Chapter 13
The Peripheral Nervous System and Reflex Activity
cord and adjacent neural crest and exit between the forming verte-
brae, and each nerve becomes associated with the adjacent muscle
mass. Te spinal nerves supply both sensory and motor fibers to
the developing muscles and help direct their maturation. Cranial
nerves innervate muscles of the head in a comparable manner.
Te distribution of cutaneous nerves to the skin follows a
similar pattern. Te trigeminal nerves innervate most of the
scalp and facial skin. Spinal nerves supply cutaneous branches
to specific (adjacent) dermatomes that eventually become der-
mal segments. As a result, the distribution and growth of the
spinal nerves correlate with the segmented body plan, which is
established by the fourth week of embryonic development.
Growth of the limbs and unequal growth of other body areas
result in an adult pattern of dermatomes with unequal sizes and
shapes and varying degrees of overlap. Because embryonic muscle
cells migrate extensively, much of the early segmental pattern is lost.
It’s crucial that physicians understand the general pattern of
sensory nerve distribution. For example, in areas where sev-
eral dermatomes overlap, two or three spinal nerves must be
blocked (anesthetized) to perform local surgery.
Sensory receptors atrophy to some degree with age, and mus-
cle tone decreases in the face and neck. Reflexes occur a bit more
slowly during old age. Tis deterioration seems to reflect a gen-
eral loss of neurons, fewer synapses per neuron, and a slowdown
in central processing rather than any major changes in the pe-
ripheral nerve fibers. In fact, peripheral nerves remain viable and
normally functional throughout life unless subjected to traumatic
injury or ischemia. Te most common symptoms of ischemia are
sensations of tingling or numbness in the affected region.
Check Your Understanding
Segmentation in the embryo gives rise to segmentation in the
adult. Name two examples of segmentation in the adult body.
For answers, see Appendix H.
Te PNS is an essential part of any functional nervous sys-
tem. Without it, the CNS would lack its rich bank of informa-
tion about events of the external and internal environments.
Now that we have connected the CNS to both of these environ-
ments, we are ready to consider the autonomic nervous system,
the topic of Chapter 14.
(sole) of the foot. Te normal response is for the toes to flex
downward (curl). However, if the primary motor cortex or cor-
ticospinal tract is damaged, the plantar reflex is replaced by an
abnormal reflex called
Babinski’s sign
, in which the great toe
dorsiflexes and the smaller toes fan laterally.
Infants exhibit Babinski’s sign until they are about a year old
because their nervous systems are incompletely myelinated. De-
spite its clinical significance, the physiological mechanism of
Babinski’s sign is not understood.
Abdominal Reflexes
Stroking the skin of the lateral abdomen above, to the side, or
below the umbilicus induces a reflex contraction of the abdomi-
nal muscles in which the umbilicus moves toward the stimu-
lated site. Tese reflexes, called
abdominal reflexes
, check the
integrity of the spinal cord and ventral rami from ±
to ±
Abdominal reflexes vary in intensity from one person to another.
However, their absence indicates lesions in the corticospinal tract.
Check Your Understanding
Name the five components of a reflex arc.
What is the role of the stretch reflex? The flexor reflex?
Juan injured his back in a fall. When his ER physician stroked
the bottom of Juan’s foot, she noted that his big toe pointed
up and his other toes fanned out. What is this response
called and what does it indicate?
For answers, see Appendix H.
Developmental Aspects of the
Peripheral Nervous System
Describe the developmental relationship between the
segmented arrangement of peripheral nerves, skeletal
muscles, and skin dermatomes.
List changes that occur in the peripheral nervous system
with aging.
Most skeletal muscles derive from paired blocks of mesoderm
(somites) distributed segmentally down the posteromedial aspect
of the embryo. Te spinal nerves branch from the developing spinal
Chapter Summary
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Te peripheral nervous system consists of sensory receptors,
nerves conducting impulses to and from the CNS, their
associated ganglia, and motor endings.
Sensory Receptors and Sensation
Sensory Receptors
(pp. 484–487)
Sensory receptors are specialized to respond to environmental
changes (stimuli).
Sensory receptors include the receptors for the general senses—
pain, touch, pressure, and temperature receptors in the skin—as
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