361
CHAPTER TEN
Nervous System I
Central nervous system
Peripheral nervous system
Cell body
Interneurons
Dendrites
Axon
terminal
Sensory
receptor
Effector
(muscle or gland)
Axon
(central process)
Axon
Axon
Axon
(peripheral process)
Sensory (afferent) neuron
Motor (efferent) neuron
Cell body
FIGURE 10.7
Neurons are classif
ed by Function as well as structure. Sensory (a±
erent) neurons carry inFormation into the central nervous system
(CNS), interneurons are completely within the CNS, and motor (e±
erent) neurons carry instructions to e±
ectors.
unipolar, as shown in f
gure 10.7, although some
are bipolar.
2.
Interneurons
(also called association or internuncial
neurons) lie within the brain or spinal cord. They are
multipolar and Form links between other neurons.
Interneurons transmit impulses From one part oF the
brain or spinal cord to another. That is, they may direct
incoming sensory impulses to appropriate regions For
processing and interpreting. Other incoming impulses
are transFerred to motor neurons.
3.
Motor neurons
(eFFerent neurons) are multipolar and
carry nerve impulses out oF the brain or spinal cord
to eFFectors—structures that respond, such as muscles
or glands. ±or example, when motor impulses reach
muscles, they contract; when motor impulses reach
glands, they release secretions.
Motor neurons oF the somatic nervous system
(see f g. 10.2) that control skeletal muscle contraction
are under voluntary (conscious) control. Those that
control cardiac and smooth muscle contraction and the
secretions oF glands are part oF the autonomic nervous
system and are largely under involuntary control.
Table 10.1
summarizes the classif
cation oF neurons.
Classif
cation oF Neuroglia
Neuroglia were once thought to be mere bystanders to neu-
ral Function, providing scaFFolding and controlling the sites
at which neurons contact one another
(f gs. 10.8
and
10.9)
.
These important cells have additional Functions. In the embryo,
neuroglia guide neurons to their positions and may stimulate
them to specialize. Neuroglia also produce the growth Factors
that nourish neurons and remove ions and neurotransmitters
that accumulate between neurons, enabling them to continue
transmitting inFormation. In cell culture experiments, certain
types oF neuroglia (astrocytes) signal neurons to Form and
maintain synapses.
Neuroglia oF the CNS
The Four types oF CNS neuroglia are astrocytes, oligodendro-
cytes, microglia, and ependyma:
1.
Astrocytes.
As their name implies, astrocytes are
star-shaped cells. They are commonly Found between
neurons and blood vessels, where they provide
support and hold structures together with abundant
cellular processes. Astrocytes aid metabolism oF
certain substances, such as glucose, and they may help
regulate the concentrations oF important ions, such
as potassium ions, in the interstitial space oF nervous
tissue. Astrocytes also respond to injury oF brain tissue
and Form a special type oF scar tissue, which f
lls spaces
and closes gaps in the CNS. These multiFunctional cells
also have a nutritive Function, regulating movement oF
substances From blood vessels to neurons and bathing
nearby neurons in growth Factors. Astrocytes play an
important role in the blood-brain barrier, which restricts
movement oF substances between the blood and the
CNS (see Clinical Application 5.1, p. 145). Gap junctions
link astrocytes to one another, Forming protein-lined
channels through which calcium ions travel, possibly
stimulating neurons.
2.
Oligodendrocytes.
Oligodendrocytes resemble
astrocytes but are smaller and have Fewer processes.
They Form in rows along myelinated axons, and produce
myelin in the brain and spinal cord.
Unlike the Schwann cells oF the PNS, oligodendro-
cytes can send out a number oF processes, each oF which
Forms a myelin sheath around a nearby axon. In this way,
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