At birth, the human spine is composed of 33 interlocking bones, called vertebrae, which stack upon each other at a slight angle to form the spine's S-shaped curve. This distinctive curvature helps the spine act as a shock absorber when we move while still supporting the body's weight. The vertebrae grow progressively larger and stronger from top to bottom, and by adulthood, most people have only 24 vertebrae because some fuse together.
The seven delicate cervical (neck) vertebrae support the head; the larger 12 thoracic (chest) vertebrae bear the weight of the arms and trunk; and the five thickest and sturdiest lumbar (lower back) vertebrae carry the weight of the entire body. Because the lumbar region endures the greatest stress, it is the most vulnerable to strain or other problems. At the base of the spine are nine fused vertebrae that make up the sacrum (the back wall of the pelvis) and coccyx (tailbone).
Each vertebra consists of a large oval block of bone (the vertebral body), pedicles, and lamina that surround the canal that the nerves run through, facets that articulate with the vertebrae above and below, two transverse bony projections (processes) that are points of attachment of spinal muscles, and a single spinous process that provides attachments for the muscles, ligaments, and tendons that connect one vertebra to another.
Between each vertebral body and the next is a flexible pad of cartilage-like tissue. This pad, which is called an intervertebral disk, cushions the vertebrae as the body moves. The high water content of the disks makes them very elastic—as we bend, twist, and move, they can expand, contract, and then return to their original shape. Thus, the disks, working in conjunction with the interlocking facet joints, give the spine its tremendous flexibility.
Yet another crucial function of the spine is to form a protective shell around the delicate spinal cord. At the rear of each vertebral body is a hole; when the vertebrae are stacked upon each other, these holes create a channel known as the spinal canal, through which the spinal cord runs. Spinal nerves, which branch off from the spinal cord through spaces (foramina) between adjacent vertebrae, extend to all parts of the body. The spinal cord ends at about waist level, dividing into a bundle of nerves that continues downward— the cauda equina (Latin for horse's tail, since its structure resembles the tail of a horse). Because of this intricate interweaving of bone and nerve tissue, problems with the vertebrae can cause symptoms from pressure on the spinal nerves or the spinal cord.
Originally published in The Johns Hopkins White Papers: Back Pain and Osteoporosis (2011)
Lee H. Riley III, M.D.
Chief, Orthopaedic Spine Division, Johns Hopkins Medicine
Suzanne M. Jan de Beur, M.D.
Chief, Division of Endocrinology, Johns Hopkins Bayview Medical Center