Anatomy

Bones           

The bony spine is made of stacked bones called vertebrae. This column of vertebrae provides structure to the entire trunk and protects the spinal cord. Vertebrae are grouped together by their location, with the vertebrae nearest the head called cervical vertebrae, followed by thoracic, lumbar, sacral, and finally, coccygeal vertebrae nearest the feet. The lumbar vertebrae are the thickest and have the greatest circumference in the spine. Vertebrae are named by section and numbered from top to bottom. For example, the lumbar vertebra nearest the head is called L1, the second is called L2, and so on. The sacral vertebrae are unique, because they are fused together to make one large, triangular bone.

Nerves

The spinal cord is surrounded and protected by the column of vertebrae. The spinal cord is the largest nerve bundle of the body. It starts from brain stem and ends as nerve roots of the lumbar spine. The cauda equina, or horse's tail of nerves, trails below the spinal cord. The nerves involved in sciatica become part of the cauda equina and leave the spinal column in the lumbar and sacral areas of the spine. Two nerves exit at the same level on either side of the spine, one descending to the right and the other to the left. Nerves in the lumbar region are named for the vertebra directly above the nerve as it exits the spinal column (e.g., the nerve exiting below the third lumbar vertebrae is called "L3.") The sacral nerves exit the spine through paired foramina in the sacrum.

The nerve roots travel a short distance until they join in a complex intersection called the lumbosacral plexus and the sacral plexus. From there, the sciatic nerve emerges to travel down to the buttock and back of the thigh. At the knee, the sciatic nerve divides into two smaller nerves, called the peroneal and tibial nerves, which serve the calf, shin, and foot. Because of these interconnections, irritation of a nerve anywhere along its course, whether in the spine or in the deep buttock, can cause leg pain and tingling (sciatica).

Each nerve arising from the lumbar and sacral spine carries motor fibers that relay messages from the brain, down the spinal cord, and to muscles, causing them to contract and relax. They also carry sensory fibers that relay sensory messages from the skin to the spinal cord and ultimately to the brain. When nerves are irritated in sciatica, these messages may be interrupted or false messages may be sent, causing pain, numbness, tingling, and weakness.

Discs

Between 2 vertebrae lies an intervertebral disc, made of layers of collagen surrounding a "jelly-like" center. The layers of collagen are called lamellae and together form the annulus fibrosus. The jelly center is called the nucleus pulposus. In a young healthy disc, the nucleus pulposus is 88% water. Like the hydraulic system of your car, the intervertebral discs absorb shock that daily activities exert on the spine. As discs age, or because of injury, the collagen fibers are replaced by fibrous tissue, resulting in decreased elasticity and shock absorption. The annulus fibrosus may sustain small tears. In old age, the disc is dry and frayed.

Muscles and Ligaments

The movements of the spine are controlled by the muscles and ligaments that surround the bony structures. The most superficial (or the outermost layer) of these muscles run the length of the spine from top to bottom and work to maintain an erect posture. Deep to these muscles lie a group of shorter muscles that run parallel or diagonal to the spine. This group of muscles maintains erect posture, stabilizes, and rotates the spine. The muscles of the deepest group are much smaller and help to maintain posture and act to exert small, precise forces on the spine at each level. Muscles are the most important stabilizer of the spine.