Regulation and Integration of the Body
Nerves and Their
Structure and Repair
Nerves and Associated Ganglia
Deﬁne ganglion and indicate the general body location of
Describe the general structure of a nerve.
Follow the process of nerve regeneration.
Structure and Classiﬁcation
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
Each axon is surrounded by
um), a delicate layer of loose connective tissue that also en-
closes the ﬁber’s associated Schwann cells.
A coarser connective tissue wrapping, the
binds groups of ﬁbers into bundles called
A tough ﬁbrous sheath, the
, 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
(eﬀerent) divisions. Nerves are also classiﬁed according to
the direction in which they transmit impulses:
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
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
±or convenience, the peripheral nerves are classiﬁed as
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
are collections of neuron
cell bodies associated with nerves in the PNS, whereas
collections of neuron cell bodies in the CNS. Ganglia associated
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
, 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
) innervate both the heart and arm, the
brain interprets these inputs as coming from the more common
shows cutaneous areas to which
visceral pain is commonly referred.
Check Your Understanding
What are the three levels of sensory integration?
What is the key difference between tonic and phasic
receptors? Why are pain receptors tonic?
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.
Map of referred pain.
This map shows the anterior
skin areas to which pain is referred from certain visceral organs.