Selected nerves of the spinal nerve plexuses Table 7.3
Plexus
Origin
Nerve
Distribution
Cervical
C1-C5
Phrenic (FREN-ik; origin between C3 and C5)
Diaphragm
Brachial
C5-C8 and T1
Musculocutaneous (mus'-ku-lo-ku-TAN-e-us;
origin between C5 and C7)
Muscles of arm
Axillary (AK-si-lar-e; origin between C5 and
C6)
Deltoid and teres minor muscles; skin over deltoid and
superior posterior aspect of arm
Median (origin between C5 and T1)
Flexors of forearm; skin of palm of hand and fingers
Radial (origin between C5 and T1)
Triceps and extensor muscles of forearm; skin of poste-
rior arm, forearm, hand, and fingers
Ulnar (origin between C8 and T1)
Flexor muscles of forearm, and most muscles of hand;
skin of hand and some fingers
Lumbar
L1-L4
Femoral (including lateral and anterior cutane-
ous branches) (origin between L2 and L4)
Largest nerve arising from lumbar plexus; flexor
muscles of hip and extensor muscles of knee; skin over
anterior and medial aspect of thigh and medial side of
leg and foot
Obturator (OB-too-ra-tor; origin between L2
and L4)
Adductor muscles of hip joint; skin over medial aspect
of thigh
Sacral
L4-L5 and S1-4
Superior and inferior gluteal (origin between
L4 and S2)
Gluteus muscles
Sciatic (origin between L4 and S3)
Actually two nerves—tibial and common fibular—bound
by a common sheath of connective tissue; splits into two
divisions, usually at the knee
Common fibular (origin between L4 and S2)
Divides into superficial and deep branches; serves lateral
aspects of leg and foot
Tibial (origin between L4 and S3)
Posterior muscles of leg and foot
Table 7.3 summarizes some selected nerves of the
spinal nerve plexuses.
During an episode of chickenpox, the varicella zoster
virus can sometimes infect a spinal nerve. The virus in-
vades the peripheral nerve cell bodies in the dorsal root
ganglion. After a period of lying dormant, the virus can
reactivate, causing shingles. Shingles is characterized by a
painful rash in the skin of the associated dermatome.
The PNS and spinal cord process certain information
without input from the brain. This is done via reflexes.
Let’s take a closer look to see how this is accomplished.
Reflex Arcs Allow Automatic
Responses to a Stimulus
If you pick up a hot pot, you may release it automati-
cally before you even realize that your hand is injured.
Similarly,
you
withdraw
your
hand
immediately
when
it
is
pricked by a sharp object before
you
consciously feel
the pain.
These responses, called
reflex-
es
, are rapid and involuntary,
and they are caused by specific
reflex
A fast re-
sponse to a change
(stimulus) in the
internal or external
environment that
attempts to restore
homeostasis.
218 CHAPTER 7
The Nervous System
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