66 Neuroanatomy: Draw It to Know It
The Thigh
Here, we will draw the innervation of the thigh. To local-
ize each form of femoral, sciatic, or obturator nerve
injury, learn at least one muscle from each muscle group.
First, label across the top of the page from left to right:
abdomen and pelvis, thigh, and leg. Th e thigh divides
into three compartments: anterior, medial, and poste-
rior, which supply the extensor, adductor, and fl exor
muscles, respectively. In accordance with the “one
compartment — one nerve” principle, indicate that the
femoral nerve innervates the anterior compartment,
which the L2–L4 nerve roots supply; the obturator
nerve innervates the medial compartment, which is also
supplied by L2–L4; and the sciatic nerve innervates the
posterior compartment, which, again, is supplied by
L4–S2. Note, though, that the tibial division of the sci-
atic nerve receives additional supply from S3 for its
innervation of the foot.
Now, let’s show the innervation of each compart-
ment’s primary muscle groups. First, indicate that
proximally, the femoral nerve innervates the iliopsoas
muscle, which comprises iliacus and the psoas major and
minor muscles. Iliopsoas is the primary hip fl exor and
attaches within the "iliac region;” when it is weak,
patients have diffi culty climbing upstairs or rising from a
low chair. Next, indicate that distally, the femoral nerve
innervates the quadriceps femoris muscles, which are
rectus femoris and the vastus muscles: vastus medialis,
vastus intermedius, and vastus lateralis. Th e quadriceps
femoris muscles provide knee extension, and when they
are weak, patients have diffi culty walking downstairs.
Next, show that the obturator nerve innervates the
adductor muscles, which are adductor longus, adductor
brevis, and adductor magnus; note that adductor magnus
is also supplied by the sciatic nerve, as we will later show.
We test the adductor muscles through hip adduction but
these muscles provide a variety of actions intrinsic to gait
and stability.
Now, show that the sciatic nerve innervates the ham-
strings muscles, which are semimembranosus, semiten-
dinosus, and the short and long heads of the biceps
femoris muscle. Th e hamstrings muscles provide knee
fl exion and hip extension.FIGURE 4-11 Iliopsoas. FIGURE 4-12 Quadriceps femoris.
FIGURE 4-13 Adductor muscles. FIGURE 4-14 H a m s t r i n g s.