PHYSIOLOGY OF AUTONOMIC NERVOUS SYSTEM
Morpho-functional organization of autonomic nervous system
a) Sympathetic nervous system (Sympathetic patr of autonomic nervous system includes paravertebral ganglions, prevertebral ganglions, sympathetic nerves. In the lateral parts of the spinal cord on the thoracic-lumbal level are present sympathetic centre of Yakobson, whose activity regulated by brain stem. Axons of neurons of sympathetic centre go out from the spinal cord in ventral roots and form white branches with the ganglions of sympathetic stems. From these stems go out postganglionic axons and go to the organs of brain, thorax, abdominal cavity and pelvis. Preganglionic axons, which goes out on the level of segments of spinal cord, innerevate a few paravertebral and prevertebral ganglions; that is why provide multiplicative central regulation of different visceral functions.
This review examines how the sympathetic nervous system plays a major role in the regulation of cardiovascular function over multiple time scales. This is achieved through differential regulation of sympathetic outflow to a variety of organs. This differential control is a product of the topographical organization of the central nervous system and a myriad of afferent inputs. Together this organization produces sympathetic responses tailored to match stimuli. The long-term control of sympathetic nerve activity (SNA) is an area of considerable interest and involves a variety of mediators acting in a quite distinct fashion. These mediators include arterial baroreflexes, angiotensin II, blood volume and osmolarity, and a host of humoral factors. A key feature of many cardiovascular diseases is increased SNA. However, rather than there being a generalized increase in SNA, it is organ specific, in particular to the heart and kidneys. These increases in regional SNA are associated with increased mortality. Understanding the regulation of organ-specific SNA is likely to offer new targets for drug therapy. There is a need for the research community to develop better animal models and technologies that reflect the disease progression seen in humans. A particular focus is required on models in which SNA is chronically elevated.
Historically, the sympathetic nervous system (SNS) has been taught to legions of medical and science students as one side of the autonomic nervous system, presented as opposing the parasympathetic nervous system. This review examines the evidence that over the past decade a new and more complex picture has emerged of the SNS as a key controller of the cardiovascular system under a variety of situations. Studies have revealed some of the central nervous system pathways underlying sympathetic control and where or how a variety of afferent inputs regulate sympathetic outflow. Our understanding of how sympathetic nerve activity regulates end organ function and blood pressure has increased along with the development of new technologies to directly record SNA in conscious animals and humans. Most importantly, increasing clinical evidence indicates a role for sympathoactivation in the development of cardiovascular diseases. Such information highlights the need to better understand how the SNS interfaces with the cardiovascular system and how this interaction may result in increased morbidity or mortality. Aspects of the SNS have been the subject of reviews in the past, and with between 1,300 and 2,000 publications published per year for the past 5 years involving various aspects of the SNS, it is not possible to cover in detail the wealth of recent information on this area. The accent of this review is on the nature of the activity present in sympathetic nerves, how it affects cardiovascular function, and how it is implicated in disease processes. It aims not to simply catalog the studies surrounding these areas, but rather attempts to distill down observations to provide future directions and pitfalls to be addressed.
SNS activity provides a critical aspect in the control of arterial pressure. By rapidly regulating the level of activity, the degree of vasoconstriction in the blood vessels of many key organs around the body is altered. This in turn increases or decreases blood flow through organs, affecting the function of the organ, peripheral resistance, and arterial pressure. In contrast to the activity present in motor nerves, sympathetic nerves are continuously active so all innervated blood vessels remain under some degree of continuous constriction. Since its first description in the 1930s sympathetic nerve activity (SNA) has engendered itself to researchers in two camps; neurophysiologists have seen its inherent properties as an opportunity to understand how areas of the central nervous system may be “wired” to generate and control such activity, while cardiovascular physiologists saw its regulation of blood flow as a means to measure the response to different stimuli, drugs, and pathological conditions. However, the innervation to almost all arterioles and actions on specific organs such as the heart and kidney is not sufficient to justify its importance. What distinguishes the SNS is the emerging evidence that overactivity is strongly associated with a variety of cardiovascular diseases. A key question is, Does this increased SNA act as a driver of the disease progression or is it merely a follower? Furthermore, how does increased SNA accelerate the disease progression? Is it simply that it results in increased vascular resistance or are there subtle structural changes induced by elevated SNA or specific actions on organs such as the kidney through its regulation of the renin-angiotensin system and/or pressure natriuresis?
It was Walter Cannon who portrayed the SNS as central to the regulation of homeostasis. Cannon showed that when an animal is strongly aroused, the sympathetic division of its autonomic nervous system “mobilizes the animal for an emergency response of flight or fight. The sympathico-adrenal system orchestrates changes in blood supply, sugar availability, and the blood's clotting capacity in a marshalling of resources keyed to the violent display of energy.” In this setting, the SNS and parasympathetic nervous system were presented as two opposing forces with the parasympathetic endorsing “rest and digest” while the SNS “flight and fight.” An unintended side effect advanced in some textbooks has been to portray the actions of sympathetic nerves as confined to extreme stimuli. As will be advanced in this review, the SNS plays a key role in the moment-to-moment regulation of cardiovascular function at all levels from quiet resting to extreme stimuli. While SNA can be quite low under quiet resting conditions, removal of all sympathetic tone via ganglionic blockade significantly lowers blood pressure. Furthermore, removal of SNA to only one organ such as the kidney can chronically lower blood pressure in some animals, indicating its importance in maintaining normal cardiovascular function.
Evidence that sympathetic nerves are tonically active was established from the 1850s with the observation that section or electrical stimulation of the cervical sympathetic nerve led to changes in blood flow in the rabbit ear. However, it was not until the 1930s that Adrian, Bronk, and Phillips published the first description of actual sympathetic discharges. They observed two obvious features: 1) that discharges occur in a synchronized fashion, with many of the nerves in the bundle being active at approximately the same time, and 2) that discharges generally occur with each cardiac cycle in a highly rhythmical fashion. They also noted that by no means was the overall activity level constant as it was increased by asphyxia or a small fall in blood pressure. This was the first direct evidence supporting Hunt's assertion in 1899 that “the heart is under the continual influence of sympathetic impulses.” These early studies answered a number of questions on the nature of multifiber discharges, such as whether the activity present in the nerve bundle reflected that of single fibers firing very rapidly, or groups of fibers firing more or less synchronously. They also showed that the synchronized activation of postganglionic nerves was not a function of the ganglia as it could be observed in preganglionic nerves and that activity was bilaterally synchronous, that is, that activity in right and left cardiac nerves was the same.
The origin of the rhythmical discharges was considered in the 1930s to be a simple consequence of phasic input from arterial baroreceptors, which had been shown to display pulsatile activity. This proposal had the effect of diminishing the role of the central nervous system to that of a simple relay station and may go some way to explaining the lack of further interest in recording SNA until the late 1960s. Green and Heffron then reexamined the question of the origin of SNA after noting a rapid sympathetic rhythm (at ?10 Hz) under certain conditions (mainly reduced baroreceptor afferent traffic) that was far faster than the cardiac rhythm. This indicated that the origin of bursts of SNA could not simply be a product of regular input from baroreceptors. Their suggestion that the fast rhythm did not have a cardiac or ganglionic origin, but was of brain stem origin, stimulated interest from neurophysiologists, who could use this phenomena for the study of the central nervous system.
Postganglionic sympathetic nerves are composed of hundreds to thousands of unmyelinated fibers, whose individual contributions to the recorded signal are exceedingly small. But fortunately, their ongoing activity can be measured from whole nerve recordings because large numbers of fibers fire action potentials at almost the same time (synchronization) to give discharges of summed spikes. Although it is possible to perform single unit recordings from postganglionic nerve fibers, the favored approach is a multiunit recording. This is obviously a much easier experimental preparation, which allows recordings in conscious animals. However, several important points can only be shown from single-unit recordings. First, while multifiber discharges can occur at quite fast rates (up to 10 Hz), the frequency of firing in the single unit is much lower. Average rates in anesthetized rabbits have been recorded between 2 and 2.5 spikes/s for renal nerves, ?1.2 spikes/s for splenic nerves in the cat, and between 0.21 and 0.5 spikes/s in the human. This slow firing rate means that the rhythmical properties of the single-unit discharges are not seen unless their activity is averaged over time against a reference such as the cardiac cycle or respiration. Single unit recordings also show the minimal firing interval for postganglionic neurons is between 90–100 ms. This indicates it is unlikely that multifiber discharges represent high frequency impulses from a single neuron, but rather the summation of impulses from multiple fibers that fire synchronously. These properties have subsequently been confirmed with single unit recordings in the human. The low firing rate of individual nerves seems to preclude the same neuron being activated more than once in each multifiber discharge. Rather, it would seem that the activated neurons are drawn from a neuronal pool. It is unlikely that the low firing rate is due to a long refractory period for the nerves, since the individual nerves can be induced to fire at quite fast rates by stimuli such as from chemoreceptors or nociceptors.
b) Parasympathetic nervous system (Parasympathetic patr of autonomic nervous system includes ganglions (present near organs-effectors or inside them), parasympathetic nerves. Bodies of the preganglionic parasympathetic neurons are in the brain stem and in the sacral level of spinal cord. Axons of preganglion neurons go to the postganglion neurons, which are present in ganglions. The parasympathetic fibers are in n.oculomotorius, n.facialis, n.glossopharyngeus, n.vagus, sacral nerves. Parasympathetic nervous system also innervates muscles of vessels, exept sex organs and may be brain.)
c) Metasympathetic nervous system (Metasympathetic patr of autonomic nervous system is intramural ganglions, which are in the organs walls. Reflector arc are present in the wall of organs too. It regulated by sympathetic and parasympathetic system. It has sensory, interneuronal, moving chain and own mediators.)
The autonomic nervous system, like the somatic nervous system, is organized on the basis of the reflex arch. Impulses initiated in visceral receptors are relayed via afferent autonomic pathways to the central nervous system, integrated within it at various levels, and transmitted via efferent pathways to visceral effectors.
The ANS is further divided into the sympathetic nervous system and the parasympathetic nervous system. Both of these systems can stimulate and inhibit effectors. However, the two systems work in opposition—where one system stimulates an organ, the other inhibits. Working in this fashion, each system prepares the body for a different kind of situation, as follows.
* The sympathetic nervous system prepares the body for situations requiring alertness or strength or situations that arouse fear, anger, excitement, or embarrassment (“fight-or-flight” situations). In these kinds of situations, the sympathetic nervous system stimulates cardiac muscles to increase the heart rate, causes dilation of the bronchioles of the lungs (increasing oxygen intake), and causes dilation of blood vessels that supply the heart and skeletal muscles (increasing blood supply). The adrenal medulla is stimulated to release epinephrine (adrenalin) and norepinephrine (noradrenalin), which in turn increases the metabolic rate of cells and stimulate the liver to release glucose into the blood. Sweat glands are stimulated to produce sweat. In addition, the sympathetic nervous system reduces the activity of various “tranquil” body functions, such as digestion and kidney functioning.
* The parasympathetic nervous system is active during periods of digestion and rest. It stimulates the production of digestive enzymes and stimulates the processes of digestion, urination, and defecation. It reduces blood pressure and heart and respiratory rates and conserves energy through relaxation and rest.
In the SNS, a single motor neuron connects the CNS to its target skeletal muscle. In the ANS, the connection between the CNS and its effector consists of two neurons—the preganglionic neuron and the postganglionic neuron. The synapse between these two neurons lies outside the CNS, in an autonomic ganglion. The axon (preganglionic axon) of a preganglionic neuron enters the ganglion and forms a synapse with the dendrites of the postganglionic neuron emerges from the ganglion and travels to the target organ. There are three kinds of autonomic ganglia:
The sympathetic trunk, or chain, contains sympathetic ganglia called paravertebral ganglia. There are two trunks, one on either side of the vertebral column along its entire length. Each trunk consists of ganglia connected by fibers, like a string of beads.
The prevertebral (collateral) ganglia also consist of sympathetic ganglia. Preganglionic sympathetic fibers that pass through the sympathetic trunk (without forming a synapse with a postganglionic neuron) synapse here. Prevertebral ganglia lie near the large abdominal arteries, which the preganglionic fibers target.
Sympathetic nervous system. Cell bodies of the preganglionic neurons occur in the lateral horns of gray matter of the 12 thoracic and first 2 lumbar segments of the spinal cord. (For this reason, the sympathetic system is also called the thoracolumbar division.) Preganglionic fibers leave the spinal cord within spinal nerves through the ventral roots (together with the PNS motor neurons). The preganglionic fibers then branch away from the nerve through white rami (white rami communicantes) that connect with the sympathetic trunk. White rami are white because they contain myelinated fibers. A preganglionic fiber that enters the trunk may synapse in the first ganglion it enters, travel up or down the trunk to synapse with another ganglion, or pass through the trunk and synapse outside the trunk. Postganglionic fibers that originate in ganglia within the sympathetic trunk leave the trunk through gray rami (gray rami communicantes) and return to the spinal nerve, which is followed until it reaches its target organ. Gray rami are gray because they contain unmyelinated fibers.
Parasympathetic nervous system. Cell bodies of the preganglionic neurons occur in the gray matter of sacral segments S2-S4 and in the brain stem (with motor neurons of their associated cranial nerves III, VII, IX, and X). (For this reason, the parasympathetic system is also called the craniosacral division, and the fibers arising from this division are called the cranial outflow or the sacral outflow, depending upon their origin.) Preganglionic fibers of the cranial outflow accompany the PNS motor neurons of cranial nerves and have terminal ganglia that lie near the target organ. Preganglionic fibers of the sacral outflow accompany the PNS motor neurons of spinal nerves. These nerves emerge through the ventral roots of the spinal cord and have terminal ganglia that lie near the target organ.
Vegetative functions are those bodily processes most directly concerned with maintenance of life. This category encompasses nutritional, metabolic, and endocrine functions including eating, sleeping, menstruation, bowel function, bladder activity, and sexual performance. These functions can be altered by a wide variety of psychologic states.
Problems in vegetative function are so frequent that every patient with an emotional disorder should be asked about disturbances in food intake, elimination, menstruation, and sleep. What the clinician primarily investigates is a psychologically induced change, which may be either increased or decreased, in the patient's usual pattern.
By the time questions related to vegetative function are explored, the physician will have already sought for evidence of anxiety, depression, or interpersonal difficulties in other parts of the psychiatric database. Then the physician determines whether there is an association between the vegetative function disturbances and emotional conflicts. In doing this, it is helpful to ask such questions as the following: "Did the bodily disturbance (e.g., anorexia) begin during a time of emotional stress? Does it become worse when emotional stress increases? Does it vary in different situations?"
With the exception of the sexual area, most patients do not find it difficult to discuss problems related to their vegetative functions. Almost everyone has experienced disturbances in these bodily functions at some time, and there is little or no stigma attached to admitting to these difficulties. There is usually a temporal and a quantifiable relationship between the emotional symptoms and disturbance in vegetative function. Increase or decrease in emotional symptoms is often accompanied by concomitant changes in the disturbance of vegetative function. Characteristically, increased emotional stress is associated with increased vegetative dysfunction.
It is also important when exploring this area to ask in a general way about any disturbances of physical function for which past physicians could find no cause. The patient can be asked: "Have you ever had any physical problem for which your physician could find no cause?" The patient could also be asked: "Have you ever been told that you were having physical symptoms as a result of nervousness, depression, or stress?"
It is important to ask patients specifically about the presence of any eating disorders, such as anorexia nervosa or bulimia, both of which are discussed later in this chapter. Patients with either disorder are often very secretive. They will almost never volunteer any information regarding their symptoms. Nevertheless, when asked directly about binge eating, self-induced vomiting, or use of cathartics or diuretics in order to lose weight, many patients will admit to these activities. In addition, the physician should always be alert for the possibility of anorexia nervosa in any female patient who appears emaciated.
The early work of investigators Flanders Dunbar, Franz Alexander, W. B. Cannon, Hans Selye, and others have provided validation of the concept that emotional conflicts can result in changes in physical function. Efforts to link specific personality types or specific psychological conflicts with specific psychophysiological disorders have been attempted many times. For example, the type A personality has been described as being particularly prone to coronary occlusion. The type A personality is typically competitive, restless, and preoccupied with time. Such individuals characteristically also have physiologic findings that include high plasma triglycerides, hyperinsulinemic response to glucose challenge, increased blood cholesterol levels, and increased levels of norepinephrine in urine. Despite the fact that many patients with coronary artery disease appear to fit the type A personality, many patients with coronary artery disease do not fit this personality type. While it seems reasonable on the basis of current investigations to view patients who have a type A personality as being more prone to coronary disease, it also seems clear that this is by no means the entire explanation for this condition.
John Nemiah and Peter Sifneos (1970) have postulated the interesting concept of alexithymia. Alexithymia refers to the condition of being unable to express feeling tones verbally. In this hypothesis, psychosomatic symptoms are developed as an alternative expression of affect as a result of the inability to express and deal with feelings verbally.
Modern neurologic research has made it much easier to understand how emotional conflicts can result in changes in vegetative function. Many of the neuronal circuits controlling emotions are centered in the limbic system of the brain. The limbic system has many pathways connecting to autonomic centers in the hypothalamus. When emotional stress leads to increased limbic system activity, there are ample neuronal connections for transmission of this increased activity into hypothalamic areas that control autonomic function. Changes in the output of these autonomic centers pass through the autonomic nervous system to end organs such as the bowel and bladder. Presumably asthma, hypertension, peptic ulcer, and other psychophysiologic disorders are the result, at least in part, of long-continued overactivity of the autonomic nervous system on the various end organs.
The extent to which usual vegetative function is disrupted by emotional conflict allows the clinician to make a rough judgment of the severity of the emotional disturbance. A psychiatric condition in which there is an accompanying disturbance in vegetative function is in general more severe than the same condition without such a bodily disturbance. The presence of a distinct change in vegetative function is of more significance than the direction of the change, since patients with the same emotional symptoms may show opposite changes in bodily function. For example, most depressed patients have decreased appetite, but some such patients overeat, as is described below.
Food is of strong emotional significance. Infants are repeatedly comforted by being offered food. Many people associate the process of eating with feelings of security, comfort, and happiness. For some, eating can become a means of alleviating mild anxiety or depression. This tendency to eat in response to stress is thought to be a factor in some cases of obesity. Although some patients react to depression by overeating, these are usually those in whom depression is mild. The majority of patients with significant depression have a distinct loss of appetite. In a somewhat similar way, an occasional patient with anxiety may react by increasing food consumption. The large majority of patients with moderate to severe anxiety have some degree of decrease in appetite, although characteristically this is not as marked as is seen in depression.
Anorexia nervosa is a particularly important disturbance of eating. Patients with this condition have an intense fear of becoming obese, and this fear does not subside as weight loss progresses. Unless adequately treated, the persistent refusal of these patients to eat may lead to death from complications of starvation. Bulimia is another eating disorder that is of clinical importance. Bulimia refers to the condition in which patients experience recurrent episodes during which large amounts of food are consumed in a short period of time. These episodes are typically referred to as binges. Patients with bulimia frequently terminate the episodes with self-induced vomiting. Patients with either anorexia nervosa or bulimia may use cathartics or diuretics in an effort to lose weight. More than 90% of patients with anorexia nervosa are female, as are a large majority of patients with bulimia. Although fatalities occur less often from bulimia than from anorexia nervosa, serious medical complications can result from bulimia, including esophagitis, dental damage, and toxicity from use of cathartics or diuretics.
Disturbances in sleep involve difficulties in getting to sleep, staying asleep, and in quality of sleep. Difficulty falling asleep occurs in many patients who have either anxiety or depression. A pattern of insomnia that occurs primarily in depression is one in which the patient is able to fall asleep but awakens after a few hours and then is unable to return to sleep. Many patients with emotional conflicts are troubled by disturbing dreams. Such patients often complain of feeling very tired when they awaken in the morning. Some patients respond to emotional stress by withdrawal. The clinician should remember that one form of withdrawal can be sleep. A minority of such patients, much more frequently depressed patients than anxious ones, will sleep excessively.
In the presence of marked emotional stress, female patients not infrequently show a change in their menstrual pattern. Menstrual abnormalities occur in several psychiatric conditions. Patients with marked depression often show a decrease in menstruation that may progress to cessation of menstruation. Amenorrhea also occurs in anorexia nervosa. In these patients, the amenorrhea is usually secondary to starvation. Amenorrhea also occurs in pseudocyesis, which is a condition of false pregnancy found in certain women who have psychologic conflicts around an intense desire to become pregnant.
Changes in bowel habits are frequent in emotional disturbances. Diarrhea often occurs during anxiety states. Constipation frequently accompanies depression.
Disturbances of genitourinary function are infrequent in depression. However, the presence of anxiety is often manifest by increased frequency of urination.
Vegetative reflexes are reflexes through the vegetative nervous system (sympathetic and parasympathetic). There is a large number of short and long vegetative reflexes which "close" the nervous circuit in the brain, the spinal cord, in the big nervous ganglia or in smaller peripheral ganglia. There are not only segmental reflexes. Many vegetative reflexes have been describe in medicine. As an example I will mention the segmental and suprasegmental reflexes that are prodused due to local biochemical changes and tissue damage in patiens with acute myocardial ischemia (AMI). This reflex is known as Bezold-Jarich reflex (abnormal vagovagal reflex) and produce severe bradycardia, peripheral vasodilation, severe hypotension and atrioventricular block. These reflexes involves afferents and efferents of both cardiac vagi and cardiac sympathetic nerves which produse sympathosympathetic reflexes. In the AMI patiens exist also suprasegmental reflex responses result from nociceptively induced stimulation of the medullary centers, hypothalamic centers, limbic structures and neuroendocrine function.
The great plexuses of the autonomic system are the cardiac; celiac, or solar; and hypogastric plexus. While these plexuses are regarded as essentially sympathetic, they also receive fibers from the parasympathetic system. The cardiac plexus lies under the arch of the aorta just above the heart. It receives branches from the cervical sympathetic ganglia and from the right and left vagal nerves (parasympathetic) and has a regulatory effect on the heart. The celiac, or solar, plexus is the largest network of cells and fibers of the autonomic system. It lies behind the stomach and is associated with the aorta and the celiac arteries. The ganglia receive the splanchnic nerves from the sympathetic system and branches of the vagus from the parasympathetic system. A blow to this region may slow the heart, reduce the flow of blood to the head, and depress the breathing mechanism.
The hypogastric plexus forms a connection between the celiac plexus above and the two pelvic plexuses below. It is located in front of the fifth lumbar vertebra and continues downward in front of the sacrum, forming the right and left pelvic plexuses. These plexuses supply the organs and blood vessels of the pelvis.
PARASYMPATHETIC, OR CRANIOSACRAL, DIVISION.
The craniosacral, or parasympathetic, division of the autonomic nervous system is associated with certain cranial and sacral nerves in which autonomic fibers are incorporated; hence the name craniosacral division. The oculomotor (Hid cranial) nerve, arising in the midbrain, innervates certain voluntary muscles that move the eyeball; in addition, it carries parasympathetic fibers to involuntary muscles within the eyeball. Preganglionic fibers are distributed to the ciliary’s ganglion located behind the eyeball. Postganglionic fibers arising in the ganglion extend to the ciliary’s muscles of the eye and to the sphincter of the pupil. The facial (Vll th cranial), glossopharyngeal (IX th cranial), vagus (X th cranial), and accessory (Xl th cranial) nerves constitute a group of cranial nerves arising from the medulla. Since they also contain parasympathetic fibers, they are a part of the craniosacral division. The vagus supplies the viscera of the thorax and abdomen; this may be the reason why there are no parasympathetic fibers arising from the thoracic or lumbar regions of the cord.
The sacral portion of this system is identified with certain sacral nerves that carry parasympathetic fibers to the pelvic viscera.
PREGANGLIONIC AND POSTGANGLIONIC FIBERS. Typically the parasympathetic preganglionic fiber extends from its nucleus in the brain or sacral region of the spinal cord to the organ supplied. The postganglionic fiber is often a very short fiber located within the organ itself. The preganglionic fiber can end in a collateral ganglion, as in the case of preganglionic fibers extending out to the ciliary’s ganglion of the eye. The postganglionic fibers in this case are longer than those incorporated within certain organs.
The parasympathetic system functions as an antagonist of the sympathetic system if an organ is supplied by both systems. If the sympathetic system is the accelerator system, as in the heart, for example, then the parasympathetic system is the inhibitor. Its function in this case is to slow the accelerated heart and thus restore the normal heart rate. Even though it acts as an inhibitor, it does not ordinarily depress the heart rate below normal unless unduly stimulated, as from the action of drugs or pressure on a nerve.
d) Difference between autonimuc and somatic
nervous system (1. Nervous centres in autonimuc nervous system are present in
mesencephalon, bulbar part of brain, thoraco-lumbal and sacral part of spinal
cord, in somatic – diffuse in all sentral nervous system; 2. Efference ways of
reflector arc in autonimuc nervous system consist of two neurons, in somatic –
CRANIAL NERVES THAT CARRY PARASYMPATHETIC FIBERS. If the oculomotor nerve is cut experimentally, the pupil dilates. The parasympathetic fibers within the oculomotor nerve carry nervous impulses that cause the pupil to constrict. Cutting the nerve destroys the balance between parasympathetic and sympathetic innervation. The sympathetic nervous impulses then cause the pupil to dilate. The "drops" placed in the eye for optical examination apparently act in much the same way by blocking the parasympathetic nerve endings.
It has been mentioned that there are four cranial nerves arising from the medulla that carry autonomic fibers and therefore are a part of the craniosacral system. These nerves are the facial glossopharyngeal, vagus, and accessory nerves. The facial nerve includes parasympathetic fibers that are secretary to the lacrimal gland and to the sublingual and submaxillary salivary glands. The lacrimal gland is supplied with postganglionic fibers from the sphenopalatine ganglion. The sublingual and submaxillary salivary glands receive postganglionic fibers arising in the submaxillary ganglion.
Preganglionic fibers in the glossopharyngeal nerve extend outward to the optic ganglion. Postganglionic fibers arise in the otic ganglion and supply the parotid salivary gland. These glands, including the lacrimal, have a double innervation. They derive their sympathetic innervation by way of the superior cervical sympathetic ganglion and carotid plexuses. The action of the two sets of nerves is not clear. Apparently they both contain secretory fibers, but the secretory action of the parasympathetic system seems to be dominant. The vagus nerve contains both motor and visceral afferent fibers. The motor fibers are long preganglionic fibers that extend out to the organ supplied. Very short postganglionic fibers are contained within the organ. Motor fibers are supplied to the larynx, trachea, bronchioles, heart, esophagus, stomach, small intestine, and some parts of the large intestine. Stimulation of the vagus acts as an inhibitor to the heart, causing its rate of beating to slow or to stop. To the muscles of the wall of the digestive tract, branches of the vagus act as accelerator nerves. Peristalsis is increased by parasympathetic stimulation. Parasympathetic fibers to the glands of the digestive tract have regulatory function on secretion, but food content of the stomach or intestine and hormones circulating in the blood can also stimulate secretion.
Parasympathetic fibers from both the right and left vagus nerves enter the great plexuses of the sympathetic system. There is, however, a definite parasympathetic nerve supply to such organs as the pancreas, liver, and kidneys. Nervous stimulation of these organs is, for the most part, merely regulatory. Hormones in the blood normally cause the pancreas and liver to secrete, but stimulation of the vagus increases the flow of pancreatic juice and bile. While sympathetic stimulation of the kidneys by way of the splanchnic nerves results in vasoconstriction and therefore reduced flow of urine, there are many other physiological factors that affect the function of the kidneys. A part of the accessory nerve contains visceral motor and cardiac inhibitory fibers. Certain types of allergy offer examples of overstimulation of the parasympathetic system. Epinephrine can be used to counteract these effects, since it is associated with the action of the sympathetic system.