The S1 nerve root is a segment of the nervous system responsible for motor and sensory functions in the lower body. It is one of the five sacral nerve roots that originate in the lower back, connecting the spinal cord to the legs and feet. The S1 nerve is a mixed nerve, containing both motor fibers that control muscle movement and sensory fibers that transmit touch, temperature, and pain signals back to the brain. Proper function of the S1 nerve root is tied directly to the ability to walk, balance, and feel sensation in the lower extremities. Problems in the lower back can therefore lead directly to noticeable neurological symptoms in the legs and feet.
The S1 Nerve Root: Location and Context
The S1 nerve root is the first of the five sacral nerves, emerging from the lowest region of the spinal column. The spinal cord ends higher up, typically between the first and second lumbar vertebrae (L1 or L2). Below this point, the nerve roots travel downward in a bundle known as the cauda equina, or “horse’s tail,” before exiting the spine.
The S1 nerve root travels within this collection of nerves before exiting the spinal canal through the first sacral foramen in the sacrum bone. S1 fibers then join with other nerve roots (L4, L5, S2, and S3) to form the sacral plexus. This plexus gives rise to the major nerves of the lower limb, including the sciatic nerve, which carries S1 fibers all the way down the leg.
Sensory Mapping: Understanding the S1 Dermatome
The area of skin supplied by a single spinal nerve root is called a dermatome. The S1 nerve root is responsible for the S1 dermatome, which covers a distinct path down the leg and foot. This sensory territory runs down the back of the thigh and the entire calf.
The sensation continues to the outer (lateral) side of the ankle and encompasses the outer border and sole of the foot, often including the heel. When the S1 nerve root is irritated or compressed, a person may experience sensory disturbances along this distribution. Symptoms include numbness, altered sensation, or a pins-and-needles feeling (paresthesia). Identifying the location of these changes helps clinicians pinpoint the affected nerve root.
Motor Function: Muscles Controlled by the S1 Myotome
The S1 nerve root supplies motor function to a specific group of muscles known as the S1 myotome. The most important action controlled by S1 is ankle plantar flexion, which is the movement of pointing the foot and toes downward. This movement is primarily executed by the large calf muscles: the gastrocnemius and the soleus.
This action is fundamental to walking, allowing a person to push off the ground during the gait cycle and stand up on their toes. S1 also contributes motor control to the gluteus maximus, a large muscle responsible for hip extension. Hip extension is involved in movements like climbing stairs and standing up from a chair. Testing the strength of plantar flexion is a direct way to assess the health of the S1 nerve root.
A specific clinical test for S1 function is the Achilles (ankle jerk) reflex. When the Achilles tendon is tapped, a healthy S1 nerve root triggers a rapid, involuntary plantar flexion of the foot. A reduced or absent Achilles reflex is a reliable sign of S1 nerve root dysfunction, making it a key diagnostic finding.
Symptoms of S1 Nerve Impingement and Injury
Compression or irritation of the S1 nerve root causes S1 radiculopathy, which produces a characteristic set of symptoms. The most common cause is a herniated disc, often at the L5/S1 level, where the disc material presses on the nerve root. Spinal stenosis (narrowing of the spinal canal) or trauma can also lead to S1 nerve impingement.
The primary symptom is sharp, radiating pain known as sciatica, which travels down the posterior leg and into the foot, following the S1 dermatome path. This pain may feel like a burning sensation or an electric shock, often worsening with movements like coughing or sneezing. Motor weakness is another consequence, specifically affecting the ability to push off the ground or stand on the toes.
In severe cases, this weakness can cause difficulty walking or standing, and the ankle jerk reflex will be diminished or lost entirely. The combination of posterior leg pain, weakness in pushing the foot down, and loss of the ankle reflex creates a distinct clinical picture pointing to S1 nerve root damage.