46 Neuroanatomy: Draw It to Know It
Radial Nerve
Here, we will draw the radial nerve. To localize each
form of radial nerve injury, learn at least one muscle from
each muscle group. To draw the radial nerve, divide the
page into the ventral rami and brachial plexus, upper
arm, forearm, and hand. First, show the proximal seg-
ment of the radial nerve. Next, indicate that the radial
nerve is derived from the C5–C8 nerve roots via the
posterior cord. Indicate that the axillary nerve originates
from the posterior cord just proximal to the derivation
of the radial nerve; the axillary nerve innervates the del-
toid and teres minor muscles. Because the axillary nerve
lies upstream from the radial nerve, it is unaff ected in
radial nerve palsy; we show it here because its anatomic
proximity to the radial nerve makes it useful to test for
clinical localization.
Unlike the median and ulnar nerves, the radial nerve
does provide important motor and sensory branches to
the upper arm. First, show that the radial nerve inner-
vates the triceps muscle, which is supplied primarily by
C6 and C7 but also by C8; it provides elbow extension.
Now, show two key anatomic sites so we can under-
stand how to use the triceps as a localizing tool in radial
nerve palsy. First, show that the axilla lies proximal to
the triceps. When the radial nerve is compressed within
the axilla, such as from using crutches, the triceps is
aff ected because it lies downstream from the axilla. Next,
show that the spiral groove lies distal to the triceps.
Along the spiral groove, the radial nerve opposes the
humerus and is susceptible to compression; injury here is
called “Saturday night palsy.” Th e triceps is unaff ected in
Saturday night palsy because the take-off for the triceps
is proximal to the spiral groove.
Where the upper arm and forearm meet, label the
elbow and show the elbow group, which the C5–C8
nerve roots supply. It comprises brachioradialis, which
C5 and C6 supply, extensor carpi radialis longus and
brevis, which C6 and C7 supply, and anconeus, which
C6–C8 supply. In regards to brachialis, the musculocu-
taneous nerve innervates the C5 and C6 portion and the
radial nerve innervates the small C7 portion.
Brachioradialis fl exes the elbow with the forearm in mid-
pronation/supination position whereas brachialis fl exes
the elbow with the arm in any position. Th e extensor
carpi radialis longus and brevis muscles extend the wrist
with lateral deviation (toward the radius bone). Anconeus
assists in elbow extension.FIGURE 3-33 Triceps. FIGURE 3-34 Brachioradialis.
FIGURE 3-35 Extensor carpi radialis.