Sympathetic Nervous System
From Classic Encyclopedia 1911
"SYMPATHETIC NERVOUS SYSTEM, in physiology (see 26.287). On studying the effects of stimulation of the sympathetic nervous system it appears that they are directed towards activating the body for fight or flight. The dilatation of the pupil increases the perception of light; the acceleration and augmentation of the heart-beat increases the blood supply; the constriction of the blood vessels in the visceral area raises the blood pressure, driving the blood from the digestive area, whose functions are simultaneously inhibited, into the skeletal and cardiac muscles, the lungs and the brain. The sweat glands are stimulated to cool the blood heated by increased muscular effort and the hairs are erected in many animals to render them more alarming. The " goose skin " experienced by man under emotional stress is similarly produced, though not now advantageous. Some emotional responses, like some bodily structures, are vestigial remains. The other division of the autonomic system, to which the name of parasympathetic has been given, serves the purpose of building up reserves and of fortifying the body against times of need and stress. Thus by contracting the pupil it shields the retina from excessive light; by slowing the rate of the heart it gives it longer periods for recuperation; it excites appetite and by promoting the assimilation and digestion of food it stores up energy. The sacral division of the parasympathetic is a mechanism for emptying, thus leading to greater comfort. The sympathetic is, therefore, katabolic, converting potential energy into kinetic, and facilitating outward manifestations of that energy; while the parasympathetic is anabolic, directing energy inwards, where it is stored up. When these two are distributed to the same structure their action is always antagonistic.
In pain, fear, rage and any intense excitement, the sympathetic neurons are brought rapidly into play and the action of the cranial division of the parasympathetic is inhibited. Anabolism is in abeyance and katabolism goes on unchecked. This is comprehensible, since these katabolic activities are defensive in origin and aided the primitive animal in its struggle with or flight from its enemy.
Of late it has been realized that each of these divisions of the autonomic system cooperates with its appropriate group of endocrine glands. The sympathetic group consists of the adrenals, the thyroid and the pituitary - each of which are accelerators of metabolism. The intimate relationship embryologically, structurally and functionally between the nervous and glandular elements is best illustrated by the adrenals. The medulla of the adrenals and the sympathetic ganglia originated from similar cells, preganglionic fibres end round both, and adrenalin, the secretion of the medullary portion, produces the same effect on any part as stimulation of the postganglionic fibres; an interesting example of parallelism between a nervous and chemical mechanism. Just as the preganglionic fibre stimulates the secretion of adrenalin, so adrenalin increases the postganglionic responses.
A similar reciprocity exists in the case of the thyroid; it is stimulated to secretion by the sympathetic and the secretion lowers the threshold to sympathetic stimulation. The existence of sympathetic secretory nerves to the pituitary has been shown, but there is no definite evidence of a reciprocal action of its secretion on the sympathetic.
The endocrine group which cooperates with the parasympathetic must be looked for chiefly in the glands of the alimentary tract and its annexes, since the building up of energy must ultimately be derived from the food. Gastric secretion is started by the appetite and the taste of food reflexly stimulating the organs, but it is continued by a hormone in the pyloric glands of the stomach which spur the fundus glands on to renewed effort. In its turn the acid gastric juice entering the duodenum acts on its mucosa to form secretion, which stimulates secretion of pancreatic juice. But the pancreas forms an internal secretion as well, which promotes the assimilation of sugar. Indeed, the metabolism of sugar gives a particularly good illustration of the general principle here laid down. Acting muscle requires three and a half times as much sugar as resting muscle, and the mobilization of sugar into the blood is a necessary preliminary to muscular effort. Accordingly sympathetic stimulation is found to produce this effect, through the adrenals, thyroid and pituitary, the secretion of each of which taken separately will lower carbohydrate tolerance and may excite glycosuria. On the other hand, the pancreas which is innervated by the parasympathetic has a precisely opposite effect, the antagonistic action of the glands being as definite as that of the associated nerves. Another example of this antagonism is seen in the continuous flow of pancreatic juice following removal of the adrenals, which can be temporarily inhibited by injection of suprarenal extract. Again, it is well known that adrenalin dilates the pupil of an excised eye but not of the intact eye of a normal individual. Therefore something inhibits the adrenalin effect. In depancreatised dogs adrenalin does dilate the pupil, and it has been found to do the same in patients suffering from pancreatic disease, since the normal antagonism of the pancreas to the adrenals is then lost. This pupillary response may also be present in hyperthyroidism, suggesting that the thyroid excess inhibits pancreatic activity, allowing adrenalin to show its dilator action unchecked.
The sympathetic nervous system, the endocrine glands and the gonads form a basic tripod entrusted with the duty both of the preservation of the individual and the continuity of the species. Their relationship is shown in disease as well as in health, and is reflected in many neuroses and psychoses. Disease is the resultant of some external action and of the reaction of the organism against it. It therefore draws on the defensive mechanisms and connotes a position of less stable equilibrium in which the body works with more friction. It may demand an increased supply of hormones to compensate for this, which may ultimately lead to exhaustion of the gland that provides it. In this way the balance between the endocrine glands is disturbed, either from overaction of a gland, or from its unopposed action through the loss of an antagonist.
The sympathetic-endocrine system is affected alike by toxic, nutritional or psychic factors, and is particularly likely to be injuriously influenced if more than one such factor is overtaxing it. As this system has to defend us against internal foes, such as bacterial infections, as well as external enemies, we find that it plays an important part in the regulation of the body temperature. Since it has been shown that the cerebral vessels are remarkably impermeable to drugs and toxins as long as they maintain their integrity, it has become impossible to explain febrile reaction to infection by a central mechanism. When the activity of the thyroid and anterior lobe of the pituitary is diminished, less heat is produced. When increased production is required, the thyroid and adrenals give histological evidence of increased secretion. Cramer has found this both in fevers and after exposure to cold. He has also experimentally produced such changes by the injection of a drug which causes fever. Infections such as gas gangrene which do not induce a febrile reaction do not produce these changes. He has traced the adrenalin into the blood vessels, whereby sympathetic nerve endings all over the body will be stimulated. At the same time more sugar is poured into the blood, largely through thyroid activity. The oxidation of this sugar increases heat production. It has long been known that the adrenals may show marked signs of exhaustion after a severe infection such as diphtheria,while the thyroid may suffer after typhoid fever. Such exhaustion appears to be an important factor in the psychoneuroses of convalescence.
The influence of nutritional factors on the apparatus is seen in the way in which (I) pregnancy enlarges the thyroid and pituitary; (2) vitamine defects cause enlargement of the adrenals and pituitary while causing some atrophy of other endocrine glands; (3) deficiency of assimilable protein in the food causes pellagra in which the adrenals suffer, while the sympathetic nervous system shows actual structural degeneration. While toxic and nutritional influences play chiefly on the glandular part of the apparatus, psychic factors naturally act primarily on the nervous part, though ultimately both parts will become affected, whichever is involved first. The sympathetic is the lowest level of the nervous system and retains several characteristically primitive features, such as peripheral ganglion cells, myenteric nerve nets, connector fibres lying outside the central nervous system, and urgent widespread responses, rather than accurately localized and discriminative ones. This is in accordance with the evolution of the nervous system for defense, and with the maintenance of this primitive function by the sympathetic. Pain has been shown by Trotter to be the specialization of the primitive sensation of lower animals, and fear might similarly be regarded as a specialization of a primitive emotion. It is to such sensation and emotion that the sympathetic nervous system preeminently responds. Both pain and fear are apt to become intense when the appropriate motor response is prevented, though they may not be appreciated when vigorous response is possible. Thus during the excitement of a fight neither fear nor pain may be experienced under conditions which would ordinarily induce them. In the civilized state the response appropriate to primitive man has often to be repressed. The effect of this repression may show itself either as an anxiety neurosis, at the psychic level, or at the sympathetic-endocrine level in certain affections of the associated glands, and in cardiac or digestive neuroses. Such repressions are particularly likely to be necessitated when the great instincts of self-preservation, reproduction and gregariousness, which relate respectively to the life of the individual, of the species and of the community, come into conflict with one another. The constant demand for adrenalin when sympathetic action is increased in fear and anxiety may lead to exhaustion of the gland. Addison's disease presents a clearcut picture of adrenal deficiency in its symptoms of muscular weakness, low blood pressure, pigmentation and vomiting. Although this is due to organic changes, slighter degrees of a similar condition are now recognized and it has been suggested that this enters into many war neuroses and other functional states characterized by vasomotor instability, low blood pressure and myasthenia. Conversely, prolonged oversecretion by the adrenals must tend to raise blood pressure through the sympathetic, tending in turn to arteriosclerosis, with all its widespread effects. The well-known influence of anxiety in producing this condition can thus be explained.
There is a close association between the thyroid, the reproductive organs and the sympathetic. The thyroid tends to enlarge at puberty, marriage and in pregnancy, while myxoedema is most apt to occur after the climacteric. Amenorrhoea is common even in the minor degrees of hyperthyroidism. The effect of sympathetic irritation in producing thyroid enlargement and Graves' disease is now recognized. Cushing showed that if in cats he sutured the phrenic nerve to the cervical sympathetic so that every respiration stimulated the latter, he could produce the symptoms of Graves' disease. The influence of distressing emotions in producing hyperthyroidism was well shown during the air-raids on London during the World War. Epidemics of Graves' disease also followed the Kishinev massacres and the San Francisco earthquake. Again, if a distressing emotion has a matrimonial origin, it is particularly likely to induce Graves' disease, for here each limb of the basic tripod is involved. The disease has been compared with a state of continuous fear, a description which tallies with its general appearance.
The pituitary body also shows the two-fold association with the reproductive organs and the sympathetic nervous system. The anterior glandular part has an effect on temperature, the growth of bone and skeletal tissues and the reproductive organs. The intermediate lobe influences carbohydrate metabolism, while the secretion of the posterior lobe is mainly a stimulant to plain muscle and to the secretion of milk. The effect of the secretion of the posterior lobe on diuresis is still a matter of controversy, but it is clear that disease of the posterior lobe is often found in diabetes insipidus and that stimulation of the sympathetic nerves to the gland will cause polyuria. Probably hysterical polyuria is thus produced.
It is also clear that some cases of glycosuria are of sympathetic nervous origin; the physiological mechanism by which this can be brought about has already been explained. Diabetes is characterized, like sympathetic stimulation, by an exaggerated katabolism. This shows its effect first on the most abundant and most easily metabolised of the food-stuffs, the carbohydrates, which are also essential for muscular action, to which sympathetic stimulation should normally be a preliminary. Sympathetic stimulation induced by various disagreeable emotions will increase metabolism generally and specially lower carbohydrate tolerance. The influence of excitement and emotion in causing glycosuria is well recognized. It is commonest in Jews, a notoriously emotional race. When stocks go down in New York, says Crile, diabetes goes up. Temporary glycosuria occurred in a number of men who merely watched a football cup-tie without participating in it. Glycosuria has been, unfortunately, comparatively common in young officers entrusted with heavy responsibilities during the war. Singer and Clark have recorded two cases in which there was alternation between glycosuria and the exhibition of mental symptoms, as if the emotional discharge asserted itself either at the metabolic or the psychic level but not at both.
Finally, the inhibitory effect of the sympathetic on the digestive processes must be remembered. Fear checks the secretion of saliva, anger stops the secretion of gastric juice. Depressing emotions acting through the sympathetic check the peristalsis of the stomach while closing the pyloric sphincter, thus leading to a dilatation of the stomach. A similar inhibition of intestinal peristalsis may occur, leading to intestinal stasis. This in its turn will lead to a drag on the sympathetic nerves in the mesentery, increasing their inhibitory effect. Thus a vicious circle is established and the persistent intestinal toxaemia that results may produce organic changes in many structures, including the group of endocrine glands which cooperate with the sympathetic.
The consideration of diseases produced through the agency of the parasympathetic lies outside the present topic, except in so far as the vagus may overact from a loss of balance produced by diminished sympathetic action. Suffice it to say that laryngeal spasm, asthma, slow or irregular action of the heart, low blood pressure, hyperchlorhydria, spastic constipation and a liability to skin disturbances of the vasodilator type are among the symptoms evoked. Since febrile reaction to disease is a function of the sympathetic, which is antagonistic to the parasympathetic, we find that those subjects with an overacting vagus do not react well to infections and, indeed, show an abnormal sensitiveness to many foreign proteins. Their tendency to undue anabolism is sometimes indicated by their tendency to flabbiness of the tissues and overgrowth of lymphoid structures. A curious point is the occurrence of sweating, since the secretion of the sweat glands is under the control of the sympathetic. But a similar discrepancy is seen in the action of a vagotropic drug such as pilocarpin.
In assessing the effects of the sympathetic nervous system in disease, it must be borne in mind that we have very little knowledge of its morbid anatomy, though a beginning has been made in the study of pellagra. The evidence so far mainly relates to function and were our knowledge of its structural alterations at all comparable to what we know of organic changes in the spinal cord, we should be on firmer ground. But enough has been learned to show that the sympathetic plays a large part in the regulation of the internal viscera, and, through the endocrine glands, in general metabolism. The balance between these glands determines to a large extent both the racial and individual characteristics. Climatic influence has a profound effect through the skin on the sympathetic nerves and hence on the endocrine glands. The development of protective pigment is an important method by which the sympathetic-endocrine system can react to this, and the unadaptibility of albinos in this respect is well known. Thus physical environment can influence both temperament and structure, and the sympathetic-endocrine system must have played a large part in developing the variability of man into different races.
Evolved in a subconscious plane the sympathetic nervous system remains for ever beyond the control of the will. Timme quotes an instance which, while apparently contradicting this, proved on further inquiry to support it. This was the case of a man who could voluntarily dilate his pupils, who could cause the pilomotor muscles to raise the hairs on his arm, and who could at will produce the phenomenon of " goose-flesh " in various parts of his body. When closely questioned he admitted that the effects were produced not immediately by his will, but always by the intermediation of some association called into being by him. Thus when dilating his pupils he always imagined himself looking far into space, under which conditions the pupil does dilate. For the goose-flesh effect he would picture to himself his arm plunged into ice-cold water, and the goose-flesh appeared. Various associations produce autonomic effects without our will, and it is reasonable to infer that, if we can recall these associations through our will, the same autonomic effects will be produced.
The higher centres of the brain show their influence on the lower chiefly in the direction of inhibition. The highest organism is the most self-controlled, but the sympathetic cannot be thus controlled. The will can only help in so far as " it can make our voluntary activities harmonize with our environment." The emotional apparatus remains, as McDougall has pointed out, the most unchanging part of our nervous equipment, though the stimuli to which it responds may vary enormously in different individuals. But once the response occurs, it is extraordinarily true to type. This is comprehensible since the apparatus retains so many features of the primitive nervous system. (W. L. B.*)