40 Neuroanatomy: Draw It to Know It
Median Nerve (Cont.)
FIGURE 3-22 Abductor pollicis brevis. FIGURE 3-23 Opponens pollicis.
FIGURE 3-24 Flexor pollicis brevis. FIGURE 3-25 L u m b r i c a l s.
Next, show that the recurrent motor branch of the
thumb innervates the thenar group, supplied by C8 and
T1. Show that this group comprises abductor pollicis
brevis, opponens pollicis, and fl exor pollicis brevis.
Abductor pollicis brevis provides thumb abduction per-
pendicular to the plane of the palm (in other words,
when the palm is up, it raises the thumb toward the ceil-
ing ). Abductor pollicis brevis provides the Up component
of the Up, In, Out triad, which is as follows. Th e median-
innervated abductor pollicis brevis moves the thumb per-
pendicular to the plane of the palm: with the palm up, it
raises the thumb up toward the ceiling. Th e ulnar-inner-
vated adductor pollicis and the radial-innervated abductor
pollicis longus move the thumb in the plane of the palm:
adductor pollicis draws the thumb in toward the side of
the palm and abductor pollicis longus moves the thumb
out away from the side of the palm.
Both the median and ulnar nerves produce the action
of thumb to little fi nger opposition; the median nerve
supplies the opponens pollicis muscle, which directs the
thumb to the little fi nger, and the ulnar nerve supplies
the opponens digiti minimi muscle, which directs the
little fi nger to the thumb. Flexor pollicis brevis attaches
to the proximal phalanx of the thumb and fl exes the
thumb against the palm. Its tendon length is shorter
(more "brief ") than that of fl exor pollicis longus, which
acts at the more distal-lying interphalangeal joint. Both
the median and ulnar nerves supply fl exor pollicis brevis.
Now, show the most distal median nerve motor
group: the terminal motor group. Indicate that it is
derived from C8–T1 and that it comprises the fi rst and
second lumbricals, which have dual actions on the second
and third digits; they extend their proximal interphalan-
geal joints and fl ex their metacarpal–phalangeal joints.
Finally, show that the median nerve provides distal
sensory innervation via digital sensory branches, which
we map along with the rest of the median nerve’s sensory
coverage of the hand in Drawing 5-1.^1 – 4 , 7 – 11