30 Neuroanatomy: Draw It to Know It
Brachial Plexus
Here, we will draw the brachial plexus. First, draw three
horizontal lines. Label them from top to bottom as the
upper, middle, and lower trunks. Next, indicate that C7
makes up the middle trunk, and then that C5 and C6
form the upper trunk (C5 and C6 join at Erb’s point
and a shoulder injury here is called an Erb’s palsy).
Th en, show that C8 and T1 form the lower trunk. Th e
brachial plexus is typically formed from the C5–T1 ven-
tral rami.
Th e trunks divide into anterior and posterior divisions
as follows. Show that the posterior divisions all join to
form the posterior cord. At the bottom, label the anterior
division (lower trunk) and then the medial cord. Next,
label the anterior division (upper trunk) and show that
the anterior division (middle trunk) joins it to form the
lateral cord. Th e cords are named by their relationship to
the second portion of the axillary artery: the lateral cord
lies lateral to the axillary artery, the medial cord lies
medial to it, and the posterior cord lies posterior to it.
Now, connect the distal lateral and medial cords and
label their union as the median nerve. Th en, show that
the medial cord becomes the ulnar nerve and that the
posterior cord becomes the radial nerve.
Next, just distal to the lateral cord, label the musculo-
cutaneous nerve, which most notably innervates the
biceps brachii (C5, C6). Biceps brachii is an important
elbow fl exor and also an important supinator. Th e role of
the biceps brachii in supination explains why supination
is at least partially preserved in radial nerve injury (when
the radial-innervated supinator muscle, itself, becomes
weakened). We’ll include the lesser muscles innervated
by the musculocutaneous nerve later.
Now, show that C5, C6, and C7 derive the long tho-
racic nerve, which innervates the serratus anterior
muscle: it pulls the scapula forward (protracts it). Th en,
show that the dorsal scapular nerve originates from C5
and C4 (not shown) and that it innervates the rhomboid
muscles, which pull the scapula in the opposite direction
of the serratus anterior muscle: toward midline and
downward. Injury to either serratus anterior, the rhom-
boids, or the trapezius, which cranial nerve 11 inner-
vates, results in scapular winging. Note that the nerves to
the serratus anterior and rhomboids are derived directly
from the nerve roots, themselves — not from the brachial
plexus, which means that in a pure brachial plexopathy,
the serratus anterior and rhomboids are spared.FIGURE 3-4 Biceps brachii. FIGURE 3-5 Serratus anterior.
FIGURE 3-6 Rhomboids.