48 Neuroanatomy: Draw It to Know It
Radial Nerve (Cont.)
Next, show that the radial nerve branches into the poste-
rior interosseous nerve. In the proximal segment of the
posterior interosseous nerve, indicate the supinator
muscle. Th e C6, C7 nerve roots supply the supinator; it
provides outward rotation (supination) of the forearm.
Note that the biceps brachii is also a major supinator of
the elbow, and therefore, forearm supination is oft en
preserved in radial neuropathy because of the unaff ected
musculocutaneous-innervated biceps brachii.
Now, show that the posterior interosseous nerve inner-
vates extensor carpi ulnaris and abductor pollicis longus,
which are both supplied by C7, C8. Extensor carpi ulnaris
extends the wrist with medial deviation (towards the ulna
bone) and abductor pollicis longus abducts the thumb in
the plane of the palm. It is the Out component of the Up,
In, Out triad, which is as follows. Th e median-innervated
abductor pollicis brevis moves the thumb perpendicular
to the plane of the palm: with the palm up, it raises the
thumb up toward the ceiling. Th e ulnar-innervated
adductor pollicis and the radial-innervated abductor pol-
licis 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.
Finally, indicate the posterior interosseous nerve
innervation of fi nger and thumb extensors, supplied by
C7, C8; they are extensor indicis proprius, extensor digi-
torum communis, and extensor digiti minimi (for the
fi ngers), and extensor pollicis longus and extensor polli-
cis brevis (for the thumb). Extensor indicis proprius
extends the second digit; extensor digitorum communis
extends the third and fourth digits; and extensor digiti
minimi extends the fi ft h digit. Th e extensor pollicis mus-
cles extend the thumb (with the palm down, they raise it
up). Extensor pollicis brevis extends the thumb at the
metacarpal–phalangeal joint whereas extensor pollicis
longus extends the thumb at the interphalangeal joint.
Th is completes the motor innervation of the radial
nerve; now, let’s address the sensory innervation. First,
just distal to the axilla, show the take-off for the proximal
radial nerve branches. Th e proximal radial sensory nerves
are the posterior cutaneous nerves to the arm and fore-
arm and the lower lateral cutaneous nerve of the arm.
Th ese nerves cover the lower upper arm and the midline
posterior arm and forearm, shown in Drawing 5-4. Next,
at the take-off of the posterior interosseous nerve, draw
the superfi cial sensory radial nerve; it provides distal
radial sensory coverage, shown in Drawing 5-1.^1 – 4 , 9FIGURE 3-36 S u p i n a t o r. FIGURE 3-37 Abductor pollicis longus.
FIGURE 3-38 Extensor digitorum communis.