Cutaneous innervation
The segmental supply (dermatomes) of the lower limb has been considered on page 14. The cutaneous nerves have been described in the preceding pages but for convenience are summarized here (Figs 3.46 and 3.47).
The skin of the buttock receives fibres that run down from the subcostal and iliohypogastric nerves, the posterior rami of the first three lumbar and first three sacral nerves, and the perforating cutaneous nerve, with an upward contribution from the posterior femoral cutaneous nerve. The latter supplies a long strip down the back of the limb to the midcalf, with lateral and medial femoral cutaneous nerves on either side and a small contribution from the obturator nerve on the medial side of the upper thigh. On the front of the thigh, the subcostal, femoral branch of genitofemoral and ilioinguinal nerves supply skin below the inguinal ligament, while the lateral, intermediate and medial femoral cutaneous nerves supply the skin of the rest of the thigh, with the obturator nerve contributing to the supply of the medial side.
The skin of the front of the knee receives branches from the medial, intermediate and lateral femoral cutaneous nerves, the lateral cutaneous nerve of the calf and the saphenous nerve. The last two supply the leg, with the saphenous nerve reaching as far as the level of the metatarsophalangeal joint of the great toe. The superficial peroneal extends over the front of the lower leg and dorsum of the foot, with the deep peroneal supplying the first toe cleft. The sural nerve takes over from the lateral cutaneous nerve of the calf on the lateral side of the back of the leg and extends along the lateral side of the foot to the little toe. Medial calcanean branches of the tibial nerve supply the heel, and medial and lateral plantar nerves the sole. The medial plantar, like the median nerve in the hand, usually supplies three and a half and the lateral plantar the rest of the digits.
Muscular innervation
The segmental innervation of lower limb muscles has been considered on page 15. In the thigh the anterior compartment is supplied by the femoral nerve and the adductor group by the obturator nerve. The tibial part of the sciatic nerve is the nerve of the posterior compartment, with only the short head of biceps supplied by the common peroneal part. In the gluteal region, the inferior gluteal nerve innervates gluteus maximus with the other two glutei receiving their supply from the superior gluteal which also supplies tensor fasciae latae. The short lateral rotator muscles behind the hip have their own nerves, with the obturator externus supplied by the obturator nerve. The tibial nerve is the nerve of the flexor compartment of the leg and its plantar branches supply the muscles of the sole. The common peroneal nerve divides into the superficial peroneal for the peroneal compartment and the deep peroneal for the anterior or extensor compartment.
Sympathetic innervation
As with the brachial plexus, a grey ramus communicans connects each nerve root of the lumbar and sacral plexuses with the appropriate ganglion of the sympathetic trunk so that postganglionic fibres can be distributed to each nerve. The preganglionic fibres for the lower limb have come from cell bodies in the lateral horn of spinal cord segments T11–L2, for the supply of blood vessels, sweat glands and arrectores pilorum muscles.
Lumbar and sacral plexuses
Summaries of these plexuses, including all the lower limb branches, begin on page 325, following descriptions of the abdomen and pelvis.