an automatic action stimulated by a specific change of
some kind. Spinal cord reflexesare those that do not
depend directly on the brain, although the brain may
inhibit or enhance them. We do not have to think
about these reflexes, which is very important, as you
will see.
Reflex Arc
A reflex arc is the pathway that nerve impulses travel
when a reflex is elicited, and there are five essential
parts:
1.Receptors—detect a change (the stimulus) and
generate impulses.
2.Sensory neurons—transmit impulses from recep-
tors to the CNS.
3.Central nervous system—contains one or more
synapses (interneurons may be part of the pathway).
4.Motor neurons—transmit impulses from the
CNS to the effector.
5.Effector—performs its characteristic action.
Let us now look at the reflex arc of a specific reflex,
the patellar(or knee-jerk) reflex, with which you are174 The Nervous System
Table 8–3 MAJOR PERIPHERAL NERVESSpinal Nerves
Nerve That Contribute Distribution
Phrenic
Radial
Median
Ulnar
Intercostal
Femoral
SciaticC3–C5
C5–C8, T1
C5–C8, T1
C8, T1
T2–T12
L2–L4
L4–S3- Diaphragm
- Skin and muscles of posterior arm, forearm, and hand; thumb and first 2 fingers
- Skin and muscles of anterior arm, forearm, and hand
- Skin and muscles of medial arm, forearm, and hand; little finger and ring finger
- Intercostal muscles, abdominal muscles; skin of trunk
- Skin and muscles of anterior thigh, medial leg, and foot
- Skin and muscles of posterior thigh, leg and foot
BOX8–2 SHINGLES
Shingles is caused by the same virus that causes
chickenpox: the herpes varicella-zoster virus.
Varicella is chickenpox, which many of us probably
had as children (there is now a vaccine). When a
person recovers from chickenpox, the virus may
survive in a dormant (inactive) state in the dorsal
root ganglia of some spinal nerves. For most peo-
ple, the immune system is able to prevent reactiva-
tion of the virus. With increasing age, however, the
immune system is not as effective, and the virus
may become active and cause zoster, or shingles.
The virus is present in sensory neurons, often
those of the trunk, but the damage caused by the
virus is seen in the skin over the affected nerve. The
raised, red lesions of shingles are often very painful
and follow the course of the nerve on the skin exter-
nal to it. Pain may continue even after the rash
heals; this is postherpetic neuralgia. Occasionally
the virus may affect a cranial nerve and cause facial
paralysis called Bell’s palsy (7th cranial) or extensive
facial lesions, or, rarely, blindness. Although not acure, some antiviral medications lessen the duration
of the illness. A vaccine is being developed for
adults. Though it may not completely prevent shin-
gles, it is expected to lessen the chance of posther-
petic neuralgia.Box Figure 8–A Lesions of shingles on skin of trunk. (From
Goldsmith, LA, Lazarus, GS, and Tharp, MD: Adult and Pediatric
Dermatology: A Color Guide to Diagnosis and Treatment. FA
Davis, Philadelphia, 1997, p 307, with permission.)