Ranvier

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Processing at the Receptor Level


In special sense organs: stimulus  receptor potential in receptor cell  release of neurotransmitter  generator potential in first-order sensory neuron  action potentials (if threshold is reached)

Adaptation of Sensory Receptors


Adaptation is a change in sensitivity in the presence of a constant stimulus
 Receptor  Receptor p

membranes become less responsive potentials decline in frequency or stop p q y p

Adaptation of Sensory Receptors


Phasic (fast-adapting) receptors signal the beginning or end of a stimulus
 Examples:

receptors for pressure, touch, and smell nociceptors and most proprioceptors



Tonic receptors adapt slowly or not at all
 Examples:

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Processing at the Circuit Level




Pathways of three neurons conduct sensory impulses upward to the appropriate brain regions First-order neurons


Conduct impulses from the receptor level to the secondorder neurons in the CNS Transmit impulses to the thalamus or cerebellum Conduct impulses from the thalamus to the somatosensory cortex (perceptual level)



Second-order neurons




Third-order neurons


Processing at the Perceptual Level




Identification of the sensation depends on the specific location of the target neurons in the sensory cortex Aspects of sensory perception:
Perceptual detection—ability to detect a stimulus (requires summation of impulses)  Magnitude estimation—intensity is coded in the frequency of impulses  Spatial discrimination—identifying the site or pattern of the stimulus (studied by the two-point discrimination test)


Main Aspects of Sensory Perception






Feature abstraction—identification of more complex aspects and several stimulus properties Quality discrimination—the ability to identify submodalities of a sensation (e.g., sweet or sour ( g, tastes) Pattern recognition—recognition of familiar or significant patterns in stimuli (e.g., the melody in a piece of music)

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Perception of Pain
 





Warns of actual or impending tissue damage Stimuli include extreme pressure and temperature, histamine, K+, ATP, acids, and bradykinin Impulses travel on fibers that release neurotransmitters glutamate and substance P Some pain impulses are blocked by inhibitory endogenous opioids

Structure of a Nerve
 

Cordlike organ of the PNS Bundle of myelinated and unmyelinated peripheral axons enclosed by connective tissue

Structure of a Nerve


Connective tissue coverings include:
 Endoneurium—loose

connective tissue that encloses axons and their myelin sheaths  Perineurium—coarse connective tissue that bundles fibers into fascicles  Epineurium—tough fibrous sheath around a nerve

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Endoneurium Perineurium Epineurium

Axon Myelin sheath

Fascicle Blood vessels

Figure 13.3b

(b)

Classification of Nerves






Most nerves are mixtures of afferent and efferent fibers and somatic and autonomic (visceral) fibers Pure sensory (afferent) or motor (efferent) nerves are rare Types of fibers in mixed nerves:
 

Somatic afferent and somatic efferent Visceral afferent and visceral efferent



Peripheral nerves classified as cranial or spinal nerves

Ganglia


Contain neuron cell bodies associated with nerves
 Dorsal  Autonomic

root ganglia (sensory, somatic) (Chapter 12) ganglia (motor, visceral) (Chapter 14)

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Regeneration of Nerve Fibers
 



Mature neurons are amitotic If the soma of a damaged nerve is intact, axon will regenerate Involves coordinated activity among:
Macrophages—remove d b i M h debris Schwann cells—form regeneration tube and secrete growth factors  Axons—regenerate damaged part
 



CNS oligodendrocytes bear growth-inhibiting proteins that prevent CNS fiber regeneration

Endoneurium

Schwann cells Droplets of myelin

1 The axon becomes fragmented at f t d t the injury site.

Fragmented axon Site of nerve damage

Figure 13.4 (1 of 4)

Schwann cell

Macrophage

2 Macrophages clean out the dead axon distal a on to the injury.

Figure 13.4 (2 of 4)

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Aligning Schwann cells form regeneration tube

3 Axon sprouts, or filaments, grow through a regeneration tube formed by Schwann cells.

Fine axon sprouts or filaments

Figure 13.4 (3 of 4)

Schwann cell

Site of new myelin sheath formation f ti

4 The axon regenerates and a new myelin sheath forms.

Single enlarging axon filament

Figure 13.4 (4 of 4)

Levels of Motor Control
  

Segmental level Projection level Precommand level

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Precommand Level (highest) • Cerebellum and basal nuclei • Programs and instructions (modified by feedback) Feedback Projection Level (middle) • Motor cortex (pyramidal system) and brain stem nuclei (vestibular, red, reticular formation, etc.) • Convey instructions to spinal cord motor neurons and send a copy of that information to higher levels Segmental Level (lowest) • Spinal cord • Contains central pattern generators (CPGs) Internal feedback

Sensory input

Reflex activity

Motor output
Figure 13.13a

(a) Levels of motor control and their interactions

Segmental Level
 



The lowest level of the motor hierarchy Central pattern generators (CPGs): segmental circuits that activate networks of ventral horn neurons to stimulate specific groups of muscles p g p Controls locomotion and specific, oft-repeated motor activity

Projection Level


Consists of:
 Upper

motor neurons that direct the direct (pyramidal) system to produce voluntary skeletal muscle movements  Brain stem motor areas that oversee the indirect (extrapyramidal) system to control reflex and CPG CPGcontrolled motor actions


Projection motor pathways keep higher command levels informed of what is happening

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Precommand Level


Neurons in the cerebellum and basal nuclei
 Regulate  Precisely

motor activity start or stop movements  Coordinate movements with posture  Block unwanted movements  Monitor muscle tone  Perform unconscious planning and discharge in advance of willed movements

Precommand Level


Cerebellum
 Acts

on motor pathways through projection areas of the brain stem  Acts on the motor cortex via the thalamus


Basal nuclei
 Inhibit

various motor centers under resting conditions

Reflexes




Inborn (intrinsic) reflex: a rapid, involuntary, predictable motor response to a stimulus Learned (acquired) reflexes result from practice or repetition, p ,
 Example:

driving skills

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Reflex Arc


Components of a reflex arc (neural path)
1. 2. 3.

4.

5.

Receptor—site of stimulus action Sensory neuron—transmits afferent impulses to the CNS Integration center—either monosynaptic or polysynaptic region within the CNS Motor neuron—conducts efferent impulses from the integration center to an effector organ Effector—muscle fiber or gland cell that responds to the efferent impulses by contracting or secreting

Stimulus

Skin

1 Receptor 2 Sensory neuron 3 Integration center 4 Motor neuron 5 Effector

Interneuron

Spinal cord (in cross section)
Figure 13.14

Spinal Reflexes


Spinal somatic reflexes
 Integration  Effectors

center is in the spinal cord are skeletal muscle



Testing of somatic reflexes is important clinically to assess the condition of the nervous system

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Stretch and Golgi Tendon Reflexes


For skeletal muscle activity to be smoothly coordinated, proprioceptor input is necessary
 Muscle

spindles inform the nervous system of the length of the muscle  Golgi tendon organs inform the brain as to the amount of tension in the muscle and tendons

Muscle Spindles




Composed of 3–10 short intrafusal muscle fibers in a connective tissue capsule Intrafusal fibers
 Noncontractile

in their central regions (lack myofilaments)  Wrapped with two types of afferent endings: primary sensory endings of type Ia fibers and secondary sensory endings of type II fibers

Muscle Spindles




Contractile end regions are innervated by gamma () efferent fibers that maintain spindle sensitivity Note: extrafusal fibers (contractile muscle fibers) y p ( ) are innervated by alpha () efferent fibers

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Secondary sensory endings (type II fiber)

Efferent (motor) fiber to muscle spindle  Efferent (motor) fiber to extrafusal muscle fibers Extrafusal muscle fiber Intrafusal muscle fibers

Primary sensory endings (type Ia fiber) Muscle spindle Connective tissue capsule

Sensory fiber

Golgi tendon organ

Tendon
Figure 13.15

Muscle Spindles


Excited in two ways:
1. 2.

External stretch of muscle and muscle spindle Internal stretch of muscle spindle:


Activating the  motor neurons stimulates the ends to contract, contract thereby stretching the spindle



Stretch causes an increased rate of impulses in Ia fibers

Muscle spindle Intrafusal muscle fiber Primary sensory (la) nerve fiber Extrafusal muscle fiber

Time (a) Unstretched muscle. Action potentials (APs) are generated at a constant rate in the associated sensory (la) fiber.

Time (b) Stretched muscle. Stretching activates the muscle spindle, increasing the rate of APs.

Figure 13.16a, b

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Muscle Spindles






Contracting the muscle reduces tension on the muscle spindle Sensitivity would be lost unless the muscle spindle is y p shortened by impulses in the  motor neurons – coactivation maintains the tension and sensitivity of the spindle during muscle contraction

Time

Time

(c) Only motor (d) - Coactivation. neurons activated. Both extrafusal and Only the extrafusal intrafusal muscle muscle fibers contract. fibers contract. The muscle spindle Muscle spindle tension is mainbecomes slack and no tained and it can APs are fired. It is still signal changes unable to signal further length changes. in length. Figure 13.16c, d

Stretch Reflexes
 

Maintain muscle tone in large postural muscles Cause muscle contraction in response to increased muscle length (stretch)

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Stretch Reflexes


How a stretch reflex works:
 Stretch  Ia

activates the muscle spindle sensory neurons synapse directly with  motor neurons in the spinal cord   motor neurons cause the stretched muscle to contract


All stretch reflexes are monosynaptic and ipsilateral

Stretch Reflexes




Reciprocal inhibition also occurs - Ia fibers synapse with interneurons that inhibit the  motor neurons of antagonistic muscles Example: In the patellar reflex, the stretched muscle p p , (quadriceps) contracts and the antagonists (hamstrings) relax

Stretched muscle spindles initiate a stretch reflex, Stretched muscle spindles initiate a stretch reflex, causing contraction of the stretched muscle and causing contraction of the stretched muscle and inhibition of its antagonist. inhibition of its antagonist.
The events by which muscle stretch is damped The events by which muscle stretch is damped
1 When muscle spindles are activated 1 When muscle spindles are activated by stretch, the associated sensory by stretch, the associated sensory neurons (blue) transmit afferent impulses neurons (blue) transmit afferent impulses at higher frequency to the spinal cord. at higher frequency to the spinal cord. 2 The sensory neurons synapse directly with alpha 2 The sensory neurons synapse directly with alpha motor neurons (red), which excite extrafusal fibers motor neurons (red), which excite extrafusal fibers of the stretched muscle. Afferent fibers also of the stretched muscle. Afferent fibers also synapse with interneurons (green) that inhibit motor synapse with interneurons (green) that inhibit motor neurons (purple) controlling antagonistic muscles. neurons (purple) controlling antagonistic muscles.

Cell body of Cell body of sensory neuron sensory neuron Initial stimulus Initial stimulus (muscle stretch) (muscle stretch) Spinal cord Spinal cord Muscle spindle Muscle spindle Antagonist muscle Antagonist muscle
3a Efferent impulses of alpha motor neurons 3a Efferent impulses of alpha motor neurons cause the stretched muscle to contract, cause the stretched muscle to contract, which resists or reverses the stretch. which resists or reverses the stretch. 3b Efferent impulses of alpha motor 3b Efferent impulses of alpha motor neurons to antagonist muscles are neurons to antagonist muscles are reduced (reciprocal inhibition). reduced (reciprocal inhibition).
Figure 13.17 (1 of 2)

Sensory Sensory neuron neuron

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The patellar (knee-jerk) reflex—a specific example of a stretch reflex
2

Quadriceps (extensors)
1

3a

3b

3b

Patella Spinal cord (L2–L4)

Muscle spindle Hamstrings (flexors) Patellar ligament

1 Tapping the patellar ligament excites

muscle spindles in the quadriceps. 2 Afferent impulses (blue) travel to the spinal cord, where synapses occur with motor neurons and interneurons.
3a The motor neurons (red) send activating impulses to the quadriceps causing it to contract, extending the knee. 3b The interneurons (green) make inhibitory synapses with ventral horn neurons (purple) that prevent the antagonist muscles (hamstrings) from resisting the contraction of the quadriceps.
Figure 13.17 (2 of 2)

+ –

Excitatory synapse Inhibitory synapse

Golgi Tendon Reflexes
  

Polysynaptic reflexes Help to prevent damage due to excessive stretch Important for smooth onset and termination of muscle contraction

Golgi Tendon Reflexes


Produce muscle relaxation (lengthening) in response to tension
Contraction or passive stretch activates Golgi tendon organs Afferent impulses are transmitted to spinal cord  Contracting muscle relaxes and the antagonist contracts (reciprocal activation)  Information transmitted simultaneously to the cerebellum is used to adjust muscle tension
 

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1 Quadriceps strongly contracts. Golgi tendon organs are activated.

2 Afferent fibers synapse with interneurons in the spinal cord.

Interneurons

Quadriceps (extensors) Spinal S i l cord d

Golgi tendon organ Hamstrings (flexors)
3a Efferent impulses
+ Excitatory synapse – Inhibitory synapse

3b Efferent

to muscle with stretched tendon are damped. Muscle relaxes, reducing tension.

impulses to antagonist muscle cause it to contract.
Figure 13.18

Flexor and Crossed-Extensor Reflexes


Flexor (withdrawal) reflex
 Initiated  Causes

by a painful stimulus automatic withdrawal of the threatened body and polysynaptic

part
 Ipsilateral

Flexor and Crossed-Extensor Reflexes


Crossed extensor reflex
 Occurs

with flexor reflexes in weight-bearing limbs to maintain balance  Consists of an ipsilateral flexor reflex and a contralateral extensor reflex
 The  The

stimulated side is withdrawn (flexed) contralateral side is extended

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+ Excitatory synapse – Inhibitory synapse

Interneurons

Afferent fiber Efferent fibers

Efferent fibers

Extensor inhibited Flexor stimulated

Arm movements

Flexor inhibited Extensor stimulated

Site of stimulus: a noxious stimulus causes a flexor reflex on the same side, withdrawing that limb.

Site of reciprocal activation: At the same time, the extensor muscles on the opposite side are activated.
Figure 13.19

Superficial Reflexes
 

Elicited by gentle cutaneous stimulation Depend on upper motor pathways and cord-level reflex arcs

Superficial Reflexes


Plantar reflex
 Stimulus:

stroking lateral aspect of the sole of the foot downward flexion of the toes  Tests for function of corticospinal tracts p
 Response:

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Superficial Reflexes


Babinski’s sign
 Stimulus:

as above dorsiflexion of hallux and fanning of toes  Present in infants due to incomplete myelination p y  In adults, indicates corticospinal or motor cortex damage
 Response:

Superficial Reflexes


Abdominal reflexes
 Cause

contraction of abdominal muscles and movement of the umbilicus in response to stroking of the skin  Vary in intensity from one person to another  Absent when corticospinal tract lesions are present

Developmental Aspects of the PNS


Spinal nerves branch from the developing spinal cord and neural crest cells
 Supply

both motor and sensory fibers to developing muscles to help direct their maturation  Cranial nerves innervate muscles of the head

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Developmental Aspects of the PNS






Distribution and growth of spinal nerves correlate with the segmented body plan Sensory receptors atrophy with age and muscle tone lessens due to loss of neurons, decreased , numbers of synapses per neuron, and slower central processing Peripheral nerves remain viable throughout life unless subjected to trauma

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