The Normal Structure of the Spine

Spinal column

The spinal column consists of 24 individual bones (vertebra), the sacrum and the coccyx. The sacrum and coccyx are formed from vertebra but these have fused. Most vertebrae share a common basic structure but also have distinct differences. The spinal column has the shape of a double ‘S’. In the neck and lower back, the spine bends slightly backwards (‘lordosis’) and in the thoracic spine and sacrum it bends forwards (‘kyphosis’). When patients are in pain, muscles go often into spasm pulling the neck and lower back straighter. Certain other conditions can cause change in the normal shape. This can alter the way muscles and ligaments have to work causing more pain. The spine is, however, straight looking at it from the front or the back. Scoliosis is a term to describe sideways bending of the spinal column. This is common to a certain degree and patients with mild scoliosis often have no symptoms. In severe cases and in adolescence this should be managed by a specialist.

Common structure

All vertebrae have a vertebral body (corpus). This is connected to the facet joints via the pedicles on each side of the back of the vertebral body. The facet joints are connected to each other at the same level via the lamina. Where the lamina joins a spike arises (spinous process) which can be felt in the back. The body (in the front), the pedicles and facet joints (at each side) and the laminae and spinous process (at the back) form a hole. All these holes lined up from top to bottom form the spinal canal. The spinal cord runs through this canal. At each level, a nerve leaves the spine on each side (spinal nerves). As it leaves it is in close contact with the facet joints and intervertebral discs. The opening where the nerve leaves the spinal canal is called intervertebral foramen. It is in this area where the nerve can get trapped due to a slipped disc or ‘wear and tear’. The facet joints on each side of the vertebral body connect the vertebra to its neighbour above and below. Wear and tear of these joints can cause pain and reduce movement. The space between the sac containing the nerves and spinal cord (dura – see below) is called epidural space. It contains small blood vessels and fat.

Cervical spine (neck)

The vertebrae are relatively small. There are seven in total but the first two are different to all other vertebrae. Most of the movement of the neck is happening in this area. The main movement of bending the head backwards and forwards (flexion and extension) is between the head and the first vertebra (‘Atlas’). Most of the rotation (looking to the side) is between the first and second vertebra (Axis). A fusion involving these structures will cause significant limitations in the range of movement of the neck. On each side of the upper six vertebral bodies in the neck runs an artery (vertebral artery). This artery supplies part of the brain and injury during surgery can be serious. As there are two of them, injury of one will not cause any symptoms but injury to both (which is extremely uncommon in any surgery!) can be life threatening.

Thoracic spine (upper back)

The main difference between the vertebrae in the thoracic spine and all others is the connection to the rib cage. This makes the thoracic spine very stiff. In front of the thoracic spine are the lungs, the heart with its big vessels. An operation to the front of the spine in this region will therefore require opening of the chest (‘thoracotomy’).

Lumbar spine (lower back)

The vertebrae are large as they have to carry a lot of weight. Strong muscles are attached from all sides which help to move the whole trunk of the body and keep it upright when walking. The pedicles (connections between vertebral body and facet joints are also large. Modern surgical fusion techniques often involve the insertion of screws from the back of the spine through these pedicles into the vertebral body. Because of the weight of the upper body resting on the lumbar spine, this part is particularly prone to wear and tear.

Intervertebral disc (‘disc’)

Discs consist of a fibrous ring (annulus fibrous) and a soft, jelly-like core (nucleus pulposus). The outer fibres are connected to the bone (vertebra) above and below and hold the disc together. The inner core is made of special cartilage cells and has a very poor blood supply. The disc acts like a cushion or shock-absorber between the vertebrae. This allows the spine to move in all directions. A disc protrusion (slipped disc) is usually caused by a rupture of the ring and the jelly protruding into the canal causing pressure on a nearby nerve.

Muscles and Ligaments

The spine is the central support structure of the body. It has to be strong and rigid but at the same time able to move in all direction. Many ligaments (band like structures) connect various parts of the vertebra to others. They dampen down movement, support muscles and give the spine a firm but flexible structure. The first two vertebrae in the neck (Atlas and Axis) are hold in position by very strong and important ligaments. Injury of these ligaments because of accident or certain medical conditions (rheumatoid arthritis, infections like TB) can cause instability leading to potentially disabling or life threatening pressure on the spinal cord. The muscles in the spine are in a complex arrangement in particular in the neck. They are responsible to move the neck in all directions. Important is especially the co-ordination between the muscles. Following injury or surgery the muscles can cause pain by going into spasm. Improving strength and co-ordination of spine muscles is an important part of physiotherapy. There are many muscles which connect to the spine but their main action is to move other parts of the skeleton, most importantly the head and upper and lower limbs. The neck muscles are connected to the back of the neck and patients with neck pain caused by muscle spasm often experience headache, starting in the back of the head and then going to the front.

Spinal cord and Nerves

The spinal cord runs leaves the head through an opening in the base (foramen magnum – Latin for big hole). It runs down the spinal canal (see above). The spinal cord usually ends at the level of the lower thoracic / upper lumbar spine. The end is called conus medularis. From this point only nerve rootlets run in the canal until they exit. This part from the conus to the lowest part of the spine (sacrum) is called cauda equina (Latin for horse tail). It contains the nerves to the legs and feet as well as  the muscles controlling bowel and bladder emptying. Compression of these nerves, for example by a large slipped disc, can cause the so-called cauda equina syndrome which is an emergency requiring urgent surgery. A lumbar puncture can only be done safely in the lower back as there is danger to injure the spinal cord if done above the conus.

Within the spinal cord, the nerve bundles (called tracts) for muscle supply (motor tracts) and various types of sensation like pain, temperature, joint position, pin prick (sensory tracts) run separately. Injury to different parts of the spinal cord can therefore lead to different clinical pictures. A complete injury to the spinal cord will lead to complete paralysis and no sensation below the level of injury. The nerves and spinal cord are within a sac (dural sac). This sac is filled with cerebrospinal fluid (CSF) which looks like water. The CSF flows freely between the brain and spine. A adult brain produces around 500ml (1pint) of CSF per day which is then reabsorbed again.