The spine, or backbone, is made of 33 individual bones called vertebrae. It provides the main support for the body, allowing the person to stand upright, bend, and twist, while protecting the spinal cord from injury. A healthy spine provides strength, is flexible, and allows movement in several directions. Strong bones and muscles, flexible tendons and ligaments, and sensitive nerves contribute to the overall health of the spine. Any of these structures affected by strain, injury, or disease can cause pain and disability.
At birth, a baby’s spine is C-shaped. As the child develops and learns to crawl and then walk, the spine gradually changes into an S-shape. Viewed from the side, the cervical (neck) and lumbar (low back) regions of the spine have a lordotic, or concave, curve. In contrast, the thoracic (mid-back) and sacral (buttock) regions have a kyphotic, or convex, curve. These spinal curves work like a coiled spring to absorb shock, maintain balance, and allow motion.
Abnormal curvatures of the spine can cause various spinal deformities which may go on to cause significant pain and disability. An abnormal curve of the thoracic spine is called kyphosis, or hunchback deformity. When seen from the front or the back, a normal spine appears straight. Sometimes the spine curves side-to-side, and this condition is called scoliosis.
The vertebrae are the 33 individual bones that are stacked up on top of each other to form the spinal column. The vertebrae are numbered and divided into regions: 7 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 4 coccygeal. Only the top 24 vertebrae can move; the vertebrae of the sacrum and coccyx are fused and do not move.
- cervical – The main function of the cervical spine is to protect the brainstem and the spinal cord and to support the weight of the head (10-15 pounds). The cervical vertebrae are numbered C1 to C7 and allow the greatest range of motion.
- Thoracic – The main function of the thoracic spine is to protect the organs of the chest by providing attachment for the rib cage. The thoracic vertebrae are numbered T1 to T12 and have limited range of motion.
- Lumbar – The main function of the lumbar spine is to bear the weight of the body. The lumbar vertebrae are numbered L1 to L5 and are much larger in size than the vertebrae in the cervical or thoracic region. The lower back is prone to injury because it bears the stress of most of the motion such as sitting/standing, pushing/pulling, and lifting weights.
- Sacral – The main function of the sacrum is to provide attachment for the hip bones and protect pelvic organs. The five sacral vertebrae are fused together and have no motion.
- Coccyx – The four fused vertebrae of the coccyx, or tailbone, do not really have a function.
Each of the 24 moveable vertebrae in the spine are separated and cushioned by an intervertebral disc. These discs are designed like radial car tires. The outer ring, called the annulus fibrosus, has criss-crossing tough fibrous bands, much like a tire tread. These bands attach to the bodies of each vertebra and contain the gel-filled center called the nucleus pulposus, much like a tire tube.
Discs function like coiled springs. The criss-crossing fibers of the annulus fibrosus pull the vertebral bodies together against the elastic resistance of the gel-filled nucleus pulposus, which acts like a ball-bearing and allows the vertebral bodies to roll over the incompressible gel. The gel-filled nucleus is composed mostly of fluid. The fluid is absorbed during the night when a person lies down and is pushed out during the day with upright stance and motion.
With age, the discs increasingly lose the ability to reabsorb fluid and become brittle and flat. This is the main reason why people get shorter as they grow older. Also, diseases like osteoarthritis and osteoporosis cause formation of bone spurs. Injury to the disc can result in intervertebral disc herniation, a condition in which the gel-like nucleus pulposus is pushed out through the tough annulus fibrosus and compresses the nerve root(s). This, in turn, causes back and/or leg pain.
Vertebral Arch and Spinal Canal
On the back of each vertebral body are bony projections that form the vertebral arch. The arch is made of two pedicles (“pillars”) and two laminae (“roof”) and, along with the vertebral body in the front, it forms the hollow spinal canal that contains the spinal cord.
There are four facet joints in each vertebra, one pair that connects to the vertebra above (superior facets) and another pair that connects to the vertebra below (inferior facets). These facet joints allow back motion.
The spinal cord is the continuation of brain and it runs within the protective spinal canal from the brainstem to the first lumbar vertebra. At the end of the spinal cord, the cord fibers separate into the cauda equina and continue down through the spinal canal until they exit the spine as nerve roots and travel to the legs and feet. The spinal cord serves as a conduit, relaying messages between the brain and the body. The brain sends motor messages to the limbs and body through the spinal cord allowing for movement. The limbs and body send sensory messages to the brain through the spinal cord about what we feel and touch.
Any damage to the spinal cord can result in a loss of sensory and motor function below the level of injury. For example, an injury to the thoracic or lumbar area may cause motor and sensory loss of the legs and trunk (called paraplegia). An injury to the cervical (neck) area may cause sensory and motor loss of the arms and legs (called tetraplegia or quadriplegia).
As described above, the spinal cord forms a collection of nerve fibers at the end called the cauda equina, or horse’s tail. If these nerve fibers become damaged, motor and sensory function to the bladder and legs can be affected, and this is called cauda equina syndrome. Cauda equina syndrome is a rare condition that occurs as a result of trauma or complications related to disc herniation. This is a medical emergency that requires immediate attention and treatment as the loss of function can be permanent if left untreated.
Thirty-one pairs of spinal nerves branch off the spinal cord. These nerves carry messages back and forth between the body and the spinal cord to control sensation and movement. The spinal nerves are numbered according to the vertebrae above which it exits the spinal canal. The 8 cervical spinal nerves are C1 through C8, the 12 thoracic spinal nerves are T1 through T12, the 5 lumbar spinal nerves are L1 through L5, and the 5 sacral spinal nerves are S1 through S5. There is 1 coccygeal nerve.
The spinal nerves innervate specific areas and form a striped pattern across the body called dermatomes.