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Sciatic Nerve is the largest nerve in humans, originating in the lower back and traveling posteriorly through the lower limb as far down as the heel of the foot. The sciatic nerve innervates a significant portion of the skin and muscles of the thigh, leg, and foot.[rx][rx]

The nerve originates from the ventral rami of spinal nerves L4 through S3 and contains fibers from both the posterior and anterior divisions of the lumbosacral plexus. After leaving the lower vertebrae, the nerve fibers converge to form a single nerve. It exits the pelvis through the greater sciatic foramen inferior to the piriformis muscle along with the pudendal nerve and vessels, inferior gluteal nerve and vessels, nerve to obturator internus, and posterior cutaneous nerve. The sciatic nerve then progresses down the posterior compartment of the thigh deep to the long head of the biceps femoris muscle, superficial to adductor magnus and short head of biceps femoris muscle, and laterally to semitendinosus and semimembranosus muscles.

Anatomy

The sciatic nerve provides motor innervation to the posterior compartment of the thigh. This includes the biceps femoris, semimembranosus, semitendinosus, and the ischial portion of the adductor magnus which allow for knee flexion and hip adduction. The tibial nerve innervates the posterior compartment of the leg and foot which includes the gastrocnemius, soleus, plantaris, popliteus, flexor hallucis longus, flexor digitorum longus, and tibialis posterior. These muscles are primarily responsible for plantarflexion and flexion of the toes. The common peroneal nerve innervates the anterior and lateral compartments of the leg and foot. The anterior compartment includes tibialis anterior, extensor hallucis longus, extensor digitorum longus, and peroneus tertius. These muscles are primarily responsible for dorsiflexion of the foot and extension of toes. The lateral compartment includes peroneus longus and brevis, which are responsible for foot eversion.[rx]

Lumbar nerve roots

There are 5 lumbar vertebrae. The lumbar nerve roots emerge from the lateral spinal recess formed by the inferior facet of the rostral vertebrae and the superior facet of the caudal vertebra. The L5 nerve root exits between the L5 and S1 vertebrae.

Lumbar plexus

The lumbar plexus is composed of the anterior rami of spinal nerves L1-L4. Multiple nerves emerge from this plexus. The iliohypogastric and ilioinguinal nerves supply the transverse abdominis and internal oblique muscles. The obturator nerve supplies the adductors of the thigh. The femoral nerve is a large nerve that supplies the quadriceps femoris group and continues as the saphenous nerve which is the sensory nerve to the medial leg.

Common fibular nerve

The common fibular nerve is the lateral terminal branch of the sciatic runs laterally across the lateral head of the gastrocnemius muscle. It then continues around the fibular head where it is subcutaneous and so is vulnerable to compression. As it passes between the fibula and fibularis longus muscle it divides into the deep and superficial fibular nerves. The deep fibular nerve innervates ankle and toe-extensors and supplies sensation to a small area at the first web space between the first and second toes.[rx] The superficial fibular nerve, supplies the main evertors of the foot, the fibularis longus, and brevis muscles. The superficial fibular sensory branch supplies sensation to the dorsum of the foot and lateral calf.

The sciatic nerve also provides sensory innervation to the skin of the foot and the lower leg (except for the medial leg which is innervated by the saphenous nerve). The tibial nerve further divides into the medial and lateral plantar nerves, which are responsible for the sensation of the sole. The common peroneal nerve further divides into the superficial peroneal nerve and deep peroneal nerve. The superficial peroneal nerve provides sensory innervation to the lateral leg and dorsum of the foot. The deep peroneal nerve is responsible for sensation in between the first and second toes. The medial and lateral sural nerves are made up of collateral branches from both the tibial and common peroneal nerves and provide sensation to the calf and a small lateral portion of the foot.

Origin and Branches of the Sciatic Nerve

The sciatic nerve starts in the lower spine and follows a long path through the buttock, down the back of the thigh and leg, and finally ends in the foot.

Origin

The sciatic nerve is formed by the combination of 5 nerves in the lumbar (lower) and sacral spine—L4, L5, S1, S2, and S3. These nerve fibers are typically responsible for motor and sensory functions of the lower body. The 5 nerves group together near the front surface of the piriformis muscle deep in the buttock and form the large, thick sciatic nerve. At its thickest portion, the nerve measures around 2 cm in diameter.

The nerve leaves the pelvis along with its surrounding nerves and blood vessels through the greater sciatic foramen below the piriformis muscle. It progresses downward between the muscles of the thigh and is surrounded by a single long fatty sheath from the pelvis to the knee, where it divides. In around 10% of the population, the nerve may divide above the knee.

Branches

At the popliteal fossa near the back of the knee, the sciatic nerve divides into 2 main branches: the tibial nerve and the common peroneal nerve. The popliteal fossa is a rhomboid-shaped space that serves as a conduit for blood vessels and nerves in the leg.

The tibial nerve – continues down the back of the leg to the heel and sole of the foot.
The common peroneal – travels sideways along the outer part of the knee to the outer border of the leg and foot.

Both these nerves finally terminate into sural nerves.

The sciatic nerve is usually undivided and leaves the pelvis through the greater sciatic foramen below the piriformis muscle (left). Sometimes, the nerve may be divided, with one part passing through the piriformis and the other below the muscle (right).

Anatomic Variants of the Sciatic Nerve

Most often the undivided sciatic nerve exits the greater sciatic foramen inferior to the piriformis, but this is not always the case.[rx] In the most common anatomic variant, the sciatic nerve splits above the piriformis and one division exits through the piriformis while the other exits below. There are 6 anatomic variants described in the current literature:

Type I: sciatic nerve exits inferior to piriformis, undivided
Type II: sciatic nerve divides above piriformis, one portion exits through the piriformis, the other inferior to it
Type III: sciatic nerve divides above piriformis, one portion courses anterior to piriformis, the other posterior to it
Type IV: sciatic nerve exits through piriformis, undivided
Type V: sciatic nerve divides above piriformis, one portion exits through the piriformis, the other superior to it
Type VI: sciatic nerve divides exists from the superior aspect of piriformis posteriorly, undivided

It is estimated that around 16% of the population may have variations in the anatomical structure of the sciatic nerve.

Associate Others variations are include

The sciatic nerve divides above the piriformis muscle; one portion passes through the piriformis, the other leaves the pelvis below the muscle. This variant is the most common among others.
The sciatic nerve divides above the piriformis muscle; one portion passes through the piriformis, the other leaves the pelvic area above the muscle.
The sciatic nerve divides above piriformis, one portion travels in front of it, the other travels behind it.
An undivided sciatic nerve exits through the piriformis muscle.
An undivided sciatic nerve exits from behind the top part of the piriformis.

In cases where the sciatic nerve divides, both portions of the nerve immediately merge again and course downward as a single nerve.

Functions of the Sciatic Nerve

The sciatic nerve supplies major parts of the thighs, legs, and feet and has both motor and sensory functions.

Motor functions of the sciatic nerve The motor functions include

Knee flexion: Bending the knee
Hip adduction: Bringing the thighs together/movement of the leg toward the midline of the body
Plantar flexion: Pointing the foot downward
Flexion of toes: Pointing the toes downward
Dorsiflexion of the foot: Pointing the foot upward
Extension of toes: Pointing the toes upward

When the sciatic nerve is compressed, it is common to experience reduced motor functions and weakness in the affected leg.

Sensory functions of the sciatic nerve

The sciatic nerve provides sensations to the skin (dermatome) over the following areas:

Front, back, and outer part of the thigh
Front, back, and outer part of the lower leg
The top and outer side of the foot
Sole of foot
The web between the first and second toes

Sensory symptoms such as burning, tingling, and/or numbness may be experienced when the sciatic nerve is inflamed or irritated.

Sciatic Nerve – Anatomy, Origin, Branches, Functions