The human spine, also known as the vertebral column, serves as the body’s central support structure. It provides shape, helps maintain posture, and allows for flexible movement, enabling actions like sitting, standing, and bending. Beyond its role in structural support, the spine also protects the delicate spinal cord and nerves that branch out to the body. It is composed of individual bones called vertebrae.
Understanding the Spine’s Divisions
The human spine is divided into distinct regions, each with a specific number of vertebrae. At the top, the cervical spine in the neck consists of seven vertebrae (C1-C7). Below this, the thoracic spine, forming the upper and mid-back, contains twelve vertebrae (T1-T12) that connect to the ribs. The lower back region, the lumbar spine, has five vertebrae (L1-L5).
The lowest sections of the spine are the sacral and coccygeal regions. The sacral region is composed of five vertebrae (S1-S5) that fuse in adults to form a single, triangular bone called the sacrum. The coccygeal region, or tailbone, consists of three to five fused vertebrae, forming the coccyx.
Pinpointing the S1 Segment
The S1 segment is the uppermost part of the sacrum, the first of its five sacral vertebrae. The sacrum is a large, triangular bone positioned at the base of the spine, fitting like a wedge between the two hip bones (iliac bones) to form the posterior wall of the pelvis. This bone is remarkably thick, effectively supporting and transferring the body’s weight.
The S1 segment, specifically its superior aspect (the sacral promontory), connects with the fifth lumbar vertebra (L5), forming the L5-S1 junction or lumbosacral joint. This junction is where the lumbar spine meets the sacrum. The S1 segment also features wing-shaped projections (alae) that articulate with the ilium at the sacroiliac joints.
The Role of S1 in Body Function
The S1 segment is functionally important, primarily due to the S1 nerve root that exits this part of the spine. This nerve root is a component of the sacral plexus, a nerve network controlling movement and sensation in the lower body. The S1 nerve directly innervates several muscles in the lower extremities, contributing to critical movements.
Specifically, the S1 nerve root plays a role in plantarflexion of the foot (pointing toes downward) and contributes to the strength of calf muscles like the gastrocnemius and soleus. It also contributes to sensation in areas of the leg and foot, including the outer side of the foot. Furthermore, the S1 nerve root is important for the Achilles reflex, where a tap on the Achilles tendon causes the foot to plantarflex. A diminished or absent Achilles reflex can suggest an issue with the S1 nerve root.