The sympathetic division of the autonomic nervous system

The sympathetic division of the autonomic nervous system functions in the production of the emotion of excitement.

The nervous system is divided into two parts, the autonomic (also called the vegetative or the involuntary) and the somatic (voluntary). The somatic division of the nervous system is composed of the brain, the spinal cord, the sensory neurones originating in the receptors and ending in the spinal cord or brain, and the outgoing motor neurones which carry the impulses from the brain or cord into the striated muscles involved in voluntary movements and into the glands. The reflex arc is the schematic unit of the functioning of this part of the nervous system. The somatic nervous system maintains homeostasis of the organism by producing approach or avoidance motor responses involving the striated muscles.

The autonomic nervous system is composed of outgoing motor neurones which innervate the unstriated muscles of the internal organs and some of the glands. This efferent system has two neurones, the preganglionic and the postganglionic. The preganglionic neurone starts in the cord or brain and joins with the postganglionic neurone in the ganglia outside of the spinal cord. From these ganglia the postganglionic neurones are distributed to the effectors involving the viscera. The smooth muscles which are innervated are those found around the stomach, colon, blood vessels, spleen, ureter, bladder, genitalia, iris, bronchioles, and the pilomotor apparatus. The heart muscle, which has the structure of the striated muscle, is the only striated muscle innervated by neurones of the autonomic nervous system. The glands which are supplied by the autonomic system are the lachrymal, salivary, gastric, pancreas, sweat, adrenal, hypophysis, thyroid, sex, and the liver. Sensory afferent fibers are also present in the autonomic nervous system.

The autonomic nervous system is divided into two divisions, the sympathetic and the parasympathetic. The preganglionic neurones of the sympathetic part of the autonomic nervous system originate in the middle part of the spinal cord (the thoracic and lumbar regions). The parasympathetic neurones originate in the head region or the cranial division (the fibers emerge through the third, seventh, ninth, tenth, and eleventh cranial nerves) and in the caudal region or sacral division.

The basis of division of the autonomic nervous system into the sympathetic and the parasympathetic is antagonism of function.

When the same organs receive both parasympathetic and sympathetic fibers, the effect of stimulation from the sympathetic or parasympathetic, with some exceptions, produces opposite responses of the effectors. Organs which are innervated both by the parasympathetic and sympathetic fibers are the salivary glands, the heart, the stomach and intestines, the bladder, the blood vessels in the digestive system, the external genitalia, the lungs, the spleen, and the muscles regulating the size of the pupil. The sympathetic impulses decrease the flow of saliva from the salivary glands, accelerate the heart rate, cause the muscles of the stomach and colon to relax, produce voiding responses of the bladder and rectum, contract the blood vessels of the digestive tract, decrease the blood supply to the genitalia, relax the bronchial muscles of the lungs, contract the spleen, releasing red cells, and cause the pupil of the eye to dilate. The parasympathetic impulses have the opposite effect on these organs.

A further difference in the functioning of the sympathetic and parasympathetic systems lies in the fact that the former has widespread effects on the whole visceral system, whereas the latter affects isolated organs. This is due to structural differences. The ganglia receiving the preganglionic fibers of the sympathetics lie near the spinal cord (the vertebral ganglia). They are arranged in two chains, one on each side of the spinal cord, and in three large groups; the cervical, the solar, and the inferior mesenteric, with their subdivisions. The sympathetic ganglia receive preganglionic fibers which make connections with from six to nine of the sympathetic ganglia. Thus stimulation of the sympathetic nervous system produces a widespread and diffuse discharge of the impulses into the postganglionic fibers. The whole sympathetic system is thrown into activity. The ganglia of the parasympathetic system, on the other hand, are near or within the organs which they innervate. They are comparatively isolated from one another so that the impulses delivered to one ganglion will affect only that organ to which it is connected. Thus the innervation of the sympathetic system which produces an increase of heart rate will also affect the other effectors supplied by this nervous system. The parasympathetic system can decrease the heart rate without affecting other organs which it innervates.

In most of its functions the parasympathetic nervous system acts to preserve the natural homeostasis of the body. It decreases the intensity of light delivered to the eye by constriction of the pupil; it decreases the heartbeat to its normal rate; and it produces the normal peristaltic movements of the stomach and colon in digestion. The sympathetic system disrupts these functions and, because of its structural interconnections, mobilizes the whole body resources for increased activity. The natural movement of the digestive tract stops; the blood, by vasoconstriction of the arteries leading to the digestive tract and vasodilation of the arteries of the striated muscles, is supplied to the organs which will be most active in removing the emotional situation; the heart rate is faster and supplies more blood to the skeletal muscles; the spleen contracts, releasing more blood corpuscles, which carry, away the products of fatigue produced by greater exertion; the liver releases the sugar which is used by metabolism during work; and the product of muscular work, lactic acid, is counteracted by epinephrine, a hormone of the adrenal gland, which is released into the blood stream. It is evident that both the sympathetic and the parasympathetic nervous systems act synergistically to maintain the homeostasis of the body. Whether one or the other is more active depends upon the external environmental conditions and the internal state of the body.

Two important hormones which aid in the mobilization of the body during the emotion of excitement and sustain the general level of activity are epinephrine and sympathin. Epinephrine (adrenin) is secreted by the medulla of the adrenal gland. It is carried in the blood stream and has almost the same effects as direct stimulation of the sympathetic nervous system. Furthermore, adrenin dissipates the lactic acid produced by muscular work and sustains the general physiological process of the emotion for a long time (amounting to 30 minutes or longer) after the situation producing the emotion has been removed.

Sympathin is secreted by the action of the postganglionic neurones of the sympathetic nervous system on the effectors. The action of sympathin mimics that of the sympathetic system, although all its effects are not similar. There is evidence that sympathin liberated in different parts of the effector system has at least two different effects, one an inhibitory action on the organs and the other an excitatory action.