58 Neuroanatomy: Draw It to Know It
Lumbosacral Plexus
Here, we will draw the lumbosacral plexus. First, label
the top of the page from left to right as follows: ventral
rami; pelvis, gluteal region, and hip; thigh; leg ; and foot.
Nine spinal nerves (L1–S4) form the lumbosacral plexus.
Although more spinal nerves are involved in the lum-
bosacral plexus than in the brachial plexus, their courses
are, generally, much simpler, which makes learning the
lumbosacral plexus comparatively easier.
First, draw the innervator of the posterior thigh: the sci-
atic nerve, which is derived from L4–S3. Show that it
passes through the pelvis and posterior thigh and then
branches into the peroneal and tibial nerves, which are the
innervators of the leg and foot. Th e peroneal nerve wraps
around the fi bular neck and innervates the anterior and lat-
eral leg and dorsal foot whereas the tibial nerve innervates
the posterior leg and plantar foot. Indicate that the L4–S2
roots supply the peroneal nerve and that the L4–S3 roots
supply the tibial nerve. We draw the details of the sciatic,
peroneal, and tibial nerves in Drawings 4-3 and 4-4.
Now, draw the innervators of the anterior thigh: the
femoral and obturator nerves, derived from L2–L4. We
draw their innervation pattern in Drawings 4-5 and 4-6.
Note that an accessory obturator nerve branch occasion-
ally exists.
Next, draw the innervators of the hip: the gluteal
nerves. Show that the inferior gluteal nerve, derived
from L5–S2, innervates gluteus maximus; and that one
level above it, the superior gluteal nerve, derived from
L4–S1, innervates gluteus medius, gluteus minimus, and
tensor fasciae latae. Gluteus medius inserts into the ilium
slightly higher than gluteus maximus, which helps us
remember that the superior gluteal nerve innervates glu-
teus medius whereas the inferior gluteal nerve innervates
gluteus maximus. Gluteus maximus provides hip exten-
sion and gluteus medius provides hip abduction.
Regarding the other superior gluteal nerve-innervated
muscles, gluteus minimus provides hip abduction and
tensor fasciae latae provides hip abduction when the hip
is in fl exion.
Above the femoral and obturator nerves, draw the lat-
eral cutaneous nerve of the thigh (aka lateral femoral
cutaneous nerve), derived from L2 and L3, which pro-
vides sensory coverage to the lateral thigh.
At the bottom, draw the pudendal nerve, which is
primarily supplied by S4 but which also receives contri-
butions from S2 and S3. Th e pudendal nerve branches
into the inferior rectal nerve, perineal nerve, and dorsal
nerve to the penis or clitoris. Th e pudendal nerve pro-
vides motor innervation to the external urethral and anal
sphincters and external genitalia and it provides sensory
coverage to the anus and external genitalia, as shown in
Drawing 5-6.FIGURE 4-3 Gluteus maximus. FIGURE 4-4 Gluteus medius.