1. 8
The Nervous System

2. Chapter 8 Learning Outcomes
8-1
Describe the anatomical and functional divisions of the nervous system.
8-2
Distinguish between neurons and neuroglia on the basis of structure and function.
8-3
Describe the events involved in the generation and propagation of an action potential.
8-4
Describe the structure of a synapse, and explain the mechanism of nerve impulse transmission at a synapse.
8-5
Describe the three meningeal layers that surround the central nervous system.
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3. Chapter 8 Learning Outcomes
8-6
Discuss the roles of gray matter and white matter in the spinal cord.
8-7
Name the major regions of the brain, and describe the locations and functions of each.
8-8
Name the cranial nerves, relate each pair of cranial nerves to its principal functions, and relate the distribution pattern of spinal nerves to the regions they innervate.
8-9
Describe the steps in a reflex arc.
8-10
Identify the principal sensory and motor pathways, and explain how it is possible to distinguish among sensations that originate in different areas of the body.
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4. Chapter 8 Learning Outcomes
8-11
Describe the structures and functions of the sympathetic and parasympathetic divisions of the autonomic nervous system.
8-12
Summarize the effects of aging on the nervous system.
8-13
Give examples of interactions between the nervous system and other organ systems.
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5. The Nervous System and the Endocrine System (Introduction)
The nervous system and endocrine system coordinate other organ systems to maintain homeostasis
The nervous system is fast, short acting
The endocrine system is slower, but longer lasting
The nervous system is the most complex system in the body
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6. Functions of the Nervous System (8-1)
Monitors the body's internal and external environments
Integrates sensory information
Coordinates voluntary and involuntary responses
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7. Divisions of the Nervous System (8-1)
Anatomical divisions are:
The central nervous system (CNS)
Made up of the brain and spinal cord
Integrates and coordinates input and output
The peripheral nervous system (PNS)
All the neural tissues outside of the CNS
The connection between the CNS and the organs
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8. Divisions of the Nervous System (8-1)
Functional divisions are:
The afferent division
Includes sensory receptors and neurons that send information to the CNS
The efferent division
Includes neurons that send information to the effectors, which are the muscles and glands
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9. Efferent Division of the Nervous System (8-1)
Further divided into:
The somatic nervous system (SNS)
Controls skeletal muscle
The autonomic nervous system (ANS)
Controls smooth and cardiac muscle, and glands
Has two parts
Sympathetic division
Parasympathetic division
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10. Figure 8-1 A Functional Overview of the Nervous System.
CENTRAL NERVOUS SYSTEM
Information
processing
PERIPHERAL
NERVOUS
SYSTEM
Sensory information
within
afferent division
Motor commands
within
efferent division
includes
Somatic
nervous
system
Autonomic
nervous system
Parasympathetic
division
Sympathetic
division
Receptors
Effectors
Smooth
muscle
Visceral sensory
receptors (monitor
internal conditions
and the status
of other organ
systems)
Somatic sensory
receptors (monitor
the outside world
and our position
in it)
Skeletal
muscle
Cardiac
muscle
Glands
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11. Checkpoint (8-1)
Identify the two anatomical divisions of the nervous system.
Identify the two functional divisions of the peripheral nervous system, and describe their primary functions.
What would be the effect of damage to the afferent division of the PNS?
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12. Neurons (8-2) Cells that communicate with one another and other cells
Basic structure of a neuron includes:
Cell body
Dendrites
Which receive signals
Axons
Which carry signals to the next cell
Axon terminals
Bulb-shaped endings that form a synapse with the next cell
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13. Neurons (8-2)
Have a very limited ability to regenerate when damaged or destroyed
Cell bodies contain:
Mitochondria, free and fixed ribosomes, and rough endoplasmic reticulum
Free ribosomes and RER form Nissl bodies and give the tissue a gray color (gray matter)
The axon hillock
Where electrical signal begins
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14. Figure 8-2 The Anatomy of a Representative Neuron.
Cell body
Mitochondrion
Golgi apparatus
Axon terminals
Axon hillock
Dendrite
Collateral
Nucleus
Axon (may be myelinated)
Nucleolus
Nerve cell body
Nucleolus
Nucleus
Axon hillock
Nissl bodies
Nissl bodies
Nerve cell body
LM x 1500
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15. Structural Classification of Neurons (8-2)
Based on the relationship of the dendrites to the cell body
Multipolar neurons
Are the most common in the CNS and have two or more dendrites and one axon
Unipolar neurons
Have the cell body off to one side, most abundant in the afferent division
Bipolar neurons
Have one dendrite and one axon with the cell body in the middle, and are rare
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16. Figure 8-3 A Structural Classification of Neurons.
Multipolar neuron
Unipolar neuron
Bipolar neuron
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17. Sensory Neurons (8-2) Somatic sensory receptors
Also called afferent neurons
Total 10 million or more
Receive information from sensory receptors
Somatic sensory receptors
Detect stimuli concerning the outside world, in the form of external receptors
And our position in it, in the form of proprioceptors
Visceral or internal receptors
Monitor the internal organs
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18. Motor Neurons (8-2) Also called efferent neurons
Total about half a million in number
Carry information to peripheral targets called effectors
Somatic motor neurons
Innervate skeletal muscle
Visceral motor neurons
Innervate cardiac muscle, smooth muscle, and glands
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19. Interneurons (8-2) Also called association neurons
By far the most numerous type at about 20 billion
Are located in the CNS
Function as links between sensory and motor processes
Have higher functions
Such as memory, planning, and learning
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20. Neuroglial Cells (8-2)
Are supportive cells and make up about half of all neural tissue
Four types are found in the CNS
Astrocytes
Oligodendrocytes
Microglia
Ependymal cells
Two types in the PNS
Satellite cells
Schwann cells
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21. Astrocytes (8-2) Large and numerous neuroglia in the CNS
Maintain the blood–brain barrier
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22. Oligodendrocytes (8-2) Found in the CNS
Produce an insulating membranous wrapping around axons called myelin
Small gaps between the wrappings called nodes of Ranvier
Myelinated axons constitute the white matter of the CNS
Where cell bodies are gray matter
Some axons are unmyelinated
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23. Microglia (8-2) The smallest and least numerous
Phagocytic cells derived from white blood cells
Perform essential protective functions such as engulfing pathogens and cellular waste
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24. Ependymal Cells (8-2)
Line the fluid-filled central canal of the spinal cord and the ventricles of the brain
The endothelial lining is called the ependyma
It is involved in producing and circulating cerebrospinal fluid around the CNS
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25. Figure 8-4 Neuroglia in the CNS.
Gray matter
White matter
CENTRAL
CANAL
Ependymal
cells
Gray
matter
Neurons
Myelinated
axons
Microglial
cell
Inter-
node
Myelin
(cut)
Astrocyte
Axon
White
matter
Oligoden-
drocyte
Node
Basement
membrane
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Capillary

26. Neuroglial Cells in PNS (8-2)
Satellite cells
Surround and support neuron cell bodies
Similar in function to the astrocytes in the CNS
Schwann cells
Cover every axon in PNS
The surface is the neurilemma
Produce myelin
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27. Figure 8-5 Schwann Cells and Peripheral Axons.
Nodes
Schwann cell nucleus
Myelin covering
internode
Neurilemma
Axons
Schwann
cell nucleus
Myelinated axon
TEM x 14,048
A myelinated axon in the PNS is covered by several Schwann cells, each of which forms a myelin sheath around a portion of the axon. This arrangement differs from the way myelin forms in the CNS; compare with Figure 8-4.
Unmyelinated
axon
TEM x 14,048
A single Schwann cell can encircle several unmy-
elinated axons. Every axon in the PNS is completely enclosed by Schwann cells.
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28. Organization of the Nervous System (8-2)
In the PNS:
Collections of nerve cell bodies are ganglia
Bundled axons are nerves
Including spinal nerves and cranial nerves
Can have both sensory and motor components
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29. Organization of the Nervous System (8-2)
In the CNS:
Collections of neuron cell bodies are found in centers, or nuclei
Neural cortex is a thick layer of gray matter
White matter in the CNS is formed by bundles of axons called tracts, and in the spinal cord, form columns
Pathways are either sensory or ascending tracts, or motor or descending tracts
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30. Figure 8-6 The Anatomical Organization of the Nervous System.
CENTRAL NERVOUS SYSTEM
GRAY MATTER ORGANIZATION
Neural Cortex
Centers
PERIPHERAL
NERVOUS SYSTEM
Gray matter on
the surface of
the brain
Collections of
neuron cell bodies
in the CNS; each
center has specific
processing
functions
GRAY MATTER
Ganglia
Collections of neuron cell bodies in the PNS
Nuclei
Collections of
neuron cell
bodies in the
interior of the
CNS
Higher Centers
The most complex
centers in the
brain
WHITE MATTER
Nerves
WHITE MATTER ORGANIZATION
Bundles of axons
in the PNS
Tracts
Columns
Bundles of CNS
axons that share
a common origin,
destination, and
function
Several tracts
that form an
anatomically
distinct mass
RECEPTORS
PATHWAYS
Centers and tracts that connect
the brain with other organs and
systems in the body
EFFECTORS
Ascending (sensory) pathway
Descending (motor) pathway
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31. Checkpoint (8-2) Name the structural components of a typical neuron.
Examination of a tissue sample reveals unipolar neurons. Are these more likely to be sensory neurons or motor neurons?
Identify the neuroglia of the central nervous system.
Which type of glial cell would increase in number in the brain tissue of a person with a CNS infection?
In the PNS, neuron cell bodies are located in ________ and surrounded by neuroglial cells called ________ cells.
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32. The Membrane Potential (8-3)
A membrane potential exists because of:
Excessive positive ionic charges on the outside of the cell
Excessive negative charges on the inside, creating a polarized membrane
An undisturbed cell has a resting membrane potential measured in the inside of the cell in millivolts
The resting membrane potential of neurons is –70 mV
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33. Factors Determining Membrane Potential (8-3)
Extracellular fluid (ECF) is high in Na+ and CI–
Intracellular fluid (ICF) is high in K+ and negatively charged proteins (Pr –)
Proteins are non-permeating, staying in the ICF
Some ion channels are always open
Called leak channels
Some are open or closed
Called gated channels
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34. Factors Determining Membrane Potential (8-3)
Na+ can leak in
But the membrane is more permeable to K+
Allowing K+ to leak out faster
Na+/K+ exchange pump exchanges 3 Na+ for every 2 K+
Moving Na+ out as fast as it leaks in
Cell experiences a net loss of positive ions
Resulting in a resting membrane charge of –70 mV
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35. © 2013 Pearson Education, Inc.
Figure 8-7 The Resting Potential Is the Membrane Potential of an Undisturbed Cell.
EXTRACELLULAR FLUID
–30
–70
+30 mV
Na+ leak
channel
K+ leak
channel
Sodium–
potassium
exchange
pump
Plasma
membrane
CYTOSOL
Protein
KEY
Sodium ion (Na+)
Protein
Protein
Potassium ion (K+)
Chloride ion (Cl–)
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36. Changes in Membrane Potential (8-3)
Stimuli alter membrane permeability to Na+ or K+
Or alter activity of the exchange pump
Types include:
Cellular exposure to chemicals
Mechanical pressure
Temperature changes
Changes in the ECF ion concentration
Result is opening of a gated channel
Increasing the movement of ions across the membrane
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37. Changes in Membrane Potential (8-3)
Opening of Na+ channels results in an influx of Na+
Moving the membrane toward 0 mV, a shift called depolarization
Opening of K+ channels results in an efflux of K+
Moving the membrane further away from 0 mV, a shift called hyperpolarization
Return to resting from depolarization: repolarizing
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38. Graded Potentials (8-3)
Local changes in the membrane that fade over distance
All cells experience graded potentials when stimulated
And can result in the activation of smaller cells
Graded potentials by themselves cannot trigger activation of large neurons and muscle fibers
Referred to as having excitable membranes
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39. Action Potentials (8-3)
A change in the membrane that travels the entire length of neurons
A nerve impulse
If a combination of graded potentials causes the membrane to reach a critical point of depolarization, it is called the threshold
Then an action potential will occur
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40. Action Potentials (8-3)
Are all-or-none and will propagate down the length of the neuron
From the time the voltage-gated channels open until repolarization is finished:
The membrane cannot respond to further stimulation
This period of time is the refractory period
And limits the rate of response by neurons
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41. Figure 8-8 The Generation of an Action Potential
SPOTLIGHT
FIGURE 8-8
The Generation of an Action Potential
Sodium channels close, voltage-gated potassium channels open, and potassium ions move out of the cell. Repolarization begins.
+30 3
D E P O L A R I Z A T I O N
R E P O L A R I Z A T I O N
Potassium channels close, and both sodium and potassium channels return to their normal states.
Resting
potential 2
–40
Voltage-gated sodium channels open and sodium ions move into the cell. The membrane potential rises to +30 mV.
Membrane potential (mV)
–60
Threshold
–70 1
Axon hillock 4
A graded
depolarization brings an area of excitable membrane to threshold (–60 mV).
First part of axon to
reach threshold
REFRACTORY PERIOD
During the refractory period, the membrane cannot respond to further stimulation.
Time (msec) 1 2 1 2 3 4
Depolarization to
Threshold
Activation of Sodium
Channels and Rapid
Depolarization
Inactivation of Sodium
Channels and Activation
of Potassium Channels
Closing of Potassium Channels
Resting
Potential
Resting
Potential
–70 mV
–60 mV
+10 mV
–90 mV
–70 mV
+30 mV
Local
current
The axon membrane contains both voltage-gated sodium channels and voltage-gated potassium channels that are closed when the membrane is at the
resting potential.
The stimulus that begins an action potential is a graded depolarization large enough to open voltage-gated sodium channels. The opening of the channels occurs at a membrane potential known as the threshold.
When the voltage-gated sodium channels open, sodium ions rush into the cytoplasm, and rapid depolarization occurs. The inner membrane surface now contains more positive ions than negative ones, and the membrane potential has changed from –60 mV
to a positive value.
As the membrane potential approaches +30 mV, voltage-gated sodium channels close. This step coincides with the opening of voltage- gated potassium channels. Positively charged potassium ions move out of the cytosol, shifting the membrane potential back toward resting levels. Repolariza- tion now begins.
The voltage-gated sodium channels remain inactivated until the membrane has repolar-
ized to near threshold
levels. The voltage-gated
potassium channels
begin closing as the
membrane reaches the
normal resting potential
(about –70 mV). Until all
have closed, potassium
ions continue to leave the
cell. This produces a brief hyperpolarization.
As the voltage-gated potassium channels close, the membrane potential returns to normal resting levels. The action potential is now over, and the membrane is once again at the resting potential.
= Sodium ion
= Potassium ion
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42. Figure 8-8 The Generation of an Action Potential
Sodium channels close, voltage-
gated potassium channels open,
and potassium ions move out of the
cell. Repolarization begins.
+30 3
D E P O L A R I Z AT I O N
R E P O L A R I Z AT I O N
Resting
potential 2
Potassium channels
close, and both sodium
and potassium chan-
nels return to their
normal states.
Voltage-gated sodium
channels open and
sodium ions move into
the cell. The membrane
potential rises to +30
mV.
–40
Membrane potential (mV)
Threshold
–60
–70 1 4
A graded
depolarization brings
an area of excitable
membrane to thresh-
old (–60 mV).
REFRACTORY PERIOD
During the refractory period,
the membrane cannot
respond to further stimulation.
Time (msec) 1 2
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43. Figure 8-8 The Generation of an Action Potential
Resting Potential
Depolarization to Threshold
Activation of Sodium
Channels and Rapid
Depolarization
–70 mV
–60 mV
+10 mV
Local
current
The axon membrane contains both voltage-gated sodium channels and voltage-gated potassium channels that are closed when the membrane is at the resting potential.
The stimulus that begins an action
potential is a graded depolarization large enough to open voltage-gated sodium channels. The opening of the channels occurs at a membrane potential known as the threshold.
When the voltage-gated sodium channels open, sodium ions rush into the cytoplasm, and rapid
depolarization occurs. The inner membrane surface now contains more positive ions than negative ones, and the membrane potential has changed from –60 mV to a positive value.
= Sodium ion
= Potassium ion
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44. Figure 8-8 The Generation of an Action Potential
Resting
Potential
Inactivation of Sodium
Channels and
Activation of Potassium Channels
Closing of Potas-
sium Channels
–70 mV
–90 mV
+30 mV
The voltage-gated sodium channels remain inactivated until the membrane has repolar-
ized to near threshold levels. The voltage-gated potassium channels begin closing as the membrane reaches the normal resting potential (about –70 mV). Until all have closed, potassium ions continue to leave the cell. This produces a brief hyperpolarization.
As the membrane
potential approaches +30 mV, voltage-gated sodium channels close. This step coincides with the opening of voltage-
gated potassium channels. Positively charged potassium ions move out of the cytosol, shifting the membrane potential back toward resting levels. Repolariza- tion now begins.
As the voltage-gated
potassium channels
close, the mem-
brane potential re-
turns to normal rest-
ing levels. The
action potential is
now over, and the
membrane is once
again at the resting
potential.
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45. Propagation of an Action Potential (8-3)
Occurs when local changes in the membrane in one site:
Result in the activation of voltage-gated channels in the next adjacent site of the membrane
This causes a wave of membrane potential changes
Continuous propagation
Occurs in unmyelinated fibers and is relatively slow
Saltatory propagation
Is in myelinated axons and is faster
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46. © 2013 Pearson Education, Inc.
Figure 8-9 The Propagation of Action Potentials over Unmyelinated and Myelinated Axons.
Action potential propagation along an
unmyelinated axon
Action potential propagation along a
myelinated axon
Stimulus
depolarizes
membrane to
threshold
Stimulus
depolarizes
membrane to
threshold
EXTRACELLULAR FLUID
EXTRACELLULAR FLUID
Myelinated
Myelinated
Myelinated
Internode
Internode
Internode
Plasma membrane
CYTOSOL
Plasma membrane
CYTOSOL
Myelinated
Myelinated
Myelinated
Internode
Internode
Internode
Local current
Local current
Myelinated
Myelinated
Myelinated
Internode
Internode
Internode
Local current
Local current
Repolarization
(refractory period)
Repolarization
(refractory period)
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47. Checkpoint (8-3)
What effect would a chemical that blocks the voltage-gated sodium channels in a neuron's plasma membrane have on the neuron's ability to depolarize?
What effect would decreasing the concentration of extracellular potassium have on the membrane potential of a neuron?
List the steps involved in the generation and propagation of an action potential.
Two axons are tested for propagation velocities. One carries action potentials at 50 meters per second, the other at 1 meter per second. Which axon is myelinated?
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48. The Synapse (8-4) A junction between a neuron and another cell
Occurs because of chemical messengers called neurotransmitters
Communication happens in one direction only
Between a neuron and another cell type is a neuroeffector junction
Such as the neuromuscular junction or neuroglandular junction
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49. A Synapse between Two Neurons (8-4)
Occurs:
Between the axon terminals of the presynaptic neuron
Across the synaptic cleft
To the dendrite or cell body of the postsynaptic neuron
Neurotransmitters
Stored in vesicles of the axon terminals
Released into the cleft and bind to receptors on the postsynaptic membrane
PLAY
ANIMATION Neurophysiology: Synapse
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50. Figure 8-10 The Structure of a Typical Synapse.
Axon of
presynaptic cell
Axon
terminal
Mitochondrion
Synaptic
vesicles
Presynaptic
membrane
Postsynaptic
membrane
Synaptic
cleft
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51. The Neurotransmitter ACh (8-4)
Activates cholinergic synapses in four steps
Action potential arrives at the axon terminal
ACh is released and diffuses across synaptic cleft
ACh binds to receptors and triggers depolarization of the postsynaptic membrane
ACh is removed by AChE (acetylcholinesterase)
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52. Figure 8-11 The Events at a Cholinergic Synapse.
An action potential arrives and
depolarizes the axon terminal
Presynaptic
neuron
Action potential
Synaptic
vesicles
EXTRACELLULAR
FLUID
Axon
terminal
AChE
POSTSYNAPTIC
NEURON
Extracellular Ca2+ enters the axon
terminal, triggering the exocytosis of ACh
ACh
Ca2+
Ca2+
Synaptic cleft
Chemically regulated
sodium ion channels
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53. Figure 8-11 The Events at a Cholinergic Synapse.
ACh binds to receptors and depolarizes
the postsynaptic membrane
Initiation of
action potential
if threshold is
reached
ACh is removed by AChE
Propagation of
action potential
(if generated)
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54. Table 8-1 The Sequence of Events at a Typical Cholinergic Synapse
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55. Other Important Neurotransmitters (8-4)
Norepinephrine (NE)
In the brain and part of the ANS, is found in adrenergic synapses
Dopamine, GABA, and serotonin
Are CNS neurotransmitters
At least 50 less-understood neurotransmitters
NO and CO
Are gases that act as neurotransmitters
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56. Excitatory vs. Inhibitory Synapses (8-4)
Usually, ACh and NE trigger depolarization
An excitatory response
With the potential of reaching threshold
Usually, dopamine, GABA, and serotonin trigger hyperpolarization
An inhibitory response
Making it farther from threshold
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57. Neuronal Pools (8-4)
Multiple presynaptic neurons can synapse with one postsynaptic neuron
If they all release excitatory neurotransmitters:
Then an action potential can be triggered
If they all release an inhibitory neurotransmitter:
Then no action potential can occur
If half release excitatory and half inhibitory neurotransmitters:
They cancel, resulting in no action
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58. Neuronal Pools (8-4)
A term that describes the complex grouping of neural pathways or circuits
Divergence
Is a pathway that spreads information from one neuron to multiple neurons
Convergence
Is when several neurons synapse with a single postsynaptic neuron
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59. Figure 8-12 Two Common Types of Neuronal Pools.
Divergence
Convergence
A mechanism for
spreading stimulation to
multiple neurons or
neuronal pools in the
CNS
A mechanism for providing
input to a single neuron from
multiple sources
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60. Checkpoint (8-4) Describe the general structure of a synapse.
What effect would blocking calcium channels at a cholinergic synapse have on synapse function?
What type of neural circuit permits both conscious and subconscious control of the same motor neurons?
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61. The Meninges (8-5)
The neural tissue in the CNS is protected by three layers of specialized membranes
Dura mater
Arachnoid
Pia mater
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