The human spine is a complex structure that provides both mobility (so you can bend and twist) and stability (so you can remain upright all day). The normal spine has an "S"-like curve when looked at from the side. This curvature allows for even distribution of weight. The "S" curve helps a healthy spine withstand stress. Even though the lower portion of the spine holds most of the body's weight, each segment relies upon the strength of the others to function properly. Ultimately, this interdependence among all sections of the spine, plus the competing demands of mobility and stability make the spine vulnerable to injury and deterioration due to aging.
Made up of 33 bones called vertebrae, the spine features three natural curves, the cervical (neck) curve, the thoracic (middle back) curve, and the lumbar (lower back) curve. In a normal spine, the cervical and lumbar sections curve forward, while the thoracic section curves backward. The cervical spine is the top 7 vertebrae in the neck area. These are smaller bones that allow turning the head freely, while the rest of the back remains stationary. The thoracic spine is made up of the 12 vertebrae in the upper back, and each thoracic vertebra is attached to a rib. The lumbar spine is made up of the next 5 vertebrae. The lower back usually receives the most stress and strain, so most spine problems occur in the lumbar region. Finally, below the lumbar region are 5 fused vertebrae of the sacrum and the 5 fused vertebrae of the coccyx.
A facet joint joins each pair of vertebra (i.e., the one above to the one below). Like hinges, the facet joints guide the movement of the spine, while also stabilizing the vertebral column. Ideally, the joints in the spine are lined up so that the back can twist and bend with little friction between the vertebrae. Between each pair of vertebrae lies a flat, circular inter-vertebral disc. The outer part of the disc, the annulus, is strong and hard. The inner portion, the nucleus pulposus, is soft and absorbs shocks to the spine during movement.
Two long ligaments run the length of the spine and help hold the vertebrae together, along with the smaller ligaments between each vertebra. The extensors and flexors are the two main muscle groups that affect the back. The extensors are small muscles in the back that span two or three vertebrae and allow the body to straighten up and lift objects. The flexor muscles, which include the abdominal muscles, are in the front and allow for bending forward as well as providing back support.
The vertebrae surround and protect the spinal cord, a column of nerves running down from the brain. Peripheral nerves branch off from the spinal cord and, with their roots passing through the vertebrae, extend all over the body. As a result, pain from a back problem may also travel to other parts of the body.