Download Chapter 7 Body Systems

Chapter 12
Nervous System Cells
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Slide 1
Introduction

The function of the nervous system, along
with the endocrine system, is to communicate

The nervous system is made up of the brain,
the spinal cord, and the nerves (Figure 12-1)
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Slide 2
Organization of the Nervous System
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Organized to detect changes in internal and external
environments, evaluate the information, and initiate an
appropriate response
Subdivided into smaller “systems” by location (Figure 12-2)
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Central nervous system (CNS)
• Structural and functional center of entire nervous system
• Consists of the brain and the spinal cord
• Integrates sensory information, evaluates it, and initiates
an outgoing response

Peripheral nervous system (PNS)
• Nerves that lie in “outer regions” of nervous system
• Cranial nerves—originate from brain
• Spinal nerves—originate from spinal cord
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Slide 3
Organization of the Nervous System

Afferent and efferent divisions
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Afferent division—consists of all incoming
sensory pathways
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Efferent division—consists of all outgoing
motor pathways
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Slide 4
Organization of the Nervous System

“Systems” according to the types of organs
they innervate
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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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Slide 5
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 and glands)

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
• Visceral sensory division—carries feedback information
to autonomic integrating centers in the CNS
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Slide 6
Cells of the Nervous System

Glia
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Glial cells support the neurons
Five major types of glia (Figure 12-3):
• Astrocytes
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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)
• Microglia


Small, usually stationary, cells
In inflamed brain tissue, they enlarge, move about, and
carry on phagocytosis
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Slide 7
Cells of the Nervous System

Five major types of glia (cont.):
• Ependymal cells

Resemble epithelial cells and form thin sheets that line
fluid-filled cavities in the CNS

Some produce fluid; others aid in circulation of fluid
• Oligodendrocytes

Smaller than astrocytes with fewer processes

Hold nerve fibers together and produce the myelin sheath
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Slide 8
Cells of the Nervous System
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Five major types of glia (cont.):
• Schwann cells
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Found only in PNS
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Support nerve fibers and form myelin sheaths
(Figure 12-4)

Gaps in the myelin sheath are called nodes of Ranvier

Neurilemma is formed by cytoplasm of Schwann cell
(neurilemmocyte) wrapped around the myelin sheath;
essential for nerve regrowth

Satellite cells are Schwann cells that cover and support
cell bodies in the PNS
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Slide 9
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 12-5)
• Cell body (perikaryon)

Ribosomes, rough endoplasmic reticulum (ER), Golgi apparatus
– 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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Slide 10
Cells of the Nervous System
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Components of neurons (cont.)
• Dendrites

Each neuron has one or more dendrites, which branch from
the cell body
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Conduct nerve signals to the cell body of the neuron
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Distal ends of dendrites of sensory neurons are receptors
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Dendritic spines—small knob-like protrusions on dendrites of some
brain neurons; serve as connection points for axons of other neurons
• Axon
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A single process extending from the axon hillock, sometimes
covered by a fatty layer called a myelin sheath (Figure 12-6)
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Conducts nerve impulses away from the cell body of the neuron
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Distal tips of axons are telodendria, each of which terminates in a
synaptic knob
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Slide 11
Cells of the Nervous System
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Components of neurons (cont.)
• Cytoskeleton
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Microtubules and microfilaments, as well as neurofibrils
(bundles of neurofilaments)

Allow the rapid transport of small organelles (Figure 12-7)
– Vesicles (some containing neurotransmitters), mitochondria
– Motor molecules shuttle organelles to and from the far ends
of a neuron
• Functional regions of the neuron (Figure 12-8)

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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Slide 12
Cells of the Nervous System
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Classification of neurons
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Structural classification—classified according to
number of processes extending from cell body
(Figure 12-9)
• 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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Slide 13
Cells of the Nervous System
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Classification of neurons (cont.)
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Functional classification (Figure 12-10)
• 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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Slide 14
Cells of the Nervous System
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Reflex arc
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A signal conduction route to and from the CNS,
with the electrical signal beginning in receptors
and ending in effectors
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Three-neuron arc—most common; consists of
afferent neurons, interneurons, and efferent
neurons (Figure 12-11)
• Afferent neurons—conduct impulses to the CNS from
the receptors
• Efferent neurons—conduct impulses from the CNS to
effectors (muscle or glandular tissue)
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Slide 15
Cells of the Nervous System
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Reflex arc (cont.)
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Two-neuron arc—simplest form; consists of
afferent and efferent neurons
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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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Slide 16
Nerves and Tracts
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Nerves—bundles of peripheral nerve fibers held
together by several layers of connective tissue
(Figure 12-12)
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Endoneurium—delicate layer of fibrous connective
tissue surrounding each nerve fiber
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Perineurium—connective tissue holding together
fascicles (bundles of fibers)
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Epineurium—fibrous coat surrounding numerous
fascicles and blood vessels to form a complete nerve
Tracts—within the CNS, bundles of nerve fibers are
called tracts rather than nerves
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Slide 17
Nerves and Tracts
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White matter
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PNS—myelinated nerves
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CNS—myelinated tracts
Gray matter
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Made up of cell bodies and unmyelinated fibers
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CNS—referred to as nuclei
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PNS—referred to as ganglia
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Slide 18
Nerves and Tracts
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Mixed nerves
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Contain sensory and motor neurons
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Sensory nerves—nerves with predominantly
sensory neurons
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Motor nerves—nerves with predominantly
motor neurons
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Slide 19
Repair of Nerve Fibers
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Mature neurons are incapable of cell division;
therefore, damage to nervous tissue can be
permanent

Neurons have limited capacity to repair
themselves
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Nerve fibers can be repaired if the damage is
not extensive, the cell body and neurilemma
are intact, and scarring has not occurred
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Slide 20
Repair of Nerve Fibers
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Stages of repair of an axon in a peripheral motor neuron
(Figure 12-13)
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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
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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Slide 21
Nerve Impulses
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Membrane potentials
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All living cells maintain a difference in the concentration of ions
across their membranes
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Membrane potential—slight excess of positively charged ions on
outside of the membrane and slight deficiency of positively
charged ions on inside of membrane (Figure 12-14)

Difference in electrical charge is called potential because it is a
type of stored energy
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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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Slide 22
Nerve Impulses
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Resting membrane potential (RMP)
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Membrane potential maintained by a nonconducting neuron’s
plasma membrane; typically –70 mV
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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 12-15)

Sodium-potassium pump (Figure 12-16)
• 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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Slide 23
Nerve Impulses
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Local potentials
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Local potentials—slight shift away from the resting membrane in
a specific region of the plasma membrane (Figure 12-17)
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Excitation—when a stimulus triggers the opening of additional
Na+ channels, allowing the membrane potential to move toward
zero (depolarization)
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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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Slide 24
Action Potential
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Action potential—the membrane potential of a neuron that
is conducting an impulse; also known as a nerve impulse
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Mechanism that produces the action potential
(Figures 12-18 and 12-19)
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An adequate stimulus triggers stimulus-gated Na+ channels to open, allowing Na+
to diffuse rapidly into the cell, producing a local depolarization

As threshold potential is reached, voltage-gated Na+ channels open and more Na+
enters the cell, causing further depolarization
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The action potential is an all-or-none response
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Voltage-gated Na+ channels stay open for only about 1 millisecond before they
automatically close
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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 sodium-potassium pumps
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Slide 25
Action Potential

Refractory period (Figure 12-20)
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Absolute refractory period—brief period (lasting
approximately half a millisecond) during which
a local area of a neuron’s membrane resists
restimulation and will not respond to a stimulus,
no matter how strong

Relative refractory period—time during which the
membrane is repolarized and is restoring the
resting membrane potential; the few milliseconds
after the absolute refractory period; membrane will
respond only to a very strong stimulus
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Slide 26
Action Potential

Conduction of the action potential
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At the peak of the action potential, the plasma membrane’s polarity
is now the reverse of the RMP
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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 12-21)

This cycle continues to repeat

The action potential never moves backward, as a consequence of
the refractory period
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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 12-22)

Speed of nerve conduction depends on diameter and on the
presence or absence of a myelin sheath
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Slide 27
Synaptic Transmission
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Two types of synapses (junctions)
(Figure 12-23):
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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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Slide 28
Synaptic Transmission
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Structure of the chemical synapse (Figure 12-21)
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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

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 postynaptic axon

Plasma membrane of a postsynaptic neuron—has protein
molecules that serve as receptors for the neurotransmitters
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Slide 29
Synaptic Transmission
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Mechanism of synaptic transmission (Figure 12-25)—
sequence of events is the following:

Action potential reaches a synaptic knob, causing calcium ions to
diffuse into the knob rapidly
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Increased calcium concentration triggers the release of
neurotransmitter via exocytosis

Neurotransmitter molecules diffuse across the synaptic cleft and
bind to receptor molecules, causing ion channels to open

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 either
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 12-26)
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Slide 30
Synaptic Transmission

Summation (Figure 12-28)

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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Slide 31
Neurotransmitters

Neurotransmitters—means by which neurons communicate with
one another; there are more than 30 compounds known to be
neurotransmitters, and dozens of others are suspected

Classification of neurotransmitters—commonly classified by
these features:

Function—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 receptor directly opens a channel or instead uses a
second messenger mechanism involving G proteins and intracellular
signals (Figures 12-29 and 12-30)

Chemical structure—mechanism by which neurotransmitters cause a
change; there are four main classes; since the functions of specific
neurotransmitters vary by location, they are usually classified according to
chemical structure
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Slide 32
Neurotransmitters

Small-molecule neurotransmitters
(Figure 12-31)

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 and at other times, an inhibitory one
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Slide 33
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, etc.

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
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Slide 34
Neurotransmitters

Small-molecule neurotransmitters (cont.)

Other small 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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Slide 35
Neurotransmitters

Large-molecule neurotransmitters—neuropeptides
(Figure 12-32)
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Peptides made up of 2 or more amino acids

May be secreted by themselves or in conjunction with a
second or third neurotransmitter; in this case,
neuropeptides act as a neuromodulator, a
“cotransmitter” 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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Slide 36
Cycle of Life: Nervous System Cells
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Nerve tissue development
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Begins in ectoderm
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Occurs most rapidly in womb and in first 2 years
Nervous cells organize into body network
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Slide 37
Cycle of Life: Nervous System Cells
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Synapses
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Form and reform 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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Slide 38
The Big Picture: Nervous System
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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Slide 39
The Big Picture: Nervous System
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, etc.
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Slide 40