Anatomy Of The Spine 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 1 / 6
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. 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, 2 / 6
the internal anal sphincter and provide erectile function. {japopup type="youtube" content="http://www.youtube.com/v/advbavblqum?hl=en&fs=1" width="640" height="380" title="cervical Spine and Disc Anatomy"} {/japopup} The spinal nerve roots are numbered very precisely. The first cervical spinal nerve root emerges between the occiput and the back of the C1 vertebra. This is a very small sensory nerve, which does not appear to have much in the way of representation on the skin surface. The next nerve root is much larger and emerges between the lamina of the C1 vertebra and C2. This is the C2 nerve root, which goes on to become the major constituent of the occipital nerves. Emerging through the C2/C3 foramen, we have the first of the neck cervical spinal nerves C3. Because there are 7 cervical vertebrae, but 8 cervical nerves, the nerve that emerges between the 7 cervical vertebra and the first thoracic vertebra and known as C8. The first thoracic nerve emerges between the first thoracic vertebra and the second (known as the T1 root). The next twelve roots emerge serially, so that the nerve that emerges between the T12 vertebra and the L1 vertebra is the T12 nerve root. The nerve that emerges between the first and second lumbar vertebrae is known at the L1 nerve root. The nerve roots are then numbered serially until we get to the nerve that emerges between the fifth lumbar vertebra and the sacrum this is the L5 nerve root. The first sacral nerve root emerges through the first sacral foramen and so on until we get to S5. The different body structures innervated by the spinal nerve roots may be summarised as follows. The first four cervical nerve roots innervate the strap muscles of the neck and C3, 4 and 5 give off a very important nerve the phrenic nerve which innervates the diaphragm. From C5 downwards (with some contribution from C4) to T1, the spinal nerves unite to form divisions, trunks and then cords creating a structure known as the brachial plexus. This brachial plexus is a structure much beloved of anatomy examiners and testing the knowledge of this structure has been used to torture medical students for centuries. In essence, the brachial plexus is responsible for the innervation of the upper limb. This innervation is roughly serial so that the more proximal muscles (around the shoulder joint) are innervated by the upper parts of the brachial plexus, whereas the intrinsic hand muscles (the little muscles within the palm) are innervated by the first thoracic nerve (or T1). The remaining thoracic nerves innervate the segmental musculature of the trunk. When we get to the leg, we find that the innervation to the muscles comes from L2 S2. The pattern of innervation (the myotomes) is given in the table. One simple way of remembering the innervation in the lower limbs is that the leg is twice the size of the arm and so each muscle receives its innervation from at least two spinal nerves. The only exception seems to be the dorsiflexors of the foot, which are remarkably sensitive to loss of 3 / 6
function within the L5 nerve root. Stability The spinal column depends upon a number of factors for its stability. The notion of stability is one that is debated amongst practitioners of spinal medicine and surgery. However, a useful working definition would be one as follows. "A spine may be regarded as stable if it allows a normal, physiological range of motion, without neurological compromise; and for growth in the spine to occur, without spinal deformity". Pathophysiology Of The Spine - Wear and tear in Discs - Spondylolysis - Spondylolisthesis - Nerve root compromise - Spinal cord compression - Tumours - Tuberculosis (patient on ward NOW) - Epidural abscess - empyema Mechanism of damage - all ischaemia. Stops the machine and wrecks the machinery. General rule - the longer and harder the nervous system is squeezed, the worse the outcome. 4 / 6
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