The human spine protects the spinal cord, which serves as the main conduit for nerve signals traveling between the brain and the body. The lower back, known as the lumbar spine (L), consists of five vertebrae, and below it lies the sacrum (S), a triangular bone made of five fused vertebrae. The L5 and S1 segments are located at this lumbosacral junction, just above the pelvis. At each level, pairs of nerve roots exit the spinal column through small openings. The L5 and S1 nerve roots are particularly relevant because they are the largest components that form the major nerves supplying the legs.
The Primary Functions of the L5 Nerve Root
The fifth lumbar nerve root, or L5, is primarily responsible for motor control that allows a person to lift the foot and toes upward, a motion known as dorsiflexion. This root innervates the muscles that enable the extension of the big toe. Weakness in the L5 nerve root often presents as a difficulty in lifting the front part of the foot while walking, a condition known as “foot drop”.
The L5 nerve root also controls the hip abductor muscles, which are necessary for stabilizing the pelvis during walking. Compression or injury to this nerve root can therefore lead to a weakened gait and instability. From a sensory perspective, L5 supplies the skin along the outer side of the lower leg. This sensory distribution continues over the top of the foot.
The sensory area for the L5 nerve is the web space located between the first and second toes. Numbness, tingling, or a burning sensation felt along this specific pathway can often point to irritation of the L5 nerve root. The combination of motor weakness in ankle and toe lift, along with sensory changes in the outer leg and top of the foot, are the distinct hallmarks of L5 nerve root involvement.
The Primary Functions of the S1 Nerve Root
The S1 nerve root governs the contrasting movement of pushing the foot downward, known as plantarflexion. This action is performed primarily by the calf muscles, which are crucial for standing on one’s toes. Damage to the S1 root typically manifests as a loss of power when attempting this “foot push” motion, which can impact balance and propulsion during walking.
The S1 nerve root controls the ankle jerk reflex, also known as the Achilles reflex. This reflex involves a sudden, involuntary contraction of the calf muscles when the Achilles tendon is tapped. An absent or significantly diminished ankle jerk reflex is a strong clinical indicator of S1 nerve root compression.
The sensory distribution of S1 is distinct from L5, following the posterior aspect of the leg. S1 supplies the skin along the back of the calf, the heel, and the lateral, or outside, edge of the sole of the foot. Patients experiencing S1 root irritation often report numbness or tingling that runs down the back of the leg and into the outer foot.
The Pathway of Sciatica: Linking L5 and S1 Symptoms
The L5 and S1 nerve roots are the most frequent sources of irritation leading to the common condition known as sciatica. Sciatica is not a diagnosis itself but a description of symptoms, typically a sharp, shooting pain that radiates from the lower back down the leg. This radiating discomfort is the result of compression or inflammation of one or more nerve roots that contribute to the formation of the large sciatic nerve.
The sciatic nerve is the longest and widest nerve in the body, formed by the joining of nerve roots L4 through S3. When L5 or S1 are irritated, the pain follows the anatomical path of the resulting sciatic nerve down the buttock and the back of the thigh. The most common cause of this irritation is a herniated intervertebral disc, where the disc material protrudes and presses directly onto the exiting nerve root.
Since the L5-S1 disc space is a common site for herniation, both nerve roots can be affected simultaneously or sequentially, leading to a combination of symptoms. For instance, a patient might experience radiating pain down the back of the leg (S1 pattern) but also struggle with lifting their foot (L5 motor deficit). The pain is often exacerbated by activities that increase pressure within the spinal canal, such as coughing, sneezing, or prolonged sitting. Sciatica synthesizes the specific motor and sensory deficits of the individual L5 and S1 roots into a pattern of radiating discomfort and corresponding neurological loss in the leg and foot.