The nerves can be considered the telephone wires of the body. Some carry messages from the brain to the organs and muscles of the body, telling them when to act or contract and how powerfully to do so. These are the motor or effector nerves. The sensory or receptor nerves send vital information from the periphery of the body to the brain and spinal cord. This enables us to become aware of changes in our surroundings. We can see, hear, smell and taste and also feel heat, cold, dryness, sharpness and even be aware of articulation and the positions of joints.
The nerves consist of axons, the projections of nerve cell bodies, and they conduct impulses. Single nerve fibres pass from the central nervous system (brain and spinal cord) to distant parts of the body. Each fibre, carrying a separate message, is individually wrapped in a fine insulating layer of a substance called myelin, which prevents messages from crossing over from one nerve fibre to another adjacent one. Many of these individual fibres are wrapped together to form a distinct nerve bundle, held together by a strong outer layer which forms the nerve sheet. An axon that is severed by an injury dies because it is a projection of the nerve cell body, in which the meta-bolical processes for maintainance are regulated. The axon needs the nerve cell body to survive. In consequence a severed nerve cannot grow back together again. Very slow regeneration can take place, how- ever, if the proximal part of the axon can intrude into an intact myelin sheet. For this.to occur, the two parts of a damaged nerve have to be brought very close together.
The spinal nerves arise in 31 pairs from the grey matter of the spinal cord. They emerge between the vertebrae, carrying sensory and motor impulses to and from the central nervous sytem. They branch along their course, one branch supplying the front, the other the back part of the body. This segmental configuration can be found in all living creatures. As evolution took place we have developed limbs. In the cervical and lumbar regions there are ‘swellings’ (intumescences) of the spinal cord. It is necessary that there must be more neurons supplying the limbs, and this greater quantity results in the thickening of the spinal cord at these levels.