The spine or spinal column (commonly called the backbone) is that part of the skeleton that runs from the skull above to the pelvis below. It is divided into five regions from top to bottom: the cervical spine in the neck, the thoracic spine corresponding to where the ribs arise, the lumbar spine in the low back, the sacrum at the base of the column and in between the two halves of the pelvis, and the coccyx hanging off the end of the sacrum (essentially all that remains in humans of what would be a tail in other species).
The spinal column is made up of a number of individual bones called vertebra which stack up one on top of the other and are joined together by ligaments, muscles, and a specialised cartilage called the intervertebral disc. The disc functions to tether one vertebra to the next but also as a shock absorber for the motion that occurs between neighbouring bones. Each vertebra has a large hole in its middle which, when joined up with that of the neighbouring vertebrae, together form a tube up and down the spinal column which is called the spinal canal.
There are usually seven vertebrae in the cervical spine, twelve in the thoracic, and five in the lumbar region. Those in the sacrum have fused together so that the original five here form one complete bone. The coccyx is rudimentary in humans and can consist of three to five bones. There can be some variations of “normal” here with so-called “segmentation anomalies” (for example, the top sacral vertebra may not be joined to the rest giving the appearance of six lumbar vertebrae). The presence of a segmentation anomaly is of no functional concern to an individual patient other than that the treating surgeon needs to recognise it when making certain diagnoses and planning treatment.
Looking from the front the normal human spine is usually very straight, although looking from the side there are three broad curves with the cervical spine in the neck being curved gently backwards (so called “lordosis”), the thoracic spine curved forwards (“kyphosis”), and the lumbar spine again curved backwards into lordosis. These curves balance each other out so that in the young adult spine the skull is position vertically over the pelvis. With aging we commonly see the head seeming to creep forward in front of the pelvis often due to “loss of lordosis” in the cervical or lumbar regions or “exaggerated kyphosis” in the thoracic spine. Although this can be a normal part of aging certain diseases, injuries, or degenerative process may lead to abnormal curves resulting in “loss of spinal balance” that may require treatment.
The pattern of vertebrae and intervertebral discs repeats throughout the vertebral column with each vertebra being named by region and number. The first vertebra in the lumbar region therefore is called “L1”, the fifth in the thoracic region T5, the third in the cervical region C3, and so on. Each intervertebral disc is named by reference to its two vertebral neighbours, so that the disc in between L4 and L5 is called the L4/5 disc, that in between L5 and S1 is called the L5/S1 disc (or less commonly the “lumbosacral disc”), that between C6 and C7 the C6/7 disc, etc. Since the sacrum is one complete bone there are no sacral discs, and the top two vertebrae in the neck (C1 and C2) are also very specialised to allow the high degree of movement between head and neck so that there are no discs here either. The highest disc level therefore is the C2/3 disc connecting C2 to C3, and the lowest is the lumbosacral disc at L5/S1.
Between each pair of vertebrae a spinal nerve (also called the nerve root) will exit from the spinal column. These are named from top to bottom so that the C1 nerve is the first to leave below the skull and above C1, then C2 coming out above C2, the C3 nerve above the C3 vertebra, etc. Things change a little at the junction between the cervical and thoracic spine at the base of the neck with the C7 nerve exiting above C7 as usual but then the C8 nerve coming out below the C7 vertebra (there are only seven bones but eight nerves in the cervical spinal column). The pattern is then consistent down the rest of the spine with each nerve leaving below its equally named vertebra so that the T1 nerve comes out below T1, T2 below T2, and so on all the way down to and through the sacrum. A spinal segment corresponding to one vertebra, its neighbour, their intervening intervertebral disc and the exiting spinal nerve is known as a spinal “level”.
Each spinal nerve will eventually look after a specific region of the body. That area where it supplies sensation is referred to as a “dermatome”, whereas that region where it controls muscle movement or motor control is called a “myotome”. Some of the spinal nerves (although not all) will also supply the so-called deep tendon reflexes such as the knee jerk reflex that a doctor may try to elicit with a tendon hammer. The importance here is that with careful assessment of sensory, motor, and reflex symptoms and signs the neurosurgeon can often identify which spinal level may be the site of a problem.