Neuronal Signaling and the Structure of the Nervous System
143
cone, which grows out to the effector organ so that function
is sometimes restored. Return of function following a periph-
eral nerve injury is delayed because axon regrowth proceeds at
a rate of only 1 mm per day. So, for example, if afferent neu-
rons from your thumb were damaged by an injury in the area
of your shoulder, it might take two years for sensation in your
thumb to be restored.
In humans, spinal injuries typically crush rather than
cut the tissue, leaving the axons intact. In this case, a primary
problem is self-destruction (apoptosis) of the nearby oligo-
dendrocytes. When these cells die and their associated axons
lose their myelin coat, the axons cannot transmit information
effectively. Severed axons within the central nervous system
may sprout, but no signifi cant regeneration of the axon occurs
across the damaged site, and there are no well-documented
reports of signifi cant return of function. Either some basic dif-
ference of central nervous system neurons or some property of
their environment, such as inhibitory factors associated with
nearby glia, prevents their functional regeneration.
Researchers are trying a variety of ways to provide
an environment that will support axonal regeneration in
the central nervous system. They are creating tubes to sup-
port regrowth of the severed axons, redirecting the axons to
regions of the spinal cord that lack growth-inhibiting factors,
preventing apoptosis of the oligodendrocytes so myelin can be
maintained, and supplying neurotrophic factors that support
recovery of the damaged tissue.
Medical researchers are also attempting to restore func-
tion to damaged or diseased brains by implanting progenitor
stem cells that will develop into new neurons and replace miss-
ing neurotransmitters or neurotrophic factors. Alternatively,
pieces of fetal brain or tissues from the patient that produce
the needed neurotransmitters or growth factors have been
implanted. For example, in patients with
Parkinson’s disease,
a degenerative nervous system disease resulting in progressive
loss of movement, the implantation of tissue from posterior
portions of a fetal brain into the affected area has been some-
what successful in restoring motor function. (Ethical concerns
have rendered the future of this technique uncertain, however.)
SECTION A SUMMARY
Structure and Maintenance of Neurons
I. The nervous system is divided into two parts. The central
nervous system (CNS) comprises the brain and spinal cord, and
the peripheral nervous system consists of nerves extending from
the CNS.
II. The basic unit of the nervous system is the nerve cell, or neuron.
III. The cell body and dendrites receive information from other
neurons.
IV. The axon (nerve fi ber), which may be covered with sections of
myelin separated by nodes of Ranvier, transmits information to
other neurons or effector cells.
Functional Classes of Neurons
I. Neurons are classifi ed in three ways:
a.
Afferent neurons
transmit information into the CNS from
receptors at their peripheral endings.
b.
Efferent neurons
transmit information out of the CNS to
effector cells.
c.
Interneurons
lie entirely within the CNS and form circuits
with other interneurons or connect afferent and efferent
neurons.
II. Neurotransmitters, which are released by a presynaptic neuron
and combine with protein receptors on a postsynaptic neuron,
transmit information across a synapse.
Glial Cells
I. The CNS also contains glial cells, which help regulate
the extracellular fl uid composition, sustain the neurons
metabolically, form myelin and the blood-brain barrier, serve
as guides for developing neurons, provide immune functions,
and regulate cerebrospinal fl
uid.
Neural Growth and Regeneration
I. Neurons develop from stem cells, migrate to their fi nal
locations, and send out processes to their target cells.
II. Cell division to form new neurons is markedly slowed after
birth.
III. After degeneration of a severed axon, damaged peripheral
neurons may regrow the axon to their target organ. In the
CNS, there is some regeneration of neurons, but it is not yet
known how signifi cant this is for function.
SECTION A KEY TERMS
afferent neuron
139
anterograde
139
apoptosis
142
astrocyte
141
axon
138
axon hillock
138
axon terminal
138
axonal transport
139
blood-brain barrier
141
cell body
138
central nervous system
(CNS)
138
collateral
138
dendrite
138
dynein
139
efferent neuron
139
ependymal cell
142
glial cell
141
growth cone
142
initial segment
138
integrator
138
interneuron
139
kinesin
139
microglia
142
myelin
138
nerve
140
nerve fi ber
138
neuron
138
neurotransmitter
138
neurotrophic factor
142
node of Ranvier
139
oligodendrocyte
138
peripheral nervous system
138
postsynaptic neuron
141
presynaptic neuron
141
process
138
retrograde
139
Schwann cell
139
sensory receptor
139
soma
138
stem cell
142
synapse
141
varicosity
138
SECTION A CLINICAL TERMS
Parkinson’s disease
143
SECTION A REVIEW QUESTIONS
1. Describe the direction of information fl ow through a
neuron in response to input from another neuron. What is
the relationship between the presynaptic neuron and the
postsynaptic neuron?
2. Contrast the two uses of the word
receptor.
3. Where are afferent neurons, efferent neurons, and interneurons
located in the nervous system? Are there places where all three
could be found?
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