Nervous System

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Overview

The nervous system of higher vertebrates is among the most complex aspects of biological evolution. Vertebrate nervous systems are conventionally broken down in two ways: spatially, into the central nervous system (CNS) and the peripheral nervous system (PNS); and functionally, into somatic and autonomic components, which both have elements in the CNS and PNS. The somatic refers primarily to voluntary or skeletal muscle action and the sensory nerves of the special senses. The autonomic (see below) enervates and provides sensory support for the body’s internal organs, such as the stomach and intestines (viscera) and blood vessels, and is concerned with the management of the body’s internal environment, through its actions on smooth muscle, cardiac muscle, and cells involved in secretion.

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The CNS/PNS Divide

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 CNS and the PNS 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 CNS 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 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 CNS 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 band-like 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.

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Autonomic Nervous System

The CNS component of the autonomic nervous system (ANS) is formed by several nuclei (masses of neurons) in the hypothalamus of the brain, and several columns (long masses of neurons) in the brainstem and spinal cord.

In the PNS, the ANS consists of three divisions, two of which, the sympathetic and parasympathetic, are more-or-less antagonistic in function and emerge from the CNS at different points of origin. The sympathetic division 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 parasympathetic division arises both above and below the sympathetic, that is, from the brain and from the lower part of the spinal cord.

The third division of the ANS is the enteric nervous system. This forms a separate part of the ANS, from which it is relatively independent (as it is from the CNS). The nerves composing the enteric nervous system are located in the walls of the oesophagus, stomach, small intestine, and large intestine, and are concerned with the control of gastric secretions and peristaltic movements of the gastrointestinal tract. Enteric neurons are grouped together into nets of intersecting ganglia with their supporting glial cells (enteric glia). The resulting plexuses, invisible to the naked eye, have specific functions, both sensory and motor, relating to the position occupied in succeeding sections of the gastrointestinal tract, for example the control of muscular activity. The enteric nervous system is connected with other autonomic ganglia in the PNS and via the vagus nerve with the CNS. However, the neurons are of local origin and regulation so that a section of small intestine removed from the body will continue to contract in the characteristic peristaltic wave.

As well carrying impulses to and from organs within the body cavities, within the PNS those groups of nerve fibres designated as part of the ANS are also responsible for controlling movements of the iris and lens of the eye, hairs in the skin, and the mechanism of sweating. Through its interactions with the cardiovascular system, endocrine system, and immune system, the ANS is cognizant with every part of the body.

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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 (Alzheimer’s disease, Parkinson’s disease, motor neurone disease), 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, probably autoimmune, 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. Slow-acting progressive infections such as the prion disease Creutzfeldt-Jakob disease, subacute sclerosing panencephalitis (SSPE), caused by latent measles virus infection in the brain, and HIV cause dementia and other neurological symptoms leading directly or indirectly to death. Another retrovirus, human T-cell lymphocytotrophic virus, causes slowly progressing leg weakness. 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.