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The Spine / The Spinal Disc
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 There are 23 intervertebral (inter = between, vertebral = of a vertebra), disks, one found between each pair of vertebrae below the first cervical, or neck (atlas), and above the second sacral (just above the tailbone). The lumbar (lower back) disks are thickest, the thoracic (chest or upper back) thinnest, and the cervical are of intermediate size. These differences are associated with the function of the disks. In general these disks have two functions: to allow movement between pairs of vertebrae and to act as buffers against the shocks caused by running, jumping, and other stresses applied to the spine. Each of these functions merits separate consideration.
 The intervertebral discs make up one fourth of the spinal column's length. As these discs degenerate, it is therefore possible that a person can lose a substantial amount of height. There are no discs between the atlas (C1) and axis (C2), and the coccygeal segments. Discs are not vascular and therefore depend on the end plates to diffuse needed nutrients. The cartilaginous layers of the end plates anchor the discs in place.
Discs are actually composed of two parts: a tough outer core called the annulus fibrosus and a soft inner core called the nucleus pulposus. The nucleus pulposus's configuration has been likened to a jelly doughnut (see Figure).
At birth, eighty percent of the disc is composed of water. With age, the discs dehydrate and become stiffer. This is a natural aging process, although in some individuals, as the disc degenerates it can become painful. The most likely reason for this is that the degeneration can produce micromotion instability and the inflammatory proteins (the soft inner core of the disc) probably leak out of the disc space and inflame the well innervated structures next to the disc (e.g. nerve roots).
Sometimes a twisting injury damages the disc and starts a cascade of events that leads to degeneration (see Figure to left). The disc itself has very few nerve endings and no blood supply. Without a blood supply the disc does not have a way to repair itself, and pain created by the damaged disc can last for years. In general, as we age there are less inflammatory proteins in the disc space and discogenic pain rarely occurs after 60 years of age.
If an intervertebral disk were the only joint between a pair of vertebrae, then one of these could move on the other in any direction; but each pair of vertebrae with an intervertebral disk has also a pair of synovial joints, one on each side of the vertebral (neural) arch. These joints limit the kinds of independent movement possible so that the thoracic vertebrae move in only two directions, and the lumbar in only three; only the cervical vertebrae below the atlas have full freedom of movement.
 All intervertebral disks allow approximation and separation of their adjacent vertebrae. This is caused partly by movement brought about by muscle action and partly by the weight of the head and the trunk transmitted to the pelvis when a person is upright. The effect of weight is of special importance. The mucoid substance in the centre of the disk behaves like a fluid. It is acted upon by the person's weight and any other pressure forces transmitted along the spine. Hence the disk flattens from above downward and expands in all other directions. After arising in the morning, and as the day progresses, a person decreases in height because of this compression of the disks. An average decrease of one millimetre in the height of each disk would mean an overall shortening of 2.3 centimetres, or about an inch. The spine lengthens again, of course, during sleep.
In the infant the greater part of the disk consists of the soft centre. Later the fibrous ring becomes relatively thicker in such a way that the soft part is nearer to the back of the disk. As middle age approaches, there is an increase in the fibrous element, the soft centre is reduced in size, and the amount of cartilage is increased. There is a tendency for the posterior part of the fibrous ring to degenerate, so that a sudden violent pressure may rupture the disk and allow the central part to protrude backward against the spinal cord. This condition is commonly referred to as slipped disk.
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