The spinal cord is a column of nerve cells and bundles of nerve fibers which connects the brain to all parts of the body. The spinal cord itself does not descend the entire length of the spinal canal. It ends at about the level of the second lumbar vertebra. Below this level, the remaining spinal nerves descend as a group of nerves resembling a horses mane, hence their name the "cauda equina". The spinal cord gives off spinal nerves in pairs: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. The spinal cord and nerves are surrounded by a sac called the dura mater which contains them in a bath of spinal fluid. Outside the dura, between it and the bony and ligamentous walls of the spinal canal, is the epidural space. It is in this space that epidural injections are given, including cortisone (steroid) injections to treat spinal disorders or novocaine for regional anesthesia for surgery or to deliver a baby.
Figure 1: Anatomy of a spinal motion segment: two vertebrae, one intervertebral disc, and ligaments
Artificial functional spinal units have been used in research and tested for mobility and strength under various conditions of fatigue and weight to understand better the stresses the human spine sustains under normal and diseased states. Your doctor can explain why more pressure is placed on the nucleus of the spinal disc when sitting as compared to standing or lying down. Or why support to the lumbar region when sitting reduces pressure on the disc and may be a good habit to apply.
Figure 2: spondylolisthesis
Each individual motion segment has a normal range of motion, the limits of which help contribute to the stability of the spine. When one or more parts of a motion segment are affected by disease, trauma, or degeneration, the tissues do not interact normally. The result may be pain, limited motion, or instability. Motion segment laxity results in an increase in vertebral column motion (instability) when the spine is under stress. The quality of spinal motion becomes diminished and you may feel unstable when bending or pushing and lifting an object. Because many tissues and factors contribute to motion segment stability, a balance among factors exists that, if attended to and managed by therapeutic exercises, limit abnormal motion segment motion and relieve pain. Early treatment may slow or prevent more advanced disease in many cases. Degenerative spondylolisthesis can result from severe laxity of a spinal motion segment due to degenerative changes in the facet joints. These joints are no longer able to keep the vertebra to which they belong from slipping forward and past the vertebra below.
[Figure 2 ]