PAIN AND SENSORY NERVE FIBRES
Sensory nerve fibres originate from clusters of cells that lie close to the spine, with one cluster or 'ganglion' for each vertebra. A special ganglion lies in the base of the skull and supplies the face, mouth and head. In the embryo, each cell puts out a short fibre which splits at a T-junction. One arm grows out into the tissue by way of the nerves. The other arm grows into the spinal cord with a large group of similar nerve fibres called the dorsal root, which contains all the fibres from the ganglion. The skin is profusely innervated with three types of sensory fibres. One group, called A beta fibres, are wrapped in a fatty protein called myelin and are sensitive to gentle pressure. The second group, called A delta fibres, are thinner and are sensitive to heavy pressure and temperature. The third group, called C fibres, are very thin and have no myelin, and respond to pressure, chemicals and temperature. Deep tissue and organs such as the heart, bladder and gut are innervated only by the thinner fibres.
Sensory nerve fibres detect events occurring at their ends in the tissue and signal them to the spinal cord by two methods. One method is by the production of nerve impulses, which are abrupt events in the membrane of the fibre that sweep over the fibre from its peripheral end all the way to its central end in the spinal cord. These impulses travel along the fibre like fire in a fuse but, unlike a fuse, the surface of the fibre rebuilds itself after the fire has passed. The impulse lasts only one thousandth of a second and travels at between one and a hundred metres per second depending on the thickness of the fibre. Some of our sensory nerve fibres are more than a metre long, running from the toes to the middle of the back; others are only a few centimetres long, running from the teeth to the hind brain.
The second method of sensory nerve communication is much slower. The ends of nerve fibres in the tissue, particularly the thin C fibres, absorb chemicals from the tissue and slowly transport these chemicals all the way to the cell bodies in the ganglia and on to the central terminals. In this way, the sensory nerves have two ways of informing the central nervous system about changes in the tissue in which the sensory nerves terminate. One way is by volleys of nerve impulses which bombard the central cells on which they end. The other way is by the slow transport of chemicals, which change the action of cell bodies in the ganglia and the central terminals. This transported signal takes a few hours to reach the ganglion cells if the injury is very close, and many days if the damage is as distant as the foot.
Cells in the damaged tissue
Each part of the body is made up of a collection of different types of cell. Some of these are special to that region, be it skin, heart or muscle, and others are common to all types of tissue, such as the cells that make the tendons and the fat. Some of these cells are very fragile, whereas others are tough, resisting anything short of devastation. A mild skin bum leaves the general structure intact while some cells have fallen to bits. Obviously, no cell can withstand a direct hit from a bullet.
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