Objectives
Functions and Divisions of the Nervous System
1. List the basic functions of the nervous system.
2. Explain the structural and functional divisions of the nervous system.
Histology of Nervous Tissue
3. List the types of neuroglia and cite their functions.
4. Define neuron, describe its important structural components, and relate each to a
functional role.
5. Differentiate between (1) a nerve and a tract, and (2) a nucleus and a ganglion.
6. Explain the importance of the myelin sheath and describe how it is formed in the
central and peripheral nervous systems.
7. Classify neurons by structure and by function.
Membrane Potentials
8. Define resting membrane potential and describe its electrochemical basis.
9. Compare and contrast graded potentials and action potentials.
10. Explain how action potentials are generated and propagated along neurons.
11. Define absolute and relative refractory periods.
12. Define saltatory conduction and contrast it to continuous conduction.
The Synapse
13. Define synapse. Distinguish between electrical and chemical synapses by structure
and by the way they transmit information.
14. Distinguish between excitatory and inhibitory postsynaptic potentials.
15. Describe how synaptic events are integrated and modified.
Neurotransmitters and Their Receptors
16. Define neurotransmitter and name several classes of neurotransmitters.
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Functions and Divisions of the Nervous System (p. 387; Figs. 11.1–11.2)
The nervous system has three basic functions:
Figure 11.1 The nervous system’s functions.
1. Sensory input:
2. Integration:
3. Motor output:
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The _________________________________________(CNS) consists of the _____________________ and
_____________________________________ and is the integrating and control center of the
nervous system.
The _______________________________________________(PNS) is outside the central nervous
system. The PNS consists mainly of the _____________________________ (bundles of axons)
that extend from the brain and spinal cord.

Spinal nerves:

Cranial nerves:
The PNS has two functional divisions:
1. _____________________________________________________________________________
2. _____________________________________________________________________________
The _______________________________, or ___________________________________, division of the
peripheral nervous system carries impulses toward the central nervous system
from sensory receptors located throughout the body.

Somatic sensory fibers carry impulses from receptors in the ___________________,
__________________________________ muscles, and _________________________.

Visceral sensory fibers carry impulses from _____________________________ within
the ventral body cavity
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The _______________________________, or _________________________________, division of the
peripheral nervous system carries impulses from the central nervous system to
effector organs, which are muscles and glands.

The ____________________________________________________ consists of somatic motor
nerve fibers that conduct impulses from the CNS to skeletal muscles and
allow conscious (voluntary) control of motor activities.

The ____________________________________________________(ANS) is an involuntary
system consisting of visceral motor nerve fibers that regulate the activity of
smooth muscle, cardiac muscle, and glands.
o The ANS has two functional subdivision:
___________________________________________________________________________
___________________________________________________________________________
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Figure 11.2 Levels of organization in the nervous system.
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Histology of Nervous Tissue (pp. 387–395; Figs. 11.3–11.5; Table 11.1)
The nervous tissue is made up of two principle types of cells:
1. _____________________________________________________________________
2. _____________________________________________________________________
Neuroglia
Neuroglia, or _________________________ cells, are closely associated with neurons,
providing a protective and supportive network.
There are _______ types of neuroglia-___________ in the CNS and _____________ in the PNS.
Neuroglia in the CNS

_________________________________________regulate the chemical environment
around neurons and exchange between neurons and capillaries.

________________________________________ cells monitor health and perform defense
functions for neurons.

_______________________________________ cells line the central cavities of the brain
and spinal cord and help circulate cerebrospinal fluid.

______________________________________ wrap around neuron fibers, forming myelin
sheaths.
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Neuroglia in the PNS

_________________________________________ cells are glial cells of the PNS whose
function is largely unknown. They are found surrounding neuron cell bodies
within ganglia.

________________________________________cells, or neurolemmocytes, are glial cells
of the PNS that surround nerve fibers, forming the myelin sheath.
Figure 11.3 Neuroglia (a-d) The four types of neuroglia of the CNS. (e)
Neuroglia of the PNS.
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Neurons
Neurons are specialized cells that conduct messages in the form of
_______________________________________impulses throughout the body.
Special characteristics of neurons:
Extreme longevity
Amitotic
High Metabolic Rate
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Figure 11.4 Structure of a motor neuron.
Neuron Cell Body
The neuron cell body, also called the ____________________________ or soma, is the major
biosynthetic center containing the usual organelles (except for centrioles):

Spherical ___________________________ with a conspicuous nucleolus surrounded
by cytoplasm.

Free ribosomes and rough ER synthesize proteins and membranes.

The rough ER of the neuron body is called the_________________________
substance or _______________________ bodies.

Mitochondria are scattered among the other organelles.

Some contain pigments.
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The cell body is the focal point for the outgrowth of neuron processes during
embryonic development.
In most neurons, the plasma membrane of the cell body also acts as
_____________________________________________________ that receives information from other
neurons.
Most neuron cell bodies are located in the ___________________, where they are
protected by bones of the skull and vertebral column.
Clusters of cell bodies in the CNS are called ______________________, whereas those that
lie along the nerves in the PNS are called ________________________ (ganglion = knot of
string, swelling).
Neuron Processes
Neurons have armlike processes that extend from the cell body.
The brain and spinal cord (CNS) contain both neuron cell bodies and their processes.
The PNS consists chiefly of _________________________________________. Bundles of neuron
processes are called _________________________ in the CNS and ____________________________
in the PNS.
The two types of neuron processes are _____________________ and _________________________.
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Dendrites
•
In motor neurons
–
100s of short, tapering, diffusely branched processes
–
Same organelles as in body
•
_____________________________________ (input) region of neuron
•
Convey incoming messages toward cell body as ___________________________
potentials (short distance signals)
•
In many brain areas fine dendrites specialized
–
Collect information with ___________________________ spines
•
Appendages with bulbous or spiky ends
The Axon: The Structure
•
One axon per cell arising from __________________ hillock
–
Cone-shaped area of cell body
•
In some axon short or absent
•
In others most of length of cell
–
Some 1 meter long
•
Long axons called ________________________________.
•
Occasional branches (axon _________________________)
•
Branches profusely at end (terminus)
•
Can be 10,000 terminal branches
•
Distal endings called axon terminals or terminal ___________________.
The Axon: Functional Characteristics
•
Conducting region of neuron
•
Generates nerve impulses
•
Transmits them along ______________________________________ (neuron cell
membrane) to ___________________________________________.
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–
Secretory region
–
________________________________ released into extracellular space
•
Either excite or inhibit neurons with which axons in close
contact
•
Carries on many conversations with different neurons at same time
•
Lacks rough ER and Golgi apparatus
–
Relies on cell body to renew proteins and membranes
–
Efficient transport mechanisms
–
Quickly decay if cut or damaged
Transport Along the Axon
•
Molecules and organelles are moved along axons by motor proteins and
cytoskeletal elements
•
Movement in both directions
–
Anterograde—
•
Examples: mitochondria, cytoskeletal elements, membrane
components, enzymes
–
Retrograde—
•
Examples: organelles to be degraded, signal molecules, viruses,
and bacterial toxins
Myelin Sheath
•
Composed of myelin
–
•
Segmented sheath around most long or large-diameter axons
–
•
Whitish, protein-lipoid substance
Myelinated fibers
Function of myelin
–
Protects and electrically insulates axon
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–
•
Increases speed of nerve impulse transmission
Nonmyelinated fibers conduct impulses more slowly
Myelination in the PNS
•
•
•
Plasma membranes of myelinating cells have less protein
–
No channels or carriers
–
Good electrical insulators
–
Interlocking proteins bind adjacent myelin membranes
Myelin sheath gaps
–
Gaps between adjacent Schwann cells
–
Sites where axon collaterals can emerge
–
Formerly called nodes of Ranvier
Myelin sheath gaps between adjacent
Schwann cells
–
•
Sites where axon collaterals can emerge
Nonmyelinated fibers
–
Thin fibers not wrapped in myelin; surrounded by Schwann cells but
no coiling; one cell may surround 15 different fibers
Figure 11.5 Nerve fiber myelination by Schwann cells in the PNS.
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Myelination in the CNS
•
Formed by multiple, flat processes of oligodendrocytes, not whole cells
•
Can wrap up to 60 axons at once
•
Myelin sheath gap is present
•
No outer collar of perinuclear cytoplasm
•
Thinnest fibers are unmyelinated
–
•
Covered by long extensions of adjacent neuroglia
_____________________________ matter
–
Regions of brain and spinal cord with dense collections of myelinated
fibers – usually fiber tracts
•
_____________________________ matter
–
Mostly neuron cell bodies and nonmyelinated fibers
Classification of Neurons
Neurons are classified both structurally and functionally.
Structural Classification
Neurons are grouped structurally according to the number of ______________________
extending from their cell body.
There are three structural classes of neurons:

________________________________ neurons have three or more processes.
o The major neuron type in the CNS.

________________________________ neurons have a single axon and dendrite.
o These rare neurons are found in some of the special sense organs.
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
________________________________ neurons have a single process extending from
the cell body that is associated with receptors at the distal end.
o Distal (peripheral) process—associated with sensory receptor.
o Proximal (central) process-enters CNS.
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Functional Classification
This scheme groups neurons according to the ______________________________ in which
the nerve impulse travels relative to the central nervous system.
There are three functional classes of neurons:

_____________________________(or afferent) neurons conduct impulses
___________________ the CNS from receptors.

_____________________________(or efferent) neurons conduct impulses _____________
the CNS to effectors.
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
________________________________________ (association neurons) conduct impulses
between sensory and motor neurons, or in CNS integration pathways.
Membrane Potentials (pp. 395–407; Figs. 11.6–11.15)
Basic Principles of Electricity
Some Definitions: Voltage, Resistance, Current
__________________________________ is a measure of the amount of difference in electrical
charge between two points, called the potential difference.
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The flow of electrical charge from point to point is called _____________________________,
and is dependent on voltage and ___________________________ (hindrance to current flow).
Substances with high electrical resistance are _______________________________, and those
with low resistance are ________________________________.
Ohm’s Law gives the relationship between voltage, current, and resistance:
Ohm’s law tells us three things:
1.
2.
3.
In the body, electrical currents are due to the movement of ions across cellular
membranes.
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Role of Membrane Ion Channels
Plasma membranes contain membrane proteins that act as ion channels. Each of
these channels is ______________________________ as to the type of ion (or ions) it allows to
pass.
____________________________________ or nongated channels are always open.
There are three main types of gated channels:

Chemical gated (ligand-gated channels)

Voltage-gated channels

Mechanically gated channels
Figure 11.6 Operation of gated channels.
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When ion channels are open, ions diffuse across the membrane along their
electrochemical gradients, creating electrical currents.
According to Ohm’s law equation:
Voltage (V) =
Ions move along chemical _______________________________________ when they diffuse
passively from an area of their ___________________________ concentration to an area of
______________________ concentration.
Ions move along __________________________________________ when they move toward an
area of opposite electrical charge.
Electrochemical gradient =
The Resting Membrane Potential (pp. 397–398; Figs. 11.7–11.8)
The membrane of a resting neuron is polarized, and the potential difference of this
polarity (approximately –70 mV) is called the resting membrane potential. The
resting membrane potential exists only across the membrane and is mostly due to
two factors: differences in ionic makeup of intracellular and extracellular fluids, and
differential membrane permeability to those ions.
Differences in Ionic Composition
The cytosol has a _________________________ concentration of Na+ and ______________________
concentration of K+ than extracellular fluid.
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Anionic ____________________________ balance the cations inside the cell, while chloride
ions mostly balance cations outside of the cell.
Differences in Plasma Membrane Permeability
Potassium ions (K+) play the most important role in generating a resting membrane
potential, since the membrane is roughly ___________ times more permeable to K+
than Na+.
Potassium ions diffuse ________________ of the cell along their concentration gradient
much more easily than sodium ions can __________________ the cell along theirs.
K+ flowing out of the cells causes the cell to become more ____________________ inside.
Na+ trickling into the cell makes the cell just slightly more ____________________________
than it would be if only K+ flowed.
The sodium-potassium pump (Na+-K+ ATPase) stabilizes the resting membrane
potential by maintaining the concentration gradients for sodium and potassium.

________ Na+ are ejected from the cell

________ K+ are transported back into the cell.
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Figure 11.8 Resting Membrane Potential
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Membrane Potentials That Act as Signals (pp. 397–405; Figs. 11.8–11.15)
Neurons use changes in membrane potential as communication signals and can be
brought on by changes in membrane permeability to any ion, or alteration of ion
concentrations on the two sides of the membrane.
Changes in membrane potential can produce two types of signals:

Graded potentials

Action potentials
Relative to the resting state, potential changes can be ___________________________, in
which the inside of the membrane becomes less negative, or
_______________________________________________, in which the inside of the membrane
becomes more negatively charged.
Figure 11.9 Depolarization and hyperpolarization of the membrane.
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Graded Potentials
Graded potentials are short-lived _________________ changes in membrane potentials,
can either be depolarizations or hyperpolarizations, and are critical to the
generation of action potentials.

Graded potentials occurring on receptors of sensory neurons are called
____________________________ potentials, or ___________________________potentials.

Graded potentials occurring in response to a neurotransmitter released from
another neuron is called a ___________________________ potential.
Action Potentials
Action potentials, or nerve impulses, occur on axons and are the principle way
neurons communicate.
An action potential is a brief reversal of membrane potential with a total amplitude
(change in voltage) of about __________mV (from - ______mV to + ______mV).
Depolarization is followed by repolarization and often a short period of
hyperpolarization.
An action potential (AP) is also called a _____________________________ and is typically
generated _______________________________________________________.
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A neuron generates a nerve impulse only when adequately stimulated. This stimulus
changes the ____________________________ of the neuron’s membrane by opening specific
voltage-gated channels on the axon.
These channels open and close in response to changes in the membrane potential.
They are activated by local currents (_______________________________________________) that
spread toward the axon along the dendritic and cell body membranes.
In many neurons, the transition from local graded potential to long-distance action
potential takes place at the _____________________________________________.
Generation of an Action Potential
Generation of an action potential involves a transient increase in Na+ permeability,
followed by restoration of Na+ impermeability, and then a short-lived increase in
K+ permeability.
1. Resting state: _____________________________________________________________
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2. Depolarization: _____________________________________________________________
3. Repolarization: _____________________________________________________________
4. Hyperpolarization: _____________________________________________________________
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Figure 11.11 Action Potential
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Threshold and the All-or-None Phenomenon
A critical minimum, or threshold, depolarization is defined by the amount of influx
of Na+ that at least equals the amount of efflux of K+.
Action potentials are all-or-none phenomena: they either happen completely, in the
case of a threshold stimulus, or not at all, in the event of a subthreshold stimulus.
Propagation of an Action Potential
If it is to serve as the neuron’s signaling device, an AP must be _________________________
along the axon’s entire length.
Figure 11.12 Propagation of an action potential (AP)
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•
Na+ influx causes local currents
–
Local currents cause depolarization of adjacent membrane areas in
direction away from AP origin (toward axon's terminals)
•
–
Local currents trigger an AP there
–
This causes the AP to propagate AWAY from the AP origin
Since Na+ channels closer to AP origin are inactivated no new AP is generated
there
•
Once initiated an AP is self-propagating
–
In nonmyelinated axons each successive segment of membrane
depolarizes, then repolarizes
–
Propagation in myelinated axons differs
Coding for Stimulus Intensity
All action potentials are alike and are independent of stimulus intensity
How does CNS tell difference between a weak stimulus and a strong one?
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Figure 11.13 Relationship between stimulus strength and action potential
frequency.
Refractory Period
The refractory period of an axon is related to the period of time required so that a
neuron can generate another action potential.
Absolute refractory period =
30
Relative refractory period =
Figure 11.14 Absolute and relative refractory periods in AP.
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Conduction Velocity
The rate of impulse propagation depends on two factors:
1. _____________________________________________________________
2. _____________________________________________________________
Axons with larger diameters conduct impulses faster than axons with smaller
diameters.
Nonmyelinated axons conduct impulses relatively slowly, while myelinated axons
have a high conduction velocity.

Continuous conduction =

Saltatory conduction =
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Figure 11.15 Action Potential propogation in nonmyelinated and myelinated
axons.
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The Synapse (pp. 407–414; Figs. 11.16–11.19; Table 11.2)
A ____________________________________ is a junction that mediates information transfer
between neurons or between a neuron and an effector cell
Axodendritic synapses =
Axosomatic synapses =
Figure 11.17 Synapses. Axodendritic, axosomatic, and axoaxonal synapses.
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Neurons conducting impulses toward the synapse are _______________________________
neurons, and neurons carrying impulses away from the synapse are
__________________________________ neurons.
Electrical Synapses
Electrical synapses have neurons that are electrically coupled via protein channels
and allow direct exchange of ions from cell to cell
Chemical Synapses
Chemical synapses are specialized for release and reception of chemical
neurotransmitters
A typical chemical synapse is made up of two parts:
1.
2.
Synaptic cleft =
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Information Transfer Across Chemical Synapses
1. Action Potential arrives at axon terminal
2. Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal
3. Ca2+ entry causes synaptic vesicles to release neurotransmitter by exocytosis.
4. Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors
on the postsynaptic membrane.
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5. Binding of neurotransmitter opens ion channels, creating graded potentials.
6. Neurotransmitter effects are terminated.
37
Figure 11.17 Chemical Synapse
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Synaptic Delay
Synaptic delay is related to the period of time required for release and binding of
neurotransmitters
Postsynaptic Potentials and Synaptic Integration (pp. 410–414; Figs. 11.18–
11.19; Table 11.2)
Neurotransmitters mediate graded potentials on the postsynaptic cell that may be
excitatory or inhibitory.
__________________________potentials on the postsynaptic cell occur when there is a net
influx of Na+ into the cell, and are called excitatory postsynaptic potentials (EPSPs).
_____________________potentials on the postsynaptic cell occur when there is an
increase in permeability to either K+ or Cl- and are called inhibitory postsynaptic
potentials (IPSPs).
Figure 11.8 Postsynaptic potential can be excitatory or inhibitory.
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Integration and Modification of Synaptic Events
Summation by the Postsynaptic Neuron
Summation by the postsynaptic neuron is accomplished in two ways: temporal
summation, which occurs in response to several successive releases of
neurotransmitter, and spatial summation, which occurs when the postsynaptic cell
is stimulated at the same time by multiple terminals.
Figure 11.19 Neural integration of EPSPs and IPSPs
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Synaptic Potentiation
Synaptic potentiation results when a presynaptic cell is stimulated repeatedly or
continuously, resulting in an enhanced release of neurotransmitter.
Presynaptic Inhibition
Presynaptic inhibition results when another neuron inhibits the release of an
excitatory neurotransmitter from a presynaptic cell.
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Neurotransmitters and Their Receptors (pp. 414–421; Figs. 11.20–11.21;
Table 11.3)
Neurotransmitters fall into several chemical classes: acetylcholine, the biogenic
amines, amino acid derived, peptides, purines, and gases and lipids. (For a more
complete listing of neurotransmitters within a given chemical class
Functional classifications of neurotransmitters consider whether the effects are
excitatory or inhibitory and whether the effects are direct or indirect, including
neuromodulators that affect the strength of synaptic transmission.
There are two main types of neurotransmitter receptors: channel-linked receptors
mediate fast synaptic transmission and result in brief, localized changes, and G
protein–linked receptors mediate indirect transmitter action resulting in slow
synaptic responses
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