Sciatic nerve (Nervus ischiadicus) is a large peripheral nerve that carries signals between the spinal cord and much of the lower limb. It arises from the sacral plexus in the pelvis, passes deep through the buttock, and descends along the back of the thigh before dividing into its main branches.
Where does the sciatic nerve originate?
The sacral plexus is a network of interconnecting nerves on the back wall of the pelvis. The sciatic nerve receives fibres from spinal nerves L4, L5, S1, S2 and S3: these labels identify nerves associated with the lumbar, or lower back, and sacral regions of the spine.
These fibres reach the plexus through anterior rami, the branches of spinal nerves that supply the limbs and the front and sides of the trunk. Within the plexus, fibres are redistributed into nerves that travel towards particular muscles and skin regions.
The sciatic nerve belongs to the peripheral nervous system, the part of the nervous system outside the brain and spinal cord. It is a mixed nerve, meaning that it contains both motor fibres, which carry signals to muscles, and sensory fibres, which carry information back towards the spinal cord.
Where does the sciatic nerve run?
The nerve usually leaves the pelvis through the greater sciatic foramen, an opening bounded by the hip bone and nearby ligaments. It usually passes below piriformis, a muscle extending from the sacrum at the base of the spine towards the upper thigh bone.
In the buttock, the nerve lies deep to gluteus maximus, the large muscle forming much of the buttock. It then enters the back of the thigh and descends towards the popliteal fossa, the space behind the knee.
The sciatic nerve usually divides into the tibial nerve and common fibular nerve near the upper corner of this space. The point of separation varies: the components may divide higher in the thigh or within the pelvis, so a single undivided trunk is not present throughout the same course in everyone.
What structures lie beside the sciatic nerve?
In the buttock, the nerve passes between the greater trochanter, the prominent outer projection of the upper thigh bone, and the ischial tuberosity, the bony prominence that bears weight when sitting. These landmarks help place its course without implying that it lies immediately beneath the skin.
Deep to the nerve are several muscles that turn the thigh outwards. Along its usual course, it crosses the tendon of obturator internus with the adjacent gemelli muscles, then quadratus femoris. A tendon is the strong connective tissue through which a muscle attaches to another structure.
In the thigh, the nerve lies behind adductor magnus, a broad muscle that draws the thigh towards the body's midline. The long head of biceps femoris, one of the muscles at the back of the thigh, overlies it for much of its course.
What are the sciatic nerve's main components?
The sciatic nerve contains tibial and common fibular components within a shared connective-tissue covering. These components remain distinct bundles even before they separate into independently named nerves. Their arrangement explains why the sciatic nerve supplies several different muscle groups through different branches.
| Nerve or component | Main course | Principal muscle supply |
|---|---|---|
| Undivided sciatic nerve | Buttock and back of thigh | Muscles at the back of the thigh and the hamstring part of adductor magnus |
| Tibial nerve | Behind the knee, through the back of the leg and into the sole | Muscles at the back of the leg and most muscles within the foot |
| Common fibular nerve | Towards the outer knee and around the neck of the fibula | Through its branches, muscles at the front and outer side of the leg |
The fibula is the slender bone on the outer side of the leg; its neck is the narrowed region below its upper end. In anatomical usage, the leg means the region between the knee and ankle, rather than the entire lower limb.
What movements does the sciatic nerve support?
Through its branches, the sciatic nerve supplies much of the muscular system of the lower limb. Its direct muscle branches supply semitendinosus, semimembranosus, both heads of biceps femoris, and the hamstring part of adductor magnus.
The muscles at the back of the thigh bend the knee, and most also move the thigh backwards at the hip. The short head of biceps femoris acts at the knee but does not cross the hip joint.
Below the knee, the tibial nerve supplies muscles that point the foot downwards and bend the toes. Branches of the common fibular nerve supply muscles that lift the foot and toes and turn the sole outwards.
Which areas carry sensory signals through the sciatic nerve?
Through its branches, the sciatic nerve carries sensation from most of the leg and foot. An important exception is the strip along the inner side of the leg and foot, supplied by the saphenous nerve, a branch of the femoral nerve.
The sciatic nerve does not directly supply the skin over the back of the thigh. That skin is mainly supplied by the posterior femoral cutaneous nerve, a separate nerve from the sacral plexus. Cutaneous means relating to the skin.
How can I locate the sciatic nerve in the 3D model?
Select the left sciatic nerve or right sciatic nerve in the model next to this article, then isolate it and rotate towards a view from behind. You can zoom in or out and rotate the view again to examine the selected nerve from different angles.
Switch layers, or fade surrounding structures, to adjust the view of the selected nerve. You can also hide selected parts, pull parts apart or cut a section along the sagittal, coronal or transverse plane, then rotate and zoom to examine the view from another angle.