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Spinal Column

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Anatomy Of The Spine

Spinal Column

The spine is a flexible column, composed of a stack of individual bones. Each bone is called a vertebra. There are seven vertebrae in the neck (cervical vertebrae) twelve in the thoracic region (thoracic vertebrae) and five in the lumbar region (lumbar vertebrae). At the top, the skull rests on the atlas vertebra (the first cervical vertebra). This is called the occipito-cervical junction. At the bottom, the fifth lumbar vertebra rests on the sacrum (a large triangular bone) which consists of five fused vertebral elements. The sacrum forms part of the pelvis. Below the sacrum, there is a small string of bones, the residual tail, called the coccyx. This is composed of up to six or seven segments.

The vertebrae are separated from each other by joints. At the front, there is a large fibro-cartilaginous joint, called a disc. At the back, there are facet joints. The bones are bound together by ligaments. The most important of these are the anterior longitudinal ligament, the posterior longitudinal ligament and the ligamentum flavum (see diagrams).

Down the centre of the spine there is a canal – the vertebral or spinal canal. In this canal, we find the spinal cord and spinal nerves.

The Spinal Canal

From the outside in, there are a number of layers. First, there is a layer of fat and large veins, found in the epidural space (epi means around – hence epidural means around the dura). The next layer is the dura mater. As its name implies, it is a very tough fibrous layer, which is resistant to penetration and ensheaths the nerves as they leave the spinal column. Just inside the dura, there is a layer of a thin silvery membrane, that looks a bit like a spider’s web. This is called the arachnoid. Inside the arachnoid layer, we find cerebro-spinal fluid, which is continuous with the cerebro-spinal fluid that bathes and supports the brain. Closely applied to the spinal cord and nerves is the pia mater, which in the spine, serves as a membrane that binds the spinal cord and the nerves into anatomical entities.Vertebra

The spinal nerves leave the spinal canal to the sides, via gaps between the bones known as foramina. The spinal nerves then go out into the neck, arm, trunk and limbs, bundled together as peripheral nerves. Each spinal nerve contains motor fibres, which innervate the muscles (and signal to them that the need to move). There are sensory nerves, which transmit sensation from the skin, joints and deeper organs back to the spinal cord. Then, there are sympathetic nerve fibres that run along with the peripheral nerves, which control a large number of peripheral functions, particularly sweating and the diameter of the blood vessels of the skin.

Finally, in the sacral region there are para-sympathetic nerves, which innervate the lower part of the gut (the hind gut) and the visceral structures of the pelvis, including the bladder, the rectum, the internal anal sphincter and provide erectile function.

Cervical Spine and Disc Anatomy