The autonomic nervous system

The nervous system is divided into two great subgroups: the cerebrospinal system, made up of the brain, spinal cord and the peripheral cranial and spinal nerves, and the autonomic system (also termed the vegetative, visceral or involuntary system), comprising the autonomic ganglia and nerves. Broadly speaking, the cerebrospinal system is concerned with the responses of the body to the external environment. In contrast, the auto-nomic system is concerned with the control of the internal environment, exercised through the innervation of the non-skeletal muscle of the heart, blood vessels, bronchial tree, gut and the pupils and the secretomotor supply of many glands, including those of the alimentary tract and its outgrowths, the sweat glands, and, as a rather special example, the suprarenal medulla.

The two systems should not be regarded as being independent of each other, for they are linked anatomically and functionally. Anatomically, autonomic nerve fibres are transmitted in all of the peripheral and some of the cranial nerves; moreover, the higher connections of the autonomic system are situated within the spinal cord and brain. Functionally, the two systems are closely linked within the brain and cord.

The characteristic feature of the autonomic system is that its efferent nerves emerge as medullated fibres from the brain and spinal cord, are interrupted in their course by a synapse in a peripheral ganglion and are then relayed for distribution as fine non-medullated fibres. In this respect they differ from the cerebrospinal efferent nerves, which pass without interruption to their terminations (Fig. 275).

The autonomic system is subdivided into the sympathetic and parasympathetic systems on anatomical, functional, and to a considerable extent, pharmacological grounds.

Anatomically, the sympathetic nervous system has its motor cell stations in the lateral grey column of the thoracic and upper two lumbar segments of the spinal cord. The parasympathetic system is less neatly defined anatomically since it is divided into a cranial outflow, which passes along the cranial nerves III, VII, IX and X, and a sacral outflow, with cell stations in the 2nd, 3rd and sometimes 4th sacral segments of the cord.

Functionally, the sympathetic system is concerned principally with stress reactions of the body. When this system is stimulated, the pupils dilate, peripheral blood vessels constrict, the force, rate and oxygen consumption of the heart increase, the bronchial tree dilates, visceral activity is

Erection Peripheral Nerves

diminished by inhibition of peristalsis and increase of sphincter tone, glycogenolysis takes place in the liver, the supradrenal medulla is stimulated to secrete, and there is cutaneous sweating and pilo-erection. The sympathetic pelvic nerves inhibit bladder contraction and are motor to the internal vesical sphincter.

Coronary blood flow is increased, partly by a direct sympathetic effect and partly produced by indirect factors, which include more vigorous cardiac contraction, reduced systole, relatively increased diastole and an increased concentration of vasodilator metabolites.

The parasympathetic system tends to be antagonistic to the sympathetic system (Table 6). Its stimulation results in constriction of the pupils, diminution in the rate, conduction and excitability of the heart, an increase in gut peristalisis with sphincter relaxation and enhanced alimentary glandular secretion. In addition, the pelvic parasympathetic nerves inhibit the vesical internal sphincter and are motor to the detrusor muscle of the bladder.

Fig. 275 The essential difference between the cerebrospinal and autonomic outflows: (a) the cerebrospinal system has its lowest efferent nerve cell stations within the c.n.s.; (b) the autonomic system has its lowest efferent cell stations in a peripheral ganglion (here illustrated by a typical sympathetic nerve ganglion). Red, afferent pathway; yellow, efferent pathway.

Table 6 Summary of effects of sympathetic and parasympathetic stimulation.

Sympathetic stimulation

Parasympathetic stimulation

Eye

Pupil dilates

Pupil constricts; accommodation of lens

Lacrimal gland

Vasoconstrictor

Secretomotor

Heart

Increase in force, rate, conduction and excitability

Decrease in force, rate, conduction and excitability

Lung

Bronchi dilate

Bronchi constrict; secretomotor to mucous glands

Skin

Vasoconstrictor Pilo-erection Secretomotor to sweat glands

Salivary glands

Vasoconstrictor

Secretomotor

Musculature of alimentary canal

Peristalsis inhibited

Peristalsis activated; sphincters relax

Acid secretion of stomach

Secretomotor

Pancreas

Secretomotor

Liver

Glycogenolysis

Suprarenal

Secretomotor

Bladder

Detrusor inhibited Sphincter stimulated

Detrusor stimulated Sphincter inhibited

Uterus

Uterine contraction Vasoconstriction

Vasodilatation

The sympathetic system tends to have a 'mass action' effect; stimulation of any part of it results in a widespread response. In contrast, parasympathetic activity is usually discrete and localized. This difference can be explained, at least in part, by differences in anatomical peripheral connections of the two systems, as will be shown below.

It is useful to think of the two systems as acting synergistically. For example, reflex slowing of the heart is effected partly from increased vagal and partly from decreased sympathetic stimulation. In addition, some organs receive their autonomic innervation from one system only; for example, the suprarenal medulla and the cutaneous arterioles receive only sympathetic fibres, whereas neurogenic gastric secretion is entirely under parasympathetic control via the vagus nerve.

Pharmacologically, the sympathetic postganglionic terminals release adrenaline and noradrenaline, with the single exception of the terminals to the sweat glands which, in common with all the parasympathetic postgan-glionic terminations, release acetylcholine.

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