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Anatomy & Physiology
Chapter 13: Nervous System Cells
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Introduction


Function of nervous system, along
with the endocrine system, is to
communicate
Nervous system made up of the brain,
spinal cord, and nerves (Figure 13-1)
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Organization of the Nervous System


Organized to detect changes in internal and
external environments, evaluate the information,
and initiate an appropriate response
Subdivided into smaller “systems” by location
(Figure 13-2)

Central nervous system (CNS)



Structural and functional center of the entire nervous
system
Consists of the brain and spinal cord
Integrates sensory information, evaluates it, and
initiates an outgoing response
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Organization of the Nervous System

Subdivided into smaller “systems” by
location (cont)

Peripheral nervous system (PNS)



Nerves that lie in the “outer regions” of the
nervous system
Cranial nerves—originate from the brain
Spinal nerves—originate from the spinal
cord
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Organization of the Nervous System

Afferent and efferent divisions


Afferent division—consists of all
incoming sensory pathways
Efferent division—consists of all
outgoing motor pathways
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Organization of the Nervous System

“Systems” categorized according to
types of organs they innervate (Figure
13-2)

Somatic nervous system (SNS)
 Somatic motor division carries
information to the somatic
effectors (skeletal muscles)
 Somatic sensory division carries
feedback information to somatic
integration centers in the CNS
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Organization of the Nervous System

“Systems” (cont)

Autonomic nervous system (ANS)

Efferent division of ANS carries information to the
autonomic or visceral effectors (smooth and
cardiac muscles, glands, and adipose and other
tissues)
 Sympathetic division—prepares the body to
deal with immediate threats to the internal
environment; produces “fight-or-flight”
response
 Parasympathetic division—coordinates the
body’s normal resting activities; sometimes
called the “rest-and-repair” division
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Organization of the Nervous System

“Systems” (cont)

Autonomic nervous system (cont)
 Visceral sensory division carries
feedback information to
autonomic integrating centers in
the CNS
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Cells of the Nervous System

Glia (neuroglia)

Glial cells support the neurons

Five major types of glia (Figure 13-3)

Astrocytes (in CNS)
 Star-shaped, largest, and most numerous type of glia
 Cell extensions connect to both neurons and capillaries
 Astrocytes transfer nutrients from the blood to the
neurons
 Form tight sheaths around brain capillaries, which, with
tight junctions between capillary endothelial cells,
constitute the blood-brain barrier (BBB)
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Cells of the Nervous System

Five major types of glia (cont)
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Microglia (in CNS)
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Small, usually stationary cells
In inflamed brain tissue, they enlarge,
move about, and carry on phagocytosis
Ependymal cells (in CNS)


Resemble epithelial cells and form thin
sheets that line fluid-filled cavities in the
CNS
Some produce fluid; others aid in
circulation of fluid
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Cells of the Nervous System

Five major types of glia (cont)
Oligodendrocytes (in CNS)
 Smaller than astrocytes with fewer
processes
 Hold nerve fibers together and
produce the myelin sheath
 Schwann cells (in PNS)
 Found only in peripheral neurons
 Support nerve fibers and form
myelin sheaths (Figure 13-4)

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Cells of the Nervous System

Five major types of glia (cont)
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Schwann cells (cont)
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Myelin sheath gaps are often called nodes of
Ranvier
Neurilemma is formed by cytoplasm of
Schwann cell (neurolemmocyte) wrapped
around the myelin sheath; essential for nerve
regrowth
Neuronal sheath is the myelin sheath plus the
neurilemma (that is, the whole Schwann
wrapping around the axon)
Satellite cells are Schwann cells that cover
and support cell bodies in the PNS
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Cells of the Nervous System
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Neurons

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Excitable cells that initiate and conduct
impulses that make possible all nervous
system functions
Components of neurons (Figure 13-5)

Cell body (perikaryon)
 Ribosomes, rough endoplasmic
reticulum (ER), Golgi apparatus
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Cells of the Nervous System
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Ribosomes, rough endoplasmic reticulum
(ER), Golgi apparatus (Cont)

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Provide protein molecules
(neurotransmitters) needed for transmission
of nerve signals from one neuron to another
Neurotransmitters are packaged into
vesicles
Provide proteins for maintaining and
regenerating nerve fibers
Mitochondria provide energy (ATP) for
neuron; some are transported to end of
axon
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Cells of the Nervous System
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Neurons (cont)

Components of neurons (cont)
 Dendrites
Each neuron has one or more dendrites, which
branch from the cell body
 Conduct nerve signals to the cell body of the
neuron
 Distal ends of dendrites of sensory neurons
are receptors
 Dendritic spines—small knoblike protrusions
on dendrites of some brain neurons; serve as
connection points for axons of other neurons

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Cells of the Nervous System

Components of neurons (cont)
 Axon
A single process extending from the axon
hillock, sometimes covered by a fatty layer
called a myelin sheath (Figure 13-6)
 Conducts nerve impulses away from the cell
body of the neuron
 Distal tips of axons are telodendria, each of
which terminates in a synaptic knob
 Axon varicosities—swellings that make
contact (synapse) with other cells

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Cells of the Nervous System
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Components of neurons (cont)

Cytoskeleton
Microtubules and microfilaments, as well as
neurofibrils (bundles of neurofilaments)
 Allow the rapid transport of small organelles
(Figure 13-7)
 Vesicles (some containing
neurotransmitters), mitochondria
 Motor molecules shuttle organelles to
and from the far ends of a neuron

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Cells of the Nervous System
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Components of neurons (cont)

Functional regions of the neuron (Figure
13-8)
 Input zone—dendrites and cell body
 Summation zone—axon hillock
 Conduction zone—axon
 Output zone—telodendria and synaptic
knobs of axon
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Cells of the Nervous System

Classification of neurons

Structural classification—classified
according to number of processes
extending from cell body (Figure 13-9)

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Multipolar—one axon and several dendrites
Bipolar—only one axon and one dendrite;
least numerous kind of neuron
Unipolar (pseudounipolar)—one process
comes off neuron cell body but divides almost
immediately into two fibers: central fiber and
peripheral fiber
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Cells of the Nervous System

Classification of neurons (cont)

Functional classification (Figure 13-10)
 Afferent (sensory) neurons—
conduct impulses to spinal cord or
brain
 Efferent (motor) neurons—conduct
impulses away from spinal cord or
brain toward muscles or glandular
tissue
 Interneurons
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Cells of the Nervous System
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Reflex arc


A signal conduction route to and from the
CNS, with the electrical signal beginning in
receptors and ending in effectors
Three-neuron arc—most common; consists
of afferent neurons, interneurons, and
efferent neurons (Figure 13-11)


Afferent neurons—conduct impulses to the
CNS from the receptor
Efferent neurons—conduct impulses from
the CNS to effectors (muscle or glandular
tissue)
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Cells of the Nervous System

Reflex arc (cont)


Two-neuron arc—simplest form; consists
of afferent and efferent neurons
Synapse



Where nerve signals are transmitted from
one neuron to another
Two types: electrical and chemical;
chemical synapses are typical in the adult
Chemical synapses are located at the
junction of the synaptic knob of one neuron
and the dendrites or cell body of another
neuron
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Nerves and Tracts

Nerves—bundles of peripheral nerve
fibers held together by several layers of
connective tissue (Figure 13-12)



Endoneurium—delicate layer of fibrous
connective tissue surrounding each
nerve fiber
Perineurium—connective tissue holding
together fascicles (bundles of fibers)
Epineurium—fibrous coat surrounding
numerous fascicles and blood vessels to
form a complete nerve
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Nerves and Tracts

Tracts—bundles of nerve fibers within
the CNS



Unlike nerves, tracts do not have
connective tissue coverings
Individual fibers may extend through
both a nerve and a tract as it passes
into or out of the CNS
White matter


PNS—myelinated nerves
CNS—myelinated tracts
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Nerves and Tracts


Gray matter

Made up of cell bodies and unmyelinated fibers

CNS—referred to as nuclei

PNS—referred to as ganglia
Mixed nerves



Contain sensory and motor neurons
Sensory nerves—nerves with predominantly
sensory neurons
Motor nerves—nerves with predominantly motor
neurons
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Repair of Nerve Fibers



Mature neurons are incapable of cell
division; therefore damage to nervous
tissue can be permanent
Neurons have limited capacity to repair
themselves
If the damage is not extensive, the cell
body and neurilemma are intact, and
scarring has not occurred; nerve fibers
can be repaired
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Repair of Nerve Fibers

Stages of repair of an axon in a
peripheral motor neuron (Figure 13-13)
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

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Following injury, distal portion of axon
and myelin sheath degenerates
Macrophages remove the debris
Remaining neurilemma and endoneurium
form a tunnel from the point of injury to
the effector
New Schwann cells grow in the tunnel to
maintain a path for regrowth of the axon
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Repair of Nerve Fibers

Stages of repair of an axon in a peripheral
motor neuron (cont)



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
Cell body reorganizes its Nissl bodies to provide the
needed proteins to extend the remaining healthy
portion of the axon
Axon “sprouts” appear
When “sprout” reaches tunnel, its growth rate
increases
The skeletal muscle cell atrophies until the nervous
connection is reestablished
In CNS, similar repair of damaged nerve fibers
is unlikely
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Nerve Impulses

Membrane potentials


All living cells maintain a difference in the
concentration of ions across their
membranes
Membrane potential—slight excess of
positively charged ions on the outside of
the membrane and slight deficiency of
positively charged ions on the inside of
the membrane (Figure 13-14)
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Nerve Impulses

Membrane potentials (cont)



Difference in electrical charge is called
potential because it is a type of stored energy
Polarized membrane—a membrane that
exhibits a membrane potential
Magnitude of potential difference between the
two sides of a polarized membrane is
measured in volts (V) or millivolts (mV); the
sign of a membrane’s voltage indicates the
charge on the inside surface of a polarized
membrane
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Nerve Impulses

Resting membrane potential (RMP)



Membrane potential maintained by a nonconducting
neuron’s plasma membrane; typically 70 mV
The slight excess of positive ions on a membrane’s
outer surface is produced by ion transport
mechanisms and the membrane’s permeability
characteristics
The membrane’s selective permeability
characteristics help maintain a slight excess of
positive ions on the outer surface of the membrane
(Figure 13-15)
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Nerve Impulses

Resting membrane potential (cont)

Sodium-potassium pump (Figure 13-16)
Active transport mechanism in plasma
membrane that transports Na+ and K+
in opposite directions and at different
rates
 Maintains an imbalance in the
distribution of positive ions, resulting in
the inside surface becoming slightly
negative with respect to its outer
surface

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Nerve Impulses

Local potentials


Defined as slight shift away from the
resting membrane in a specific region of
the plasma membrane (Figure 13-17)
Excitation—when a stimulus triggers the
opening of additional Na+ channels,
allowing the membrane potential to move
toward zero (depolarization)
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Nerve Impulses

Local potentials (cont)


Inhibition—when a stimulus triggers the
opening of additional K+ channels,
increasing the membrane potential
(hyperpolarization)
Local potentials are called graded
potentials because the magnitude of
deviation from the resting membrane
potential is proportional to the magnitude
of the stimulus
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Action Potential


Action potential—the membrane potential of
a neuron that is conducting an impulse; also
known as a nerve impulse
Mechanism that produces the action
potential (Figures 13-18 and 13-19)


When an adequate stimulus triggers stimulusgated Na+ channels to open, allowing Na+ to
diffuse rapidly into the cell, a local
depolarization is produced
As threshold potential is reached, voltagegated Na+ channels open and more Na+ enters
the cell, causing further depolarization
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Action Potential

Mechanism (cont)




As more Na+ rushes into cell, the membrane moves
rapidly toward and continues in a positive direction to
the peak of the action potential
Voltage-gated Na+ channels stay open for only about
1 msec before they automatically close; action
potential is an all-or-none response
After action potential peaks, membrane begins to
move back toward the resting membrane potential
when K+ channels open, allowing outward diffusion of
K+; process is known as repolarization
Brief period of hyperpolarization occurs, and then the
resting membrane potential is restored by the sodiumpotassium pumps
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Action Potential

Refractory period (Figure 13-20)


Absolute refractory period—brief period
(lasting approximately 0.5 msec) during
which a local area of a neuron’s membrane
resists re-stimulation and will not respond to
a stimulus, no matter how strong
Relative refractory period—time during which
the membrane is repolarized and restoring
the resting membrane potential; the few
milliseconds after the absolute refractory
period; will respond only to a very strong
stimulus
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Nerve Impulses



At the peak of the action potential, the plasma
membrane’s polarity is now the reverse of the
RMP
The reversal in polarity causes electrical
current to flow between the site of the action
potential and the adjacent regions of
membrane and triggers voltage-gated Na+
channels in the next segment to open; this next
segment exhibits an action potential (Figure
13-21)
This cycle continues to repeat, producing
continuous conduction
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Nerve Impulses



The action potential never moves backward
because of the refractory period
In myelinated fibers, action potentials in the
membrane only occur at the nodes of
Ranvier; this type of impulse conduction is
called saltatory conduction (Figure 13-22)
Speed of nerve conduction depends on
diameter and on the presence or absence of
a myelin sheath
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Synaptic Transmission

Two types of synapses (junctions) (Figure
13-23)


Electrical synapses occur where cells
joined by gap junctions allow an action
potential to simply continue along
postsynaptic membrane
Chemical synapses occur where
presynaptic cells release chemical
transmitters (neurotransmitters) across a
tiny gap to the postsynaptic cell, possibly
inducing an action potential there
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Synaptic Transmission

Structure of the chemical synapse
(Figure 13-25)


Synaptic knob—tiny bulge at the end
of a terminal branch of a presynaptic
neuron’s axon that contains vesicles
housing neurotransmitters
Synaptic cleft—space between a
synaptic knob and the plasma
membrane of a postsynaptic neuron
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Synaptic Transmission

Structure of the chemical synapse (cont)

Arrangements of synapses




Axodendritic—axon signals postsynaptic
dendrite; common
Axosomatic—axon signals postsynaptic
soma; common
Axoaxonic—axon signals postsynaptic
axon; may regulate action potential of
postsynaptic axon
Plasma membrane of a postsynaptic
neuron—has protein molecules that serve
as receptors for the neurotransmitters
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Synaptic Transmission

Mechanism of synaptic transmission (Figure 1325)—sequence of events is as follows:



Action potential reaches a synaptic knob, causing
calcium ions to diffuse into the knob rapidly
Increased calcium concentration triggers the
release of neurotransmitter by way of exocytosis
Neurotransmitter molecules diffuse across the
synaptic cleft and bind to receptor molecules,
causing ion channels to open
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Synaptic Transmission

Mechanism of synaptic transmission (cont)


Opening of ion channels produces a postsynaptic
potential, either an excitatory postsynaptic
potential (EPSP) or an inhibitory postsynaptic
potential (IPSP)
The neurotransmitter’s action is quickly terminated
by neurotransmitter molecules being transported
back into the synaptic knob (reuptake) and/or
metabolized into inactive compounds by enzymes
and/or diffused and taken up by nearby glia
(Figure 13-26)
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Synaptic Transmission

Summation (Figure 13-28)


Spatial summation—adding together the
effects of several knobs being activated
simultaneously and stimulating different
locations on the postsynaptic membrane,
producing an action potential
Temporal summation—when synaptic
knobs stimulate a postsynaptic neuron in
rapid succession, their effects can
summate over a brief period of time to
produce an action potential
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Synaptic Transmission

Synapses and memory


Memories are stored by facilitating
(or inhibiting) synaptic transmission
Short-term memories (seconds or
minutes) may result from axoaxonic
facilitation or inhibition of the
presynaptic axon terminal
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Synaptic Transmission

Synapses and memory


Intermediate long-term memory (minutes to
weeks) happens when serotonin blocks potassium
channels in the presynaptic terminal—thus
prolonging the action potential and increasing the
amount of neurotransmitter released
Long-term memories (months or years) require
structural changes at the synapse—for example,
more vesicles, more vesicle release sites, more
presynaptic terminals, more sensitive postsynaptic
membranes
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Neurotransmitters

Neurotransmitters—means by which
neurons communicate with one
another; there are more than 50
compounds known to be
neurotransmitters, and dozens of
others are suspected
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Neurotransmitters

Classification of neurotransmitters—
commonly classified by the following:

Function—the function of a neurotransmitter is
determined by the postsynaptic receptor; two
major functional classifications are excitatory
neurotransmitters and inhibitory
neurotransmitters; can also be classified
according to whether the receptor directly
opens a channel or instead uses a second
messenger mechanism involving G-protein–
coupled receptors (GPCRs) and intracellular
signals (Figures 13-29 and 13-30)
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Neurotransmitters

Classification of neurotransmitters—
commonly classified by the following
(cont):

Chemical structure—the mechanism by
which neurotransmitters cause a change;
there are four main classes; because the
functions of specific neurotransmitters
vary by location, they are usually
classified according to chemical structure
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Neurotransmitters

Small-molecule neurotransmitters (Figure 1331)

Acetylcholine



Unique chemical structure; acetate (acetyl
coenzyme A) with choline
Acetylcholine is deactivated by
acetylcholinesterase, with the choline molecules
being released and transported back to
presynaptic neuron to combine with acetate
Present at various locations, sometimes in an
excitatory role; other times, inhibitory
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Neurotransmitters

Small-molecule neurotransmitters
(cont)

Amines
 Synthesized from amino acid
molecules
 Two categories: monoamines and
catecholamines
 Found in various regions of the
brain, affecting learning,
emotions, motor control, and so
on
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Neurotransmitters

Small-molecule neurotransmitters (cont)

Amino acids



Believed to be among the most common
neurotransmitters of the CNS
In the PNS, amino acids are stored in
synaptic vesicles and used as
neurotransmitters
Other small-molecule transmitters


Nitric oxide (NO) derived from an amino acid
NO from a postsynaptic cell signals the
presynaptic neuron, providing feedback in a
neural pathway
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Neurotransmitters

Neuropeptides—large-molecule
neurotransmitters


Neuropeptides are short strands of
amino acids called polypeptides or
peptides (Figure 13-32)
Peptides first discovered to have
regulatory effects in the digestive tract;
also act as neurotransmitters in the
brain
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Neurotransmitters

Neuropeptides (cont)


May be secreted by themselves or in
conjunction with a second or third
neurotransmitter; in this case,
neuropeptides act as a neuromodulator, a
“co-transmitter” that regulates the effects of
the neurotransmitter released along with it
Neurotrophins (neurotrophic [nerve growth]
factors) stimulate neuron development but
also can act as neurotransmitters or
neuromodulators
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Role of Nervous System Cells


Neuron doctrine—proposes neuron is
basic structural and functional unit of the
nervous system and neurons are
independent units connected by
chemical synapses
Reticular theory—proposes nervous
system is best understood as a large
integrated network
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Role of Nervous System Cells


Today, the dominant neuron doctrine has
expanded to include concepts of the reticular
theory—neurons are distinct units but also
participate in a network; some neurons are
functionally connected by electrical
synapses, and chemical signals can move in
two directions at the same synapse
Because neuroglia also participate in the
neural network, the whole concept of
nervous system cell function is rapidly
expanding
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Cycle of Life: Nervous System Cells

Nerve tissue development

Begins in ectoderm

Occurs most rapidly in womb and in first 2 years

Nervous cells organize into body network

Synapses



Form and re-form until nervous system is intact
Formation of new synapses and strengthening or
elimination of old synapses stimulate learning and
memory
Aging causes degeneration of the nervous system,
which may lead to senility
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The Big Picture: Nervous System
Cells and the Whole Body



Neurons act as the “wiring” that connects
structures needed to maintain homeostasis
Sensory neurons—act as receptors to detect
changes in the internal and external
environment; relay information to integrator
mechanisms in the CNS
Information is processed, and a response is
relayed to the appropriate effectors through
the motor neurons
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The Big Picture: Nervous System
Cells and the Whole Body



At the effector, neurotransmitter triggers
a response to restore homeostasis
Neurotransmitters released into the
bloodstream are called hormones
Neurons are responsible for more than
just responding to stimuli; circuits are
capable of remembering or learning new
responses, generation of thought, and
so on
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