We have all heard about (or suffered in our own flesh) the discomfort caused by a disorder such as sciatica.

The main cause of this characteristic pain is compression of the ischial nerve, which causes severe pain and numbness of the extremities. It is precisely this very important nerve that we will talk about throughout the article.

We explain what it is and where it is located, and what its main functions are . We will also talk about the different disorders associated with ischial nerve injury.

Ischial nerve: definition, structure and location

The ischial nerve, also called the sciatic nerve, is the largest and longest peripheral nerve in humans and other vertebrate animals. It begins in the pelvis, at the bottom of the sacral plexus, formed by the anterior roots of several spinal nerves, and continues through the hip joint, down the leg.

In humans, the ischial nerve is formed from segments L4 and S3 of the sacral plexus, whose fibers join to form a single nerve in front of the pyriform muscle. The nerve then passes under this muscle and through the greater sciatic foramen, exiting the pelvis.

From there it travels along the posterior thigh to the popliteal fossa (colloquially known as the “hump”). The nerve runs through the posterior compartment of the thigh behind the adductor major muscle, in front of the long head of the biceps femoris muscle.

The ischial nerve, in the lower thigh area and above the knee (at the back), is divided into two nerves: the tibial nerve, which continues its downward path to the feet and is responsible for innervating the heel and sole; and the peroneal nerve, which runs laterally along the outside of the knee and up to the upper foot area.

As we will see later, this nerve provides the connection to the nervous system for almost all the skin of the leg , the muscles of the back of the thigh and those of the leg and foot. Next, we will see what functions this important nerve takes care of.


The ischial nerve is the one that allows movement, reflexes, motor and sensory functions and strength to the leg, thigh, knee , calf, ankle, fingers and toes. Specifically, it serves as a connection between the spinal cord and the outer thigh, the hamstring muscles at the back of the thigh, and the muscles of the lower leg and feet.

Although the ischial nerve passes through the gluteal region, it does not innervate any muscle there. However, it does directly innervate the muscles in the posterior compartment of the thigh and the hamstring portion of the adductor muscle. Through its two terminal branches, it innervates the calf muscles and some of the foot muscles, as well as those in the front and side of the leg, and some other intrinsic muscles of the foot.

On the other hand, although the ischial nerve does not have skin functions as such, it does provide indirect sensory innervation through its terminal branches by innervation of the anterolateral posterolateral sides of the leg and sole of the foot, as well as the lateral part of the leg and dorsal area of the foot.

Related disorders: sciatica

Sciatica is the result of damage or injury to the ischial nerve and is characterised by a sensation that can manifest itself as symptoms of moderate to severe pain in the back, buttocks and legs. Weakness or numbness may also occur in these areas of the body. Typically, the person experiences pain that flows from the lower back, through the buttocks, and into the lower extremities.

The symptoms are often made worse by sudden movement (e.g., getting out of bed), by certain positions (e.g., sitting for a long time), or by heavy exercise (e.g., moving a piece of furniture or holding a bag). Among the most common causes of sciatica are the following:

1. Herniated disks

The vertebrae are separated by pieces of cartilage , which is filled with a thick, transparent material that ensures flexibility and cushioning when we move. The herniated discs are produced when this first layer of cartilage is torn.

The substance inside can compress the ischial nerve, resulting in pain and numbness in the lower extremities. It is estimated that between 1 and 5 percent of the population will suffer from back pain caused by a herniated disc at some point in their lives.

2. Spinal Stenosis

Spinal stenosis, also called lumbar spinal stenosis, is characterized by abnormal narrowing of the lower spinal canal . This narrowing puts pressure on the spinal cord and its sciatic nerve roots. Symptoms that may be experienced are: weakness in the legs and arms, lower back pain when walking or standing, numbness in the legs or buttocks, and balance problems.

3. Spondylolisthesis

Spondylolisthesis is one of the associated conditions of degenerative disc disorder . When one vertebra extends forward over another, the extended spinal bone can pinch the nerves that form its ischial nerve.

Although it is a painful condition, it is treatable in most cases. Symptoms include: stiffness in the back and legs, persistent low back pain, thigh pain, and stiffness of the hamstrings and buttock muscles.

4. Piriform syndrome

Piriform syndrome is a rare neuromuscular disorder in which the piriform muscle contracts or tightens involuntarily, causing sciatica. This muscle is the one that connects the lower spine to the thigh bones. When tensed, it can put pressure on the ischial nerve .

The clinical features of the syndrome include: root pain, numbness and muscle weakness, and tenderness in the buttocks. Pain can sometimes be exacerbated by internal rotation of the lower hip.

The usual treatment is either surgical, with the aim of releasing the pyriform muscle; or non-surgical, with the injection of corticosteroid drugs, the application of analgesic drugs and physiotherapy.

Bibliographic references:

  • Cardinali, D.P. (2000). Manual of Neurophysiology. Madrid: Ediciones Díaz de Santos.

  • Olmarker, K., & Rydevik, B. (1991). Pathophysiology of sciatica. The Orthopedic clinics of North America, 22(2), 223-234.

  • Sobotta, J. (2006). Atlas of Human Anatomy (Vol. 2). Panamerican Medical Publishing.