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Even the simplest animal - a protozoon made of a single cell - responds to a stimulus by withdrawing itself. In more advanced creatures some of the cells in the outer layer of the body are adapted to receive stimuli and transmit them to other cells adapted for movement. Cells that transmit impulses to other cells are called nerve cells or neurones. In all higher animals the neurones are gathered together to form an anatomical unit, the nervous system, derived entirely from the outermost layer of the embryo (ectoderm), which also forms the outer coat of the skin.
In the embryo of a vertebrate (such as man) the future neurones are gathered in the mid-line of the surface of the back. This strip, running the length of the embryo, is first depressed to form a groove and then submerged, as the tissues of the back close over it, to form a tube. Later this neural tube is surrounded by the skull and vertebrae, to form the brain and the spinal cord, together making the central nervous system.
A neurone consists of a cell-body with several thread-like projections or processes. There are generally several branched processes, dendrites, and a single long process, the axon, which is the main conducting fibre. An axon may be very long, extending for example, from a cell-body in the spinal cord at the bottom of the back, to a muscle in the foot. Similarly, sensory processes extend from the periphery to their parent cells in the nerve roots (spinal nerves). The nerves of the body are simply bundles of conducting fibres, each with its parent cell in the central nervous system. Each fibre has a coat of myelin, a fatty insulating substance The myelin sheath accelerates conduction and is essential to repair if the fibre is damaged. Fibres within the central nervous system have rudimentary myelin sheaths and no power of healing.
Twelve pairs of cranial nerves spring from the under-surface of the brain and 31 pairs of spinal nerves from the spinal cord. Their parent cells are only the outer links of complex chains of neurones within the central nervous system. A group of muscle cells is activated by a single motor neurone in the spinal cord, but this neurone may receive impulses through its dendrites from hundreds of other neurones, of which some stimulate and others inhibit. Only a few of these, in the upper part of the brain, are involved in conscious activity or sensation. The rest operate reflex actions.
Herophilus (Alexandria, 3rd century BC) recognized the nerves had to do with sensation and movement, and he described the brain in some detail. His contemporary Erasistratus taught that the nerves were hollow and acted by transmitting vital spirit, a belief that was not seriously challenged until late in the Renaissance. Vital spirit flowed through nerves into the muscles and caused them to swell and shorten. Two thousand years later Descartes was expounding the same theory. He even described and drew valves within the hollow of the nerves. Swammerdam disproved the theory in the 17th century, but his work was not published until 1737. The first modern account of the nervous system was by Albrecht von Haller in 1766. Having shown the tendency of muscle fibres to contract in response to stimuli, Hailer proved that some nerve fibres carried impulses from the central nervous system which stimulated the muscles, while others carried sensory impulses to the brain. Although the idea that nerves might be sensory or motor was very ancient - it was put forward by Herophilus and elaborated by Galen (2nd century AD) - sensation as a property of the brain and not of the organ stimulated was a revolutionary concept. A pricked finger does not feel pain; it only transmits an impulse along a nerve to be interpreted in the brain.
The nervous system functions largely by reflex action. Descartes was the first to suggest this. It was a lucky guess with no shred of evidence behind it. Descartes was concerned not with the workings of the body but with the human soul. Having denied the existence of a soul in animals he had to postulate something like reflex action to explain animal behaviour. Descartes's theory was confirmed by the experiments of Hales, published in 1761, showing that the withdrawal of a limb when the skin was pricked was determined by the spinal cord. Marshall Hall (1833) described spinal reflexes and their modification by impulses from the brain. The current view of the nervous system as an integrated whole is largely due to Sherrington's work around the turn of this century.
Johannes Müller's discovery (c. 1830) that a given nerve fibre, however stimulated, can produce only one kind of effect was the first step towards understanding the nature of nervous impulses, though it now appears that there are exceptions to Müller's rule. In 1852 Helmholtz recorded the speed of conduction in a nerve. Later knowledge is due largely to Lucas, Sherrington, Dale, Adrian, Hodgkin, and Huxley in England, and to Erlanger and Gasser in America. Excepting Lucas, all these physiologists have been awarded Nobel Prizes.
In all vertebrates, a special system of nerves regulates the organs of bloodcirculation, respiration, digestion, excretion, and reproduction. Since its activity is wholly reflex and appears to be independent of the brain, it was named the autonomic system by J. N. Langley (Cambridge, 1898). Functionally the system is in two parts: the sympathetic system, arising from the thoracic and upper lumbar spinal nerves, and the parasympathetic system from certain cranial nerves of which the most important is the vagus (controls the heart), and from the sacral nerves. The autonomic motor nerve cells are in groups or ganglia outside the central nervous System. The sympathetic ganglia form a chain at either side of the backbone, and the parasympathetic ganglia are actually on the organs supplied.
When the neural tube is formed in the embryo (see above) some of the developing neurones are stranded between the tube and the back of the embryo. They form the neural crest. These neurones migrate, some to become the autonomic nerves and the medulla of the adrenal gland (which is a modified sympathetic ganglion) and others to the sensory ganglia of the spinal nerves. Despite appearances the workings of the autonomic and central nervous systems are intimately related. Autonomic activity is determined partly by the vital centres in the brain-stem and partly by centres in the floor of the forebrain which are also concerned with emotion the outward signs of emotional change such as blushing, pallor, sweating, palpitation, are autonomic reflexes.
Galen described the vagus nerve as part of a single nerve comprising the 9th, 1Oth (vagus) and 11th cranial nerves. Error is to his credit because it proves that he had studied the matter: the three nerves are quite distinct in the neck, but if they are traced back to their origin in the brain-stem they are seen to be intimately related. Galen also gave some account of the sympathetic nerves and their ganglia, but the first accurate description of this system was by Eustachius in the 16th century. Numerous physiologists, notably Claude Bernard, studied the autonomic system during the 19th century; the first to recognize it as a co-ordinated system was W H. Gaskell of Cambridge, who named it the involuntary nervous system. More recent studies have been mainly concerned with the connections of the system in the brain, where autonomic and central nervous systems are no longer distinct.
A living nerve fibre at rest is electrically charged. The potential difference between the inside and outside of the fibre is about 80 millivolts. It depends on the different concentrations of potassium inside and outside: the ratio is about 30:1. The two elements potassium and sodium are more or less interchangeable in a dead structure. If a nerve fibre were inert, both would diffuse freely until their concentrations were the same inside and out. But the living fibre actively rejects sodium by some unknown mechanism (the 'sodium pump'). The chemical stability of the cell requires one or other element - which one does not matter and since sodium is lacking, potassium is retained and with it an electric potential.
If the electric potential is lost the nerve is said to be depolarized. This happens if the lining membrane becomes freely permeable: sodium then enters and the surplus of potassium is released. A nerve impulse is simply a wave of depolarization, started by various kinds of stimulus, e.g. an electric current, heat, pressure, or certain chemicals. Once the process has begun, the depolarization of any part of a neurone is a sufficient stimulus to depolarize the next segment. Thus the impulse travels to the end of the nerve fibre. This is quite different from ordinary electrical conduction, and much slower. The thickest fibres conduct at a rate of 100 metres per second, and some very slow fibres at only about 1 metre per second. This is easily demonstrated because pain is conveyed in fast and slow fibres. A mildly painful stimulus such as a pinprick or touching a hot kettle causes an immediate rather indefinite sensation, followed a second or so later by a slowly conducted but precise sense of pain.
When the impulse reaches the end of the fibre it has to be passed to the next nerve cell (for all activities depend on more than one neurone) or to an effector organ such as a muscle (reflex). This is done by releasing a chemical transmitter from the nerve-ending. The gap between a nerve-ending and its target is a synapse. Some chemical transmitters are known. Muscle fibres and most autonomic nerve cells respond to acetylcholine. Sympathetic nerve-endings usually release noradrenaline. Within the central nervous system there are certainly other transmitters as well as these. Serotonin appears to be one. And since there are inhibitory nerves there are presumably inhibitory transmitters. It now seems clear that gamma-amino-butyric acid (GABA) is one of these neuro-transmitters.
Two kinds of mechanisms control bodily functions: nervous activity, and chemical activity by hormones released from glands such as the pituitary and thyroid. The distinction is convenient, but less clear-cut than it appears to be, for the function of a nerve is to release a chemical transmitter when it is stimulated. The difference between a nerve and a gland is that a nerve releases its chemical transmitter at a given point, where it is destroyed as soon as it has acted, whereas a gland releases its transmitter into the blood-stream to act throughout the body. The adrenal medulla is a collection of sympathetic nerve cells behaving as a gland; it releases adrenaline into the blood-stream to activate other sympathetic nerves in distant organs. A nerve cell returns to its resting state almost immediately after transmitting an impulse. Within one or two milliseconds its charge is restored and ready to be fired again. The process can be repeated indefinitely. Waking or sleeping, the nervous system is constantly active throughout life. No fresh cells, it seems, are formed after birth. The original stock serves for a lifetime, and the cells that wear out and die are not replaced.
The central nervous system is protected by the skull and backbone and their lining membranes, the -meninges, which enclose the -cerebrospinalfluid. The fluid acts as a shock-absorber. A neurone is the most helpless of living cells. It is like the proverbial genius, pre-eminent in his chosen specialty but incapable of looking after himself A neurone keeps no stores. If it is deprived of blood, and so of glucose and oxygen, for more than a few seconds it dies. If the blood stops flowing, the stagnant blood in the vessels carries enough supplies to keep nerve cells alive for a few minutes.
There is no ordinary connective tissue in the nervous system. Instead the neurones are supported by a network of spidery cells, the neuroglia, or glia, which appears to do for them most of the routine chemical work that less delicate cells in other tissues do for themselves. Nerve fibres are insulated with a layer of myelin. In the actual nerves, outside the brain and spinal cord, the myelin is formed by Schwann cells, which wrap each fibre in layer upon layer of myelin. If a nerve fibre is cut, the Schwann cells can form a new tube into which a new fibre can grow, provided that the cellbody of the neurone is intact. But in the brain and spinal cord the myelin is laid down by the neuroglia, which does not lay down new paths for damaged fibres. Here, then, any damage is permanent.
Localized disorders are discussed elsewhere (brain; spinal cord; nerve). Mental illnesses are presumably disturbances of the brain (a few are known to be), but this is open to dispute because nobody can say where or what the mind is. Disease of the nervous system is usually disease of its supporting structures. Since nerve cells cannot divide they cannot form tumours. Apart from the tumours of embryonic nerve cells, occurring very rarely in infancy, tumours of nervous tissue arise from the neuroglia. Diseases as different as syphilis, arteriosclerosis, and diabetes affect the nervous system by injuring its small blood vessels (diabetes probably has other effects as well). If the myelin sheaths are damaged conduction is impaired. Many disorders are of this kind, including damage to nervous tissue by diphtheria, alcohol, and multiple sclerosis. The, actual nerve cells remain healthy, but they are ineffective unless the myelin recovers. All parts of a neurone are vulnerable to pressure. Even slight pressure on a nerve disturbs sensation and causes tingling or 'pins and needles', and if the pressure continues the fibres cease to conduct - everyone has experienced numbness in a limb after sitting awkwardly. Nerve fibres recover when the pressure is relieved, but the cell-bodies in the central nervous system can be killed by pressure. Normally the cerebrospinal fluid is kept at a constant pressure, but a part of the brain or spinal cord may be compressed by bleeding, abscess, tumour, or a fractured bone. If the compression is not relieved, there is likely to be permanent damage.