The Autonomic Nervous System
Anatomical Structure of the System
Transmission of Autonomic Stimuli
Functions of the Autonomic Nervous System
(a) The Parasympathetic System
(b) The Sympathetic Nervous System
(c) The Adrenal Medulla
Anatomical Structure of the System
The nervous system comprises the brain and various types of nerves, including afferent nerves (from the Latin, ad = towards; ferro = I carry), which carry sensory impulses from all parts of the body to the brain and efferent nerves (ex = from; ferro = I carry) through which "messages" are conducted from the brain to the muscles and all of the organs of the body. The somatic part of the nervous system has sensory components which convey sensations from the eyes, the nose and other sensory organs to the brain (mainly the cerebral cortex) where most of the impulses reach our awareness, and motor components transmitting impulses to the skeletal muscles in the limbs and trunk permitting voluntary control of movements. The autonomic nervous system conveys sensory impulses from the blood vessels, the heart and all of the organs in the chest, abdomen and pelvis through nerves to other parts of the brain (mainly the medulla, pons and hypothalamus). These impulses often do not reach our consciousness, but elicit largely automatic or reflex responses through the efferent autonomic nerves, thereby eliciting appropriate reactions of the heart, the vascular system, and all the organs of the body to variations in environmental temperature, posture, food intake, stressful experiences and other changes to which all individuals are exposed.
There are two major components of the autonomic nervous system, the sympathetic and the parasympathetic systems. The afferent nerves subserving both systems convey impulses from sensory organs, muscles, the circulatory system and all the organs of the body to the controlling centers in the medulla, pons and hypothalamus. From these centers efferent impulses are conveyed to all parts of the body by the parasympathetic and sympathetic nerves. The impulses of the parasympathetic system reach the organs of the body through the cranial nerves # 3, 7, 9, & 10, and some sacral nerves to the eyes, the gastrointestinal system, and other organs. The sympathetic nerves reach their end-organs through more devious pathways down the spinal cord to clusters of sympathetic nerve bodies (ganglia) alongside the spine where the messages are relayed to other nerve bodies (or neurons) that travel to a large extent with the blood vessels to all parts of the body. Through these nervous pathways, the autonomic nerves convey stimuli resulting in largely unconscious, reflex, bodily adjustments such as in the size of the pupil, the digestive functions of the stomach and intestines, the rate and depth of respiration and dilatation or constriction of the blood vessels.
Transmission of Autonomic Stimuli
Like other nerves, those of the autonomic nervous system convey their messages to the appropriate end organs (blood vessels, viscera, etc.) by releasing transmitter substances to which the receptors of the target cells are responsive. The most important of these transmitters in the autonomic nervous system are acetylcholine and norepinephrine. In the parasympathetic system, acetylcholine is responsible for most of these transmissions between the afferent and efferent nerves of the system and between the efferent nerve endings and the cells or organs that they subserve. Acetylcholine also serves to transmit nerve-to-nerve messages in the afferent nerves and the brain centers of the sympathetic nervous system. However, the final transmission of messages from the sympathetic nerves to the end-organs or cells that they innervate is conveyed by the release of norepinephrine (noradrenaline) with at least one important exception, namely the sympathetically conveyed stimulus to the sweat glands which is transmitted by acetylcholine. A stimulus to contraction of the blood vessels is required in order to maintain the blood pressure when we arise from bed in the morning, so as to prevent fainting from excessive pooling of blood in the lower body. This stimulus is conveyed by norepinephrine release within the walls of the blood vessels from the nerve endings of the sympathetic nerves that innervate each blood vessel.
Functions of the Autonomic Nervous System
(a) The Parasympathetic System
When a stimulus arises in an organ, such as a bright light shone into the eyes, the message is conducted through sensory fibers to the midbrain to give rise to an appropriate stimulus that travels through the parasympathetic fibers of the oculomotor (third cranial) nerves to the pupils, resulting in automatic contraction of the pupillary muscles to constrict the aperture and so reduce the amount of light reaching the sensory cells in the retinae of the eyes. Similarly, the stimuli associated with the entry of food into the stomach are conveyed by afferent fibers of the vagus nerve to the command station or nucleus of the vagus in the brain whence messages are automatically conveyed through efferent fibers of the vagus back to the stomach. These stimulate the secretion of gastric juices and peristaltic contractions of the stomach to mix the food with the secreted digestive juices and gradually to convey the gastric contents into the intestines where a similar process is initiated through essentially the same parasympathetic nerve pathways. Fortunately, emptying of the rectum and of the urinary bladder is not entirely automatic but is subject to parasympathetic impulses that are voluntarily controlled. Thus, filling of the urinary bladder with urine stimulates stretch-sensitive receptors in the wall of the bladder whence the message is conveyed to the midbrain where the stimulus to bladder contraction and opening of the sphincters is voluntarily initiated to allow the discharge of the contained urine.
Similarly the very complex requirements of giving birth to a baby are initiated by stimuli to dilatation of the cervix, and involuntary contractions of the uterine musculature with delivery of the fetus assisted by voluntary contraction of the abdominal muscles.
(b) The Sympathetic Nervous System
The sympathetic nervous system is even more automatic and only exceptionally susceptible to any voluntary control. When the environmental temperature is raised on a hot summers day, the increased temperature initiates several automatic responses. Thermal receptors convey stimuli to sympathetic control centers of the brain from which inhibitory messages travel along the sympathetic nerves to the blood vessels of the skin resulting in dilatation of the cutaneous blood vessels, thereby greatly increasing the flow of blood to the surface of the body from where heat is lost by radiation from the surface of the body. Dilatation of the blood vessels in this way tends to lower the blood pressure and to promote oozing or transudation of the fluid from the capillaries which may result in swelling of the dependent limbs. Thus, fine adjustments in sympathetic control of vascular contraction and "tone" are required to prevent excessive vascular dilatation and undue reduction in blood pressure. Otherwise, this might result in severe gravitational pooling of blood in the lower limbs thereby reducing blood flow to the brain and causing fainting spells, to which individuals with impaired sympathetic nervous functions are very susceptible. The sympathetic nervous system responds to environmental heat in another important way. The rise in body temperature is sensed by the hypothalamic center from which stimuli emanate via sympathetic nerves to the sweat glands, resulting in appropriate sweating. This serves to cool the body by the loss of heat resulting from evaporation of the sweat, aided by a cool breeze. The only really voluntary input that we have to facilitate cooling in a warm environment is to get into a pool, a cold shower, or an air-conditioned room! We cannot voluntarily influence the dilatation of our blood vessels or the adequacy of our sweating in response to heat in other ways.
Control of the rate and strength of cardiac contractions is also under the predominant control of the sympathetic nervous system. Thus, a fall in blood pressure resulting from traumatic injury causing blood loss is sensed by pressure-sensitive parts of the arteries called baroreceptors. Evidence of reduced arterial distension is sensed by these baroreceptors and conveyed by the parasympathetic (mainly the glossopharyngeal) nerves to the cardiovascular control center in the medulla, called the nucleus tractus solitarii. From these nuclei sympathetic stimuli conveyed by the cardiac nerves cause acceleration of the heart rate, probably complemented by simultaneous reduction in the parasympathetic stimuli via the vagus nerves which slow the heart rate. Although pain, anxiety, fear and injuries or blood loss would involuntarily increase the sympathetic stimulation to cardiac acceleration, most of us are unable to influence either this effect or the consequences of blood loss per se on cardiac acceleration.
(c) The Adrenal Medulla
The central part of the adrenal glands (the adrenal medulla) contains a collection of sympathetic nerve cells specialized in at least two important respects. Because of their proximity to the adrenal cortex which surrounds the medulla and secretes hydrocortisone (or cortisol), the neurons of the medulla are able to synthesize not only norepinephrine but also, by attaching a methyl group to this compound, epinephrine (or adrenaline). The adrenal medulla is the only source of more than trivial amounts of epinephrine that enters the blood stream. The second aspect of specialization of the adrenal medulla is in its responses, via the sympathetic efferent nerves that reach it, to specific types of stimuli that have little or no effect on the rest of the autonomic nervous system. Thus, whereas changing from recumbency to the upright posture activates mainly the sympathetic neurons of the blood vessels where norepinephrine is released with resulting elevation mainly of plasma norepinephrine levels, a fall in blood sugar induced by an injection or excessive release of insulin causes a predominant increase in plasma epinephrine, the concentration of which may rise to 3 or 4 times the concomitant level of plasma norepinephrine. Situations such as emotional excitement, fear, apprehension, psychic distress, panic reactions, sexual activity and fight-or-flight stimuli probably activate many parts of the sympathetic nervous systems including the adrenal medullae.
It is evident, therefore, that while we are not constantly aware of the activity of the autonomic nervous system as we are of unusual sensory and motor events, the normal functioning of the autonomic nervous system day and night, from heart-beat to heart-beat, plays a largely unconscious but vital role in our livelihood. It is not surprising, therefore, that autonomic abnormalities, though they are usually more difficult to recognize than a severe pain, a sensory loss or paralysis of a limb, may be even more important in impairing the quality and even jeopardizing the continuation of life.
David H.P. Streeten, MB, DPhil, FRCP, FACP
Emeritus Professor of Medicine
SUNY Health Science Center
Syracuse, NY 13210
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