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Neurology => Neurophysiology => Nervous System
Nervous System
INTRODUCTION
Nervous System, those elements within the animal organism that are concerned with the reception of stimuli, the transmission of nerve impulses, or the activation of muscle mechanisms.
ANATOMY AND FUNCTION
The reception of stimuli is the function of special sensory cells. The conducting elements of the nervous system are cells called neurons; these may be capable of only slow and generalized activity, or they may be highly efficient and rapidly conducting units. The specific response of the neuron-the nerve impulse-and the capacity of the cell to be stimulated make this cell a receiving and transmitting unit capable of transferring information from one part of the body to another.
° Nerve Cell
Each nerve cell consists of a central portion containing the nucleus, known as the cell body, and one or more structures referred to as axons and dendrites. The dendrites are rather short extensions of the cell body and are involved in the reception of stimuli. The axon, by contrast, is usually a single elongated extension; it is especially important in the transmission of nerve impulses from the region of the cell body to other cells. See Neurophysiology.
° Simple Systems
Although all many-celled animals have some kind of nervous system, the complexity of its organization varies considerably among different animal types. In simple animals such as jellyfish, the nerve cells form a network capable of mediating only a relatively stereotyped response. In more complex animals, such as shellfish, insects, and spiders, the nervous system is more complicated. The cell bodies of neurons are organized in clusters called ganglia. These clusters are interconnected by the neuronal processes to form a ganglionated chain. Such chains are found in all vertebrates, in which they represent a special part of the nervous system, related especially to the regulation of the activities of the heart, the glands, and the involuntary muscles.
° Vertebrate Systems
Vertebrate animals have a bony spine and skull in which the central part of the nervous system is housed; the peripheral part extends throughout the remainder of the body. The brain is the part of the nervous system located in the skull; the spinal cord is that found in the spine. The brain and spinal cord are continuous through an opening in the base of the skull; both are also in contact with other parts of the body through the nerves. The distinction made between the central nervous system and the peripheral nervous system is based on the different locations of the two intimately related parts of a single system. Some of the processes of the cell bodies conduct sense impressions and others conduct muscle responses, called reflexes, such as those caused by pain.
In the skin are cells of several types called receptors; each is especially sensitive to particular stimuli. Free nerve endings are sensitive to pain and are directly activated. The neurons so activated send impulses into the central nervous system and have junctions with other cells that have axons extending back into the periphery. Impulses are carried from processes of these cells to motor endings within the muscles. These neuromuscular endings excite the muscles, resulting in muscular contraction and appropriate movement. The pathway taken by the nerve impulse in mediating this simple response is in the form of a two-neuron arc that begins and ends in the periphery. Many of the actions of the nervous system can be explained on the basis of such reflex arcs, which are chains of interconnected nerve cells, stimulated at one end and capable of bringing about movement or glandular secretion at the other.
° The Nerve Network
The cranial nerves run from the head and neck to the brain by passing through openings in the skull, or cranium. Spinal nerves are the nerves associated with the spinal cord and pass through openings in the vertebral column. Both cranial and spinal nerves consist of large numbers of processes that convey impulses to the central nervous system and also carry messages outward; the former processes are called afferent, the latter are called efferent. Afferent impulses are referred to as sensory; efferent impulses are referred to as either somatic or visceral motor, according to what part of the body they reach. Most nerves are mixed nerves made up of both sensory and motor elements.
The cranial and spinal nerves are paired; the number in humans are 12 and 31, respectively. Cranial nerves are distributed to the head and neck regions of the body, with one conspicuous exception: the tenth cranial nerve, called the vagus. In addition to supplying structures in the neck, the vagus is distributed to structures located in the chest and abdomen. Vision, auditory and vestibular sensation, and taste are mediated by the second, eighth, and seventh cranial nerves, respectively. Cranial nerves also mediate motor functions of the head, the eyes, the face, the tongue, and the larynx, as well as the muscles that function in chewing and swallowing. After they exit from the vertebrae, spinal nerves are distributed in a bandlike fashion to regions of the trunk and to the limbs. Interconnecting extensively, they form the brachial plexus, which runs to the upper extremities, and the lumbar plexus, which passes to the lower limbs.
° Autonomic Nervous System
Among the motor fibres may be found groups that carry impulses to organs within the body cavities such as the stomach and intestines (viscera). These fibres are designated as the autonomic nervous system. This consists of two divisions, more or less antagonistic in function, that emerge from the central nervous system at different points of origin. One division, the sympathetic, arises from the middle portion of the spinal cord, joins the sympathetic ganglionated chain, courses through the spinal nerves, and is widely distributed throughout the body. The other division, the parasympathetic, arises both above and below the sympathetic, that is, from the brain and from the lower part of the spinal cord. These two divisions control the functions of the respiratory, circulatory, digestive, and urogenital systems.
DISORDERS OF THE NERVOUS SYSTEM
Neurology deals with the study and treatment of disorders of the nervous system. Psychiatry deals with behavioural disturbances of a functional nature. The division between these two medical specialities is not sharply defined because neurological disorders often manifest both organic and mental symptoms. For a discussion of functional mental illness, see Mental Disorders.
Diseases of the nervous system include genetic malformations, poisonings, metabolic defects, vascular disorders, inflammations, degeneration, and tumours, and they involve either nerve cells or their supporting elements. Vascular disorders, such as cerebral haemorrhage or other forms of stroke, are among the most common causes of paralysis and other neurological complications. Some diseases exhibit peculiar geographic and age distribution. In temperate zones, multiple sclerosis is a common degenerative disease of the nervous system, but is rare in the tropics.
The nervous system is subject to infection by a great variety of bacteria, parasites, and viruses. For example, meningitis, the infection of the meninges (membranes lining the brain and spinal cord), can be caused by many different agents. On the other hand, one specific virus causes rabies. Some viruses causing neurological ailments affect only certain parts of the nervous system. For example, the virus causing poliomyelitis commonly affects the spinal cord; viruses causing encephalitis attack the brain.
Inflammations of the nervous system are named according to the part affected. Myelitis is an inflammation of the spinal cord; neuritis is an inflammation of a nerve. It may be caused not only by infection but also by poisoning, alcoholism, or injury. Tumours originating in the nervous system usually are composed of meningeal tissue or neuroglia (supporting tissue) cells, depending on the specific part of the nervous system affected, but other types of tumour may metastasize (spread) to or invade the nervous system (see Cancer). In certain disorders of the nervous system, such as neuralgia, migraine, and epilepsy, no evidence may exist of organic damage. Another disorder, cerebral palsy, is associated with brain damage before, during, or following birth.
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