490
UNIT 3
Regulation and Integration of the Body
13
PART 2
Transmission Lines:
Nerves and Their
Structure and Repair
Nerves and Associated Ganglia
Define ganglion and indicate the general body location of
ganglia.
Describe the general structure of a nerve.
Follow the process of nerve regeneration.
Structure and Classification
A
nerve
is a cordlike organ that is part of the peripheral nervous
system. Nerves vary in size, but every nerve consists of parallel
bundles of peripheral axons (some myelinated and some not) en-
closed by successive wrappings of connective tissue
(Figure 13.4)
:
Each axon is surrounded by
endoneurium
(en
0
do-nu
9
re-
um), a delicate layer of loose connective tissue that also en-
closes the fiber’s associated Schwann cells.
A coarser connective tissue wrapping, the
perineurium,
binds groups of fibers into bundles called
fascicles
.
A tough fibrous sheath, the
epineurium
, encloses all the fas-
cicles to form the nerve.
Axons constitute only a small fraction of a nerve’s bulk. Te bal-
ance consists chiefly of myelin, the protective connective tissue
wrappings, blood vessels, and lymphatic vessels.
Recall that the PNS is divided into
sensory
(afferent) and
mo-
tor
(efferent) divisions. Nerves are also classified according to
the direction in which they transmit impulses:
Mixed nerves
contain both sensory and motor fibers and
transmit impulses both to and from the central nervous system.
Sensory (afferent) nerves
carry impulses only toward the CNS.
Motor (efferent) nerves
carry impulses only away from
the CNS.
Most nerves are mixed. Pure sensory or motor nerves are rare.
Because mixed nerves oFen carry both somatic and auto-
nomic (visceral) nervous system fibers, the fibers in them may
be classified according to the region they innervate as
somatic
afferent
,
somatic efferent
,
visceral afferent
, and
visceral efferent
.
±or convenience, the peripheral nerves are classified as
cra-
nial
or
spinal
depending on whether they arise from the brain or
spinal cord. Although we mention autonomic efferents of cra-
nial nerves, in this chapter we will focus on somatic functions.
We will defer the discussion of the autonomic nervous system
and its visceral functions to Chapter 14.
Recall from Chapter 11 that
ganglia
are collections of neuron
cell bodies associated with nerves in the PNS, whereas
nuclei
are
collections of neuron cell bodies in the CNS. Ganglia associated
with
afferent
nerve fibers contain cell bodies of sensory neurons.
Visceral and Referred Pain
Visceral pain results from noxious stimulation of receptors in
the organs of the thorax and abdominal cavity. Like deep so-
matic pain, it is usually a vague sensation of dull aching, gnaw-
ing, or burning. Important stimuli for visceral pain are extreme
stretching of tissue, ischemia (low blood flow), irritating chemi-
cals, and muscle spasms.
Te fact that visceral pain afferents travel along the same
pathways as somatic pain fibers helps explain the phenomenon
of
referred pain
, in which pain stimuli arising in one part of the
body are perceived as coming from another part. ±or example,
a person experiencing a heart attack may feel pain that radi-
ates along the medial aspect of the leF arm. Because the same
spinal segments (²
1
–²
5
) innervate both the heart and arm, the
brain interprets these inputs as coming from the more common
somatic pathway.
Figure 13.3
shows cutaneous areas to which
visceral pain is commonly referred.
Check Your Understanding
3.
What are the three levels of sensory integration?
4.
What is the key difference between tonic and phasic
receptors? Why are pain receptors tonic?
5.
Your cortex decodes incoming action potentials from sensory
pathways. How does it tell the difference between hot and
cold? Between cool and cold? Between ice on your finger
and ice on your foot?
For answers, see Appendix H.
Heart
Lungs and
diaphragm
Stomach
Kidneys
Ovaries
Small intestine
Ureters
Urinary
bladder
Colon
Pancreas
Liver
Appendix
Gallbladder
Figure 13.3
Map of referred pain.
This map shows the anterior
skin areas to which pain is referred from certain visceral organs.
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