Introduction to human anatomy - nervous system part 3

Segmental Innervation of the Skin
The area of skin supplied by a single spinal nerve, and therefore a
single segment of the spinal cord, is called a dermatome. On the
trunk, adjacent dermatomes overlap considerably; to produce a
region of complete anesthesia, at least three contiguous spinal
nerves must be sectioned. Dermatomal charts for the anterior and
posterior surfaces of the body are shown in Figures 1.23 and 1.24.
In the limbs, arrangement of the dermatomes is more complicated
because of the embryologic changes that take place as
the limbs grow out from the body wall.
A physician should have a working knowledge of the segmental
(dermatomal) innervation of skin, because with the help
of a pin or a piece of cotton he or she can determine whether the
sensory function of a particular spinal nerve or segment of the
spinal cord is functioning normally.
Segmental Innervation of Muscle
Skeletal muscle also receives a segmental innervation. Most of
these muscles are innervated by two, three, or four spinal nerves
and therefore by the same number of segments of the spinal
cord. To paralyze a muscle completely, it is thus necessary to
section several spinal nerves or to destroy several segments of
the spinal cord.
Learning the segmental innervation of all the muscles of the
body is an impossible task. Nevertheless, the segmental innervation
of the following muscles should be known because they can be
tested by eliciting simple muscle reflexes in the patient (Fig. 1.25):
?? Biceps brachii tendon reflex: C5 and 6 (flexion of the elbow
joint by tapping the biceps tendon)
?? Triceps tendon reflex: C6, 7, and 8 (extension of the elbow
joint by tapping the triceps tendon)
?? Brachioradialis tendon reflex: C5, 6, and 7 (supination of the
radioulnar joints by tapping the insertion of the brachioradialis
tendon)
?? Abdominal superficial reflexes (contraction of underlying
abdominal muscles by stroking the skin): Upper abdominal
skin T6 to 7, middle abdominal skin T8 to 9, and lower abdominal
skin T10 to 12
?? Patellar tendon reflex (knee jerk): L2, 3, and 4 (extension of
the knee joint on tapping the patellar tendon)
?? Achilles tendon reflex (ankle jerk): S1 and 2 (plantar flexion
of the ankle joint on tapping the Achilles tendon)