The nervous system uses spinal nerve roots as communication pathways extending from the spinal cord to the rest of the body. The S1 nerve root, or first sacral nerve, is one of five pairs of sacral nerves emerging from the lowest region of the spinal column. It controls movement and processes sensation for a large portion of the lower body, including the leg and foot. Understanding its location and functions helps recognize the effects of potential damage or irritation.
Where the S1 Nerve Root is Located
The S1 nerve root originates in the sacral region of the spinal cord, located within the bony sacrum at the base of the spine. Unlike higher nerve roots, the S1 root travels downward inside the spinal canal before exiting. This collection of descending nerves is known as the cauda equina, or “horse’s tail.”
The nerve root exits the spinal canal through the neural foramen, typically between the first and second sacral vertebrae (S1 and S2). However, the S1 nerve root is most commonly affected by compression as it traverses the L5-S1 disc space, which sits just above the sacrum. After exiting, the S1 root joins other nerve roots to form the sacral plexus, which gives rise to the large sciatic nerve.
Key Functions of the S1 Nerve Root
The S1 nerve root is a mixed nerve, carrying both motor signals to muscles and sensory information back to the spinal cord. Its motor function primarily involves the muscles controlling the ankle and foot, particularly plantar flexion (pointing the toes downward). The gastrocnemius and soleus muscles in the calf are innervated by S1, enabling a person to stand on their tiptoes.
The nerve also contributes to hip extension and knee flexion, coordinating these larger movements with other nerve roots. The sensory role of S1 is defined by its dermatome, the specific area of skin sensation it controls. This area includes the back of the leg, the outer side of the foot, and the sole of the foot, providing touch and temperature feedback.
A primary function of S1 is its role in the Achilles reflex, also known as the ankle jerk reflex. This deep tendon reflex causes the foot to plantarflex involuntarily when the Achilles tendon is tapped. This reflex arc relies on the S1 nerve root to transmit the signal, making it a reliable clinical indicator of the nerve’s health.
Common Causes of S1 Nerve Root Compression
The S1 nerve root is frequently susceptible to irritation or compression, a condition termed radiculopathy. The most frequent cause of S1 radiculopathy is a lumbar disc herniation, particularly at the L5-S1 level. When the soft inner material of the disc pushes outward, it can directly press on the S1 nerve root, leading to inflammation.
Another anatomical issue that can compress the S1 nerve root is spinal stenosis, a narrowing of the spinal canal or the neural foramen. This narrowing can be caused by the development of bone spurs or degenerative changes associated with aging. Spondylolisthesis, where one vertebra slips forward over the one below it, can also reduce the space available for the S1 nerve root, causing irritation.
Less commonly, the S1 nerve can be irritated further along its path by conditions such as piriformis syndrome, though this is not a true radiculopathy. The piriformis muscle, located deep in the buttock, lies close to the sciatic nerve, which contains S1 fibers. Spasm or tightness in this muscle can mimic S1 nerve root symptoms by irritating the nerve.
Recognizing Symptoms of S1 Nerve Damage
When the S1 nerve root is compromised, symptoms correspond directly to the loss of its normal motor, sensory, and reflex functions. A common symptom is pain, often sharp or burning, which radiates from the lower back or buttock down the back of the leg and into the outer side of the foot. This radiating pain is a form of sciatica, as the S1 root is a component of the sciatic nerve.
Sensory changes are noticeable along the S1 dermatome, presenting as numbness, tingling (paresthesia), or a Pins and Needles sensation in the outer foot and heel. Motor weakness is observed as difficulty with plantar flexion, making it hard to stand on one’s tiptoes or push off the ground while walking. This weakness can sometimes cause a “toe drop,” affecting the gait pattern.
A physical examination often reveals a diminished or absent Achilles tendon reflex. The loss of this involuntary ankle jerk response is a specific clinical sign of S1 nerve root dysfunction. A combination of these symptoms—pain, sensory changes, motor weakness, and reflex loss—suggests that the S1 nerve root is the source of the problem.