nervous system

nervous system There are probably more than 100 000 million nerve cells in the body. The nervous system is the sum total of all these, together with their nerve fibres, which ramify throughout the body, and the various supporting components of nervous tissue.

The nervous system is subdivided into the central nervous system (CNS) and the peripheral nervous system (PNS). Basically, the brain and spinal cord form the CNS, while the rest is PNS. The CNS is well protected inside the skull and vertebral column. The PNS is essentially the nerves, which run through most of the tissues of the body. The function of the nervous system is to collect information from the body and the outside world, through the sense organs, to process it in the CNS, and to distribute relevant commands to the muscles and glands throughout the body.

Like other body tissues, the nervous system is composed of cells, similar in general form to other cells in the body, but with some important modifications. One might imagine that nervous tissue consists of nerve cells and very little else. In fact a multitude of other components are essential to proper functioning of the nervous system, and form an integral part of it. Still, the most important cells of the nervous system are the nerve cells (neurons). Their most distinctive feature is their thin processes, called fibres or axons, which transmit impulses (action potentials) and which contact muscles or glands, or, in most cases, other nerve cells. So the nervous system can be looked upon as an enormous series of ‘chains’ or circuits of neurons, each receiving excitatory and inhibitory messages from other neurons, and each sending impulses along its axon if the balance of incoming signals is in favour of excitation. A typical neuron in the brain may receive 10 000 terminals from incoming axons.

Many other cell types are necessary to support the neurons. Blood vessels supply blood to the nervous tissue and drain it away into the major veins. A large percentage of the human race will die from diseases associated with cerebral blood vessels, while many more people will be permanently handicapped, especially by stroke (blockage or rupture of blood vessels).

The most important — certainly the most numerous — other supporting cells in the nervous system are the glial cells, or glia (from the Greek for glue). Amazingly, there are about 10 times as many glia as neurons in the nervous system. The most distinctive glial cells in the PNS are the Schwann cells, which wrap themselves around peripheral nerves to produce the fatty, insulating sheath called myelin. In the CNS various types of glial cells are involved in myelination, the transfer of nutrients from capillaries to neurons, and are also components of the defence system of the CNS, protecting against infection and helping remove degenerated neurons.

In the PNS, groups of, usually, a few hundred axons form bundles, several of which are united into a nerve trunk. Individual axons are well protected and peripheral nerves are fairly flexible. They even stretch somewhat, which is necessary if they run near a limb joint, or when a surgeon wishes to suture together two divided nerve stumps. Larger nerves have their own tiny blood vessels.

The nervous system also includes the special sense organs (eyes, ears, etc.) and sensory axons throughout the body. The essential feature of a sense organ is the specialized neurons, called receptor cells, whose membranes include molecular mechanisms for detecting particular events outside the cell (such as the presence of particular chemicals, or light, or pressure on the membrane). Receptor cells ‘transduce’ the energy of these events into electrical changes inside the cell, which eventually produce a set of nerve impulses that race along axons towards the CNS. In the skin there are free nerve endings, specialized to signal touch, pain, and temperature. Muscle spindles and Golgi tendon organs are receptor organs found inside skeletal muscle, which are stimulated by stretch of the muscle or tension on the tendon. They help inform the CNS about the state of activity of the muscles and therefore the position and balance of the body. Such information can either be conscious (involving signals reaching the cerebral cortex of the brain) or unconscious (being used for example in spinal reflexes).

In the special sense organs, such as the eye and the ear, highly specialized receptors respond to light and sound. Sensory information also comes from the viscera and blood vessels. Although viscera can produce conscious sensation, such as pain when they are distended, visceral sensation is mainly used unconsciously by the autonomic nervous system.

The autonomic nervous system has both central and peripheral components. It is concerned with the automatic control of bodily function. It is subdivided into sympathetic and parasympathetic portions. To some extent, these two systems have opposing actions. For instance, sympathetic activity classically prepares for ‘fight or flight’, raising blood pressure and heart rate, facilitating breathing, dilating the pupils, and deviating blood from the skin and gastrointestinal tract to skeletal muscles. Parasympathetic activity, in contrast, adapts the body for rest and digestion. Cell bodies of sympathetic neurons are in the middle levels of the spinal cord. Their axons leave the cord and end on nerve cells in the sympathetic trunk, a long nerve tract beside the vertebral column. Thence the axons of these relaying nerve cells join, and are distributed with, other nerves of the peripheral system, to reach glands and blood vessels in all parts of the body (except the CNS itself). Others run to the eyes, to the heart and lungs, and to the abdominal and pelvic organs. Parasympathetic cell bodies are in the brain stem, with axons running in the tenth cranial (vagus) nerves, reaching glands around the mouth and throat, and extending down to the heart and lungs and to most of the abdominal organs. There is a second set of parasympathetic nerve cells in the lowest segments of the spinal cord, that send out fibres to the pelvic organs.

Thus the nervous system is responsible for rapid conduction of information throughout the body. Neurons are highly differentiated and, except in early fetal life, are generally incapable of division or mitosis to reproduce themselves. This means that if lost through disease or injury they cannot be replaced. On the other hand, axons regenerate readily in the PNS (as anyone who has cut a cutaneous nerve knows). One of the most important goals of neuroscience research in the years to come will be to understand why this is, and whether damaged neurons in the CNS can be persuaded to repair themselves.

Illustration

Laurence Garey

See also autonomic nervous system; brain; central nervous system; nerves; neurotransmitter; synapse.