Nerves of Lower Limb; courses and innervation

Peripheral Nerves in the Lower Quarter

The lumbosacral plexus terminates in three primary peripheral nerves, which are responsible for innervating the tissues of the lower extremity. They are the femoral and obturator nerves from the lumbar plexus and the sciatic nerve from the sacral plexus.

Femoral Nerve: L2-4

The femoral nerve  arises from the three posterior divisions of the lumbar plexus. It emerges from the lateral border of the psoas muscle superior to the inguinal ligament and descends underneath the ligament to the femoral triangle, lateral to the femoral artery, to innervate the sartorius and quadriceps muscle group. The iliopsoas is supplied superior to the ligament. Injuries to the nerve may occur with trauma, such as fractures of the upper femur or pelvis, during reduction of congenital dislocation of the hip, or from pressure during a forceps labor and delivery—resulting in weakness of hip flexion and loss of knee extension. Symptoms may occur from neuritis in diabetes mellitus.

Obturator Nerve: L2-4

The obturator nerve arises from the three anterior divisions of the lumbar plexus. It descends through the obturator canal in the medial obturator foramen to the medial side of the thigh to innervate the adductor muscle group and obturator externus. Isolated injury to this nerve is rare, although uterine pressure and damage during labor may cause the injury. If damaged, adduction and external rotation of the thigh are impaired, with the individual having difficulty crossing his or her legs.

Sciatic Nerve: L4,5, S1–3

The sciatic nerve emerges from the sacral plexus as the largest nerve in the body; its component parts, the tibial and common peroneal nerves, can be differentiated in the common sheath. Muscles in the buttock region (external rotators and gluteal muscles) are innervated by small nerves from the sacral plexus, which emerge proximal to formation of the sciatic nerve. The sciatic nerve exits the pelvis through the greater sciatic foramen and typically courses below, although sometimes through, the piriformis muscle. Piriformis syndrome may occur from a shortened muscle, causing compression and irritation of the nerve at this site. The nerve is protected under the gluteus maximus as it courses between the ischial tuberosity and greater trochanter, although injury may occur in this region with hip dislocation or reduction. The tibial portion of the sciatic nerve innervates the biarticular hamstring muscles and a portion of the adductor magnus; the common peroneal portion innervates the short head of the biceps femoris. Proximal to the popliteal fossa, the sciatic nerve terminates when the tibial and common peroneal nerves emerge as separate structures.

Tibial/Posterior Tibial Nerve: L4,5, S1–3)

The tibial nerve forms from the anterior primary rami of the sacral plexus, courses with the common peroneal nerve as the sciatic nerve, and then emerges as a separate nerve proximal to the popliteal fossa. After coursing through the popliteal fossa, it sends a branch that joins a branch from the common peroneal nerve to form the sural nerve and continues on as the posterior tibial nerve. In the leg, it innervates the muscles of the posterior compartment, including the plantar flexors, popliteus, tibialis posterior, and extrinsic toe flexors. In its approach to the foot, the nerve occupies a groove behind the medial malleolus along with the tendons of the tibialis posterior, flexor hallucis longus, and flexor digitorum longus; the groove is covered by a ligament, forming a tunnel. Entrapment usually from a space-occupying lesion is known as tarsal tunnel syndrome. The nerve then divides into the medial and lateral plantar and calcaneal nerves.

Plantar and calcaneal nerves

 The plantar and calcaneal nerves may become entrapped as they turn under the medial aspect of the foot and pass through openings in the abductor hallucis muscle, especially with overpronation of the foot, which stresses the nerves against the fibrousedged openings in the muscle. Symptoms elicited are similar to acute foot strain (tenderness at the posteromedial plantar aspect of the foot), painful heel (inflamed calcaneal nerve), and pain in a pes cavus foot. The medial and lateral plantar nerves innervate all the intrinsic muscles of the foot except the extensor digitorum brevis. The innervation pattern of the lateral plantar nerve in the foot corresponds to the ulnar nerve in the hand, and the medial plantar nerve corresponds to the median nerve. Weakness and postural changes in the foot such as pes cavus and clawing of the toes may occur with nerve compression or injury.

Common Peroneal Nerve: L4,5, S1,2

After it bifurcates from the sciatic nerve in the knee region, the common peroneal nerve passes between the biceps femoris tendon and lateral head of the gastrocnemius muscle, sends a branch to join the tibial nerve and form the sural nerve, and then comes laterally around the fibular neck and passes through an opening in the peroneus longus muscle. Pressure or force against the nerve in this region can cause neuropathy, including sensory changes and weakness in the muscles of the anterior and lateral compartments of the leg. Injury also occurs subsequent to fracture of the head of the fibula, from rupture of the lateral collateral ligament of the knee, or from a tightly applied cast. Also, most people have experienced their “foot falling asleep” from sustained pressure when crossing their legs. The common peroneal nerve bifurcates just below the neck of the fibula into the superficial and deep peroneal nerves.

Superficial peroneal nerve

 The superficial peroneal nerve descends along the anterior part of the fibula, innervating the peroneus longus and brevis muscles and continues on with cutaneous innervations. Injury to just this nerve primarily affects eversion. Over time, equinovarus may develop from unopposed inversion.

Deep peroneal nerve

 The deep peroneal nerve descends the leg along the interosseous membrane and distal tibia, innervating the ankle dorsiflexors, toe extensors, and peroneus tertius. In the foot it innervates the extensor digitorum brevis. Injury to the deep peroneal nerve results in foot drop and unopposed eversion during gait. Over time, pes valgus may develop.