Ischiatic (sciatic) nerve: anatomy, functions and pathologies

We have all heard about (or have 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 intense pain and numbness in the extremities. It is precisely this 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 sciatic nerve injury.

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Ischial nerve: definition, structure and location

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

From there it travels down the posterior thigh to the popliteal fossa (colloquially known as the “hamstring”). The nerve courses in the posterior compartment of the thigh behind the adductor magnus muscle, in front of the long head of the biceps femoris muscle.

The sciatic nerve, in the lower thigh and above the knee (at the back), divides into two nerves: the tibial nerve, which continues its descending course towards the feet and is responsible for innervating the heel and plant; and the peroneal nerve, which run laterally along the outside of the knee and to the top of the foot.

As we will see later, this nerve provides the connection to the nervous system for almost all of the skin on 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 is responsible for.

Functions

The sciatic 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 part of the thigh, the muscles hamstrings that are found in the back of the thigh, and the muscles of the lower part of the leg and feet.

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

On the other hand, although the sciatic nerve does not have proper cutaneous functions, it does provide indirect sensory innervation through its branches. terminals by innervating the posterolateral anterolateral sides of the leg and sole of the foot, as well as the lateral aspect of the leg and dorsal aspect of the foot. foot.

Related disorders: sciatica

Sciatica is the result of damage or injury to the sciatic nerve and is characterized by a sensation that can manifest with 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.

Symptoms are often worsened by sudden movement (eg. eg getting out of bed), by certain positions (eg. eg sitting for a long time) or when performing physical exercise with weights (eg. eg move a piece of furniture or pick up a bag). Among the most common causes of sciatica we can highlight the following:

1. herniated discs

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

The substance inside can compress the sciatic 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 include: 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 make up your sciatic nerve.

Although it is a painful condition, it is treatable in most cases. Symptoms include: stiff back and legs, persistent lower back pain, thigh pain, and tightness of the hamstrings and gluteal muscles.

4. piriformis syndrome

Piriformis syndrome is a rare neuromuscular disorder in which the piriformis muscle contracts or tightens involuntarily, causing sciatica. This muscle is the one that connects the lower part of the spine with the thigh bones. When tense, it can put pressure on the sciatic nerve.

Clinical features of the syndrome include: radicular pain, muscle numbness and weakness, and buttock tenderness. Occasionally, the pain can be exacerbated by internal rotation of the lower extremity of the hip.

The usual treatment is usually surgical, with the aim of releasing the piriformis muscle; or non-surgical, with the injection of corticosteroid drugs, the application of pain medication and physiotherapy.

Bibliographic references:

• Cardinali, D.P. (2000). Manual of neurophysiology. Madrid: Diaz de Santos Editions.

• 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). Pan American Medical Ed.