42 Neuroanatomy: Draw It to Know It
Ulnar Nerve
Here, we will draw the ulnar nerve. To localize each form
of ulnar nerve injury, learn at least one muscle from each
muscle group. First, divide the page into the ventral rami
and brachial plexus, upper arm, forearm, and hand. Draw
a line across the page to represent the course of the ulnar
nerve. Indicate that the ulnar nerve is formed from the
medial cord of the brachial plexus, which the C8–T1
nerve roots of the lower trunk supply. Th e C8–T1 nerve
roots supply all of the ulnar nerve-innervated muscles.
First, show that the medial cutaneous nerves to the
arm and forearm (aka medial brachial and medial ante-
brachial cutaneous nerves) are direct branches from the
medial cord (and are not part of the ulnar nerve); they
provide sensory coverage to the medial arm and forearm.
Th e ulnar nerve sensory coverage, itself, is confi ned to
the medial hand.
Now, show that the most proximal muscle group the
ulnar nerve innervates is the forearm muscle group,
which comprises fl exor carpi ulnaris and fl exor digitorum
profundus 4 and 5. Flexor carpi ulnaris fl exes the wrist
with medial deviation (in the direction of the ulna bone).
Flexor digitorum profundus 4 and 5 fl exes the distal
interphalangeal joints of the fourth and fi ft h digits.
Next, at the elbow, draw the cubital tunnel — the most
common ulnar nerve entrapment site. All of the motor
and sensory components of the ulnar nerve lie distal to
the cubital tunnel; therefore, in a cubital tunnel syn-
drome, all of the components of the ulnar nerve are
aff ected.
Now, at the wrist, show that the ulnar nerve passes
through Guyon’s canal (aka Guyon’s tunnel). Th is is the
other major ulnar nerve entrapment site. Variations of
Guyon’s canal entrapments exist aff ecting a variety of
diff erent distal ulnar components.
Next, show that proximal to Guyon’s canal, the ulnar
nerve derives the palmar and dorsal ulnar cutaneous
nerves; these branches are unaff ected in Guyon’s canal
entrapments because they lie upstream from it. We map
their sensory coverage, along with the sensory coverage
of the rest of the hand, in Drawing 5-1.
As the ulnar nerve enters the hand, it splits into the
superfi cial sensory division, which is purely sensory
except that it provides motor innervation to palmaris
brevis, which corrugates the hypothenar eminence, and
the deep branch, which is purely motor and innervates
muscle groups across the hand.FIGURE 3-26 Flexor carpi ulnaris. FIGURE 3-27 Flexor digitorum profundus 4 and 5.
FIGURE 3-28 Palmaris brevis.