The human body’s central support structure is the vertebral column, commonly known as the spine. It is an arrangement of small, irregularly shaped bones called vertebrae, stacked one upon the other. Between the majority of these bony segments are the intervertebral discs, specialized structures that act as resilient spacers and flexible connections. These fibrocartilaginous pads bind the vertebrae together, allowing for a wide range of movement, bending, and twisting.
The Exact Count: Intervertebral Discs vs. Vertebrae
A typical adult human spine contains exactly 23 intervertebral discs. This number often causes confusion because the spine is composed of 33 individual bony segments, or vertebrae, in total. However, discs are only found between segments designed for movement.
The count of 23 is explained by the absence of discs in specific locations. There is no disc between the base of the skull and the first cervical vertebra (C1), or between C1 and the second cervical vertebra (C2). Furthermore, the lowest segments of the spine—the sacrum and the coccyx—do not have separate discs because their bones are permanently fused together.
Therefore, the 23 discs begin below the axis (C2) and continue down to the first sacral segment (S1). These structures collectively comprise approximately one-quarter of the total height of the spinal column.
Mapping the Spine: Location and Regional Distribution
The 23 intervertebral discs are distributed across the three major mobile regions of the spine, reflecting the different mechanical demands placed on each section.
Cervical Spine
The uppermost region, the cervical spine (neck), contains six discs, located between the C2 and C7 vertebrae.
Thoracic Spine
The middle portion, the thoracic spine, is connected to the rib cage and contains 12 discs (T1 through T12). Movement in this region is limited due to the stiffness provided by the attached ribs.
Lumbar Spine
The lowest section is the lumbar spine, which bears the greatest amount of the body’s weight and contains five discs (L1 to L5). The thickness of the discs generally increases as they descend the spine, with the lumbar discs being the thickest and largest. This size increase is an adaptation to withstand the compressive forces associated with standing, lifting, and walking.
Mechanical Role: How Discs Enable Movement and Flexibility
The spine’s capacity to move, absorb impacts, and bear load depends entirely on the unique structure of the disc. Every intervertebral disc is composed of two distinct parts that manage mechanical stress.
The outer layer is a tough, concentric ring of fibrous cartilage called the annulus fibrosus. This ring consists of multiple layers of collagen fibers organized to resist rotational forces and contain the inner material.
Encased within this strong outer ring is the nucleus pulposus, a soft, gel-like substance in the center of the disc. It is highly hydrophilic, rich in water content (66% to 86% of its mass), allowing it to function like a hydraulic cushion.
When a compressive force is applied, the nucleus pulposus distributes pressure evenly outward. The surrounding annulus fibrosus contains this pressure, effectively dispersing the load across the vertebral bodies. This teamwork allows for shock absorption, prevents vertebrae from grinding, and enables complex movements like bending and twisting.
When Discs Fail: Understanding Common Issues
Despite their resilience, intervertebral discs are susceptible to wear and tear over time, leading to common spinal issues.
One widespread condition is disc degeneration, a natural process related to aging. As a person gets older, the nucleus pulposus loses water content, causing the disc to become less elastic and thinner.
A more acutely painful condition is a disc herniation, often inaccurately called a “slipped disc.” The disc does not slip out of place because it is tightly anchored between the vertebrae.
A herniation occurs when the tough outer annulus fibrosus develops a tear. The soft, gel-like nucleus pulposus pushes outward through this tear, causing a bulge or extrusion. This protruding material can press against nearby spinal nerves, leading to pain or other symptoms.