86 Neuroanatomy: Draw It to Know It
Cutaneous Nerves — Trunk (Advanced)
Here, we will map the cutaneous innervation of the
trunk and related anatomic regions. Before sketching
their sensory coverage, let’s address the origins of the
sensory nerves of the thorax, abdomen, and back. Show
that the anterior and posterior nerve roots form a mixed
spinal nerve, which splits into posterior and anterior
rami. Th e posterior ramus derives the posterior cutane-
ous sensory branch and the anterior ramus derives the
intercostal nerve, which provides the lateral cutaneous
branch and the anterior cutaneous branch.
Now, draw the anterior trunk (the thorax and abdo-
men) and posterior trunk (the back). Also include the
pelvic and gluteal junctional zones in our posterior trunk
diagram and also the back of the head and neck, as well,
given their regional relevance. Show that the supraclavic-
ular nerve covers the supraclavicular chest and also the
posterior shoulders. Th en, show that thoracic anterior
cutaneous branches cover the midline thorax and abdo-
men, and then that the thoracic lateral cutaneous
branches cover the lateral thorax and abdomen. Note
that T1 is uninvolved in sensory coverage of the trunk: it
supplies the medial cutaneous nerves of the arm and
forearm, instead; also, note that the lateral cutaneous
branch of T2 is called the intercostobrachial nerve and it
covers the axilla; and, fi nally, note that T12 is called the
subcostal nerve, which innervates the upper/lateral but-
tock, and has important connections with the cutaneous
branches of the lower abdomen, which we draw next.
Now, show that the iliohypogastric nerve provides
sensory coverage to the suprapubic area and the upper-
lateral gluteal region. Next, show that the ilioinguinal
nerve provides sensory coverage to the superior-medial
thigh and proximal external genitalia. Th en, show that
the genital branch of the genitofemoral nerve provides
additional sensory coverage to the proximal external
genitalia and that the femoral branch of the genitofemo-
ral nerve covers the femoral triangle. Next, show that the
lateral cutaneous nerve of the thigh covers the lateral
thigh. And fi nally, show that the pudendal nerve covers
the distal external genitalia, provides even further cover-
age to the proximal external genitalia, and also covers
the anus.
Next, show that the posterior cutaneous rami provide
most of the sensory innervation to the posterior trunk.
First, indicate that the thoracic rami cover from the scap-
ulae to the iliac crests and then that the lumbosacral and
coccygeal rami cover the buttocks. Note that the sensory
innervators of the buttocks are oft en called the cluneal
nerves. Now, show that the inferior aspect of each but-
tock receives additional coverage from the anterior rami-
derived posterior cutaneous nerve of the thigh and
perforating cutaneous nerve.
Finally, show that the posterior cervical rami cover
the back of the neck and that the greater occipital nerve,
which is supplied by the posterior ramus of C2, covers
the back of the head.^1 , 2 , 4 , 8 , 11