Overview
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.
Gross Anatomy
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).
Clinical Anatomy
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.
Quick Anatomy
Key Facts
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).
Aide-Memoire
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).
Summary
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).