32 Neuroanatomy: Draw It to Know It
Brachial Plexus (Cont.)
Next, show that the suprascapular nerve originates from
the upper trunk and indicate that it innervates the
supraspinatus muscle (C5, C6), which is responsible for
the fi rst 20 to 30 degrees of arm abduction, and also
show that the suprascapular nerve innervates the infraspi-
natus muscle (C5, C6), which is the primary external
rotator of the arm (the other is teres minor).
Draw the axillary nerve off the posterior cord and
show that it innervates the deltoid muscle. Whereas
supraspinatus is responsible for the fi rst 20 to 30 degrees
of arm abduction, the deltoid muscle (C5, C6) is respon-
sible for the latter 70 to 80 degrees of arm abduction. We
will complete the musculature innervated by the axillary
nerve at the end.
Next, more proximally off the posterior cord, draw
the thoracodorsal nerve; it innervates latissimus dorsi
(C6, C7, C8), which provides shoulder adduction, most
notably.
Now, we will draw the pectoral nerves. Off the lateral
cord, draw the lateral pectoral nerve, and off the medial
cord, draw the medial pectoral nerve; they innervate the
pectoralis major muscle, which as a whole, provides
shoulder adduction and shoulder internal rotation. Th e
lateral pectoral nerve innervates the clavicular head of
pectoralis major (C5 (most notably) and C6), which
additionally provides shoulder fl exion and the medial
pectoral nerve innervates the sternal head of the pecto-
ralis major muscle (C6, C7 (most notably), C8, and T1),
which additionally provides shoulder extension. For
completeness, show that the pectoral nerves (mostly the
medial pectoral nerve) also innervate pectoralis minor,
which provides scapula depression.
Now, draw the medial brachial cutaneous and medial
antebrachial cutaneous nerves; they are sensory nerves
that cover the medial aspect of the upper arm and fore-
arm, respectively. In ulnar nerve injuries, medial arm
and forearm sensation is spared due to sparing of these
cutaneous nerves; in contrast, in medial cordopathies,
these nerves are oft en injured and medial upper arm and
forearm sensation is impaired.FIGURE 3-7 Infraspinatus. FIGURE 3-8 Deltoid.
FIGURE 3-9 Latissimus dorsi. FIGURE 3-10 Pectoralis major.