60 Neuroanatomy: Draw It to Know It
Lumbosacral Plexus (Cont.)
Next, go back beneath the sciatic nerve and draw the
posterior cutaneous nerve of the thigh (aka the posterior
femoral cutaneous nerve), derived from S1–S3. It
provides sensory coverage to the midline back of the
thigh.
Now, we will draw the less commonly considered
neuroanatomic components of the lumbosacral plexus.
Above the lateral cutaneous nerve of the thigh, draw the
genitofemoral nerve, derived from L1–L2. Show that
the genital branch innervates the cremaster muscle and
provides sensory coverage to the scrotum or labia and
that the femoral branch is purely sensory; it provides
sensory coverage to the femoral triangle.
Next, at the top of the diagram, draw the iliohypogas-
tric and ilioinguinal nerves, derived from L1. Both
nerves provide motor innervation to the internal oblique
and transversus abdominis muscles. Th e iliohypogastric
nerve provides sensory coverage to the suprapubic and
upper-lateral gluteal areas, and the ilioinguinal nerve
provides sensory coverage to the superior-medial por-
tion of the thigh and proximal external genitalia.
Next, show that L5–S2 supplies the short rotators of
the hip. Th e short rotators provide external rotation of
the hip when it is in extension and hip abduction when
it is in fl exion. Th eir innervation is as follows: the nerve
to quadratus femoris innervates quadratus femoris and
gemellus inferior; the nerve to obturator internus inner-
vates obturator internus and gemellus superior; the nerve
to piriformis innervates piriformis; and the obturator
nerve innervates obturator externus (note that obturator
externus is supplied by L3, L4).
Now, from S2 and S3 show the pelvic splanchnic
nerves and the perforating cutaneous nerve. And from
S4 show the nerves to levator ani and coccygeus, which
form the pelvic diaphragm, and also show the sphincter
ani externus.^1 – 5