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
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.
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
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.
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.,
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
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.
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
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
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
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
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
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
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
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
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)
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
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
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
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
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
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
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
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.
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