The sciatic nerve (ventral rami of L4-S3) is the nerve of the posterior thigh and leg. It is the largest branch of the sacral plexus, and in addition to supplying all of the posterior musculature of the lower limb, also supplies sensation over parts of the leg and foot.
The sciatic nerve has two major branches, the tibial and common fibular nerves. It leaves the pelvis through the greater sciatic notch, below the piriformis muscle along with the inferior gluteal artery and nerve, pudendal nerve, nerve to piriformis, posterior femoral cutaneous nerve and nerve to obturator internus. The sciatic nerve then descends down the thigh, superficial to adductor magnus, but deep to the long head of biceps femoris. The nerve usually divides above the popliteal fossa, into the tibial and common fibular nerves.
It is the tibial branch of the sciatic nerve that supplies all the muscles of the posterior thigh and leg (aside from the short head of biceps femoris, which is supplied by the common fibular nerve). These muscles are the semitendinosus, semimembranosus, and biceps femoris (long head). The tibial division (L4-S2) continues down the posterior leg, and supplies the extensors of the ankle i.e. posterior compartment of the leg. These muscles include popliteus, gastrocnemius, soleus, flexor hallucis longus, flexor digitorum longus, and tibialis posterior. The tibial nerve passes behind and below the medial malleolus in the tarsal tunnel, and divides into the medial and lateral plantar nerves. The medial plantar nerve runs with the artery of the same name, and supplies the first lumbrical, abductor hallucis, flexor hallucis brevis and flexor digitorum brevis. It also supplies sensation to the medial sole and three and a half toes as well as the nail beds of these toes dorsally (analogous to the median nerve). The lateral plantar nerve runs with the lateral plantar artery, and supplies the remaining muscles of the foot (quadratus plantae, flexor digiti minimi, adductor hallucis, the interossei, three lateral lumbricals and the abductor digiti minimi. It also provides sensation to the lateral one a half sole and associated toes (analogous to the ulnar nerve).
The common fibular (peroneal nerve) descends from its origin, and runs laterally to wind around the neck of the fibula (deep to the biceps femoris, and lateral collateral ligament of the knee). At this point it divides into a deep branch and a superficial branch. The deep branch enters the anterior compartment of the leg, and supplies the muscles (extensor digitorum longus, extensor hallucis longus, and tibialis anterior). The nerve also supplies sensation to the first dorsal web-space. The superficial branch enters the lateral compartment of the leg, and supplies fibularis longus and brevis. These muscle are everters of the ankle (eversion and inversion occurs at the subtalar joint, and not the ankle joint, which simply allows for plantar flexion and dorsiflexion). The nerve also supplies sensation to the lateral and anterior surfaces of the leg, as well as the majority of the dorsum of the foot (excluding the first web-space).
Sciatic nerve compression- The sciatic nerve emerges from the greater sciatic nerve, superior to the piriformis muscle in the vast majority of people. In some the nerve passes through the piriformis, and can be affected in piriformis syndrome. The nerve is vulnerable to compression as it leaves the greater sciatic foramen, and also to spinal disc herniation, which frequently occurs at the L5-S1 junction. The disc herniation may impinge the roots of the nerve (L4-S3), as may degenerative joint disease affecting the intervertebral foramina, or from spinal stenosis narrowing the spinal canal. Spondylolisthesis is caused by pars intercularis fracture causing a shifting of the vertebral column anteriorly, which may damage the nerve at the roots.
Common Peroneal nerve palsy- This affects the extensor of the ankle, so the major presenting symptom is foot drop. This usually occurs following wearing of a tight cast or clothing, which compresses the nerve as it winds around the neck of the fibula. Sensation over the lateral and anterior leg, as well as the dorsum of the foot may also be affected.
Developmental precursor- Alar and basal plate of L4-S3
Origin- Ventral rami of L4-S3
Branches- Tibial and common fibular nerves
Muscles supplied- posterior compartment of thigh (hamstrings), posterior, anterior and lateral compartments of the leg, intrinsic muscles of the foot.
Dermatome- Lateral leg (lower part via sural nerve and upper part via superficial branch of the common fibular nerve, dorsum of the foot by superficial fibular nerve, and first dorsal web-space by deep fibular nerve).
The structures supplies by the medial plantar nerve can be remembered with the LAFF acronym (first Lumbrical, Abductor hallucis brevis, Flexor hallucis brevis and Flexor digitorum brevis).
The sciatic nerve (ventral rami L4-S3) supplies the posterior compartment of the thigh (tibial nerve), all compartments of the leg (anterior by the deep fibular and lateral by the superficial fibular), and intrinsic feet muscles (via medial and lateral plantar nerves). It also supplies sensation to the lateral leg (sural and superficial fibular nerves), as well as the dorsum and sole of the foot (dorsum by the deep and superficial fibular nerves, sole by medial and lateral plantar nerves).