Autonomic Nervous System - Part 1
The sympathetic division of the autonomic nervous system influences the .. This figure shows the connection between autonomic fibers and the target effectors. The autonomic nervous system (ANS) is a network of nerves and ganglia that The sympathetic division regulates the use of metabolic resources and . arrest have been reported in association with the Arnold‐Chiari malformation type 1. The Autonomic Nervous System (ANS) is the involuntary division of the nervous system. Sympathetic nervous system (SNS) - the SNS triggers what is commonly the PNS have a complementary relationship, rather than one of opposition.
Instead, it projects through one of the splanchnic nerves to a collateral ganglion or the adrenal medulla not pictured. An axon from the central neuron that projects to a sympathetic ganglion is referred to as a preganglionic fiber or neuron, and represents the output from the CNS to the ganglion.
Because the sympathetic ganglia are adjacent to the vertebral column, preganglionic sympathetic fibers are relatively short, and they are myelinated. A postganglionic fiber—the axon from a ganglionic neuron that projects to the target effector—represents the output of a ganglion that directly influences the organ.
Compared with the preganglionic fibers, postganglionic sympathetic fibers are long because of the relatively greater distance from the ganglion to the target effector. These fibers are unmyelinated.
The problem with that usage is that the cell body is in the ganglion, and only the fiber is postganglionic. Typically, the term neuron applies to the entire cell. One type of preganglionic sympathetic fiber does not terminate in a ganglion.
These are the axons from central sympathetic neurons that project to the adrenal medulla, the interior portion of the adrenal gland. These axons are still referred to as preganglionic fibers, but the target is not a ganglion. The adrenal medulla releases signaling molecules into the bloodstream, rather than using axons to communicate with target structures. The cells in the adrenal medulla that are contacted by the preganglionic fibers are called chromaffin cells.
These cells are neurosecretory cells that develop from the neural crest along with the sympathetic ganglia, reinforcing the idea that the gland is, functionally, a sympathetic ganglion. The projections of the sympathetic division of the autonomic nervous system diverge widely, resulting in a broad influence of the system throughout the body. As a response to a threat, the sympathetic system would increase heart rate and breathing rate and cause blood flow to the skeletal muscle to increase and blood flow to the digestive system to decrease.
Sweat gland secretion should also increase as part of an integrated response.
All of those physiological changes are going to be required to occur together to run away from the hunting lioness, or the modern equivalent. This divergence is seen in the branching patterns of preganglionic sympathetic neurons—a single preganglionic sympathetic neuron may have 10—20 targets.
An axon that leaves a central neuron of the lateral horn in the thoracolumbar spinal cord will pass through the white ramus communicans and enter the sympathetic chain, where it will branch toward a variety of targets. At the level of the spinal cord at which the preganglionic sympathetic fiber exits the spinal cord, a branch will synapse on a neuron in the adjacent chain ganglion. Some branches will extend up or down to a different level of the chain ganglia.
Other branches will pass through the chain ganglia and project through one of the splanchnic nerves to a collateral ganglion. Finally, some branches may project through the splanchnic nerves to the adrenal medulla.
All of these branches mean that one preganglionic neuron can influence different regions of the sympathetic system very broadly, by acting on widely distributed organs. The parasympathetic system can also be referred to as the craniosacral system or outflow because the preganglionic neurons are located in nuclei of the brain stem and the lateral horn of the sacral spinal cord. The preganglionic fibers from the cranial region travel in cranial nerves, whereas preganglionic fibers from the sacral region travel in spinal nerves.
The targets of these fibers are terminal ganglia, which are located near—or even within—the target effector. These ganglia are often referred to as intramural ganglia when they are found within the walls of the target organ. The postganglionic fiber projects from the terminal ganglia a short distance to the target effector, or to the specific target tissue within the organ.
Comparing the relative lengths of axons in the parasympathetic system, the preganglionic fibers are long and the postganglionic fibers are short because the ganglia are close to—and sometimes within—the target effectors. The cranial component of the parasympathetic system is based in particular nuclei of the brain stem. In the midbrain, the Edinger—Westphal nucleus is part of the oculomotor complex, and axons from those neurons travel with the fibers in the oculomotor nerve cranial nerve III that innervate the extraocular muscles.
The preganglionic parasympathetic fibers within cranial nerve III terminate in the ciliary ganglion, which is located in the posterior orbit. The postganglionic parasympathetic fibers then project to the smooth muscle of the iris to control pupillary size. In the upper medulla, the salivatory nuclei contain neurons with axons that project through the facial and glossopharyngeal nerves to ganglia that control salivary glands.
Tear production is influenced by parasympathetic fibers in the facial nerve, which activate a ganglion, and ultimately the lacrimal tear gland. Neurons in the dorsal nucleus of the vagus nerve and the nucleus ambiguus project through the vagus nerve cranial nerve X to the terminal ganglia of the thoracic and abdominal cavities. Parasympathetic preganglionic fibers primarily influence the heart, bronchi, and esophagus in the thoracic cavity and the stomach, liver, pancreas, gall bladder, and small intestine of the abdominal cavity.
The postganglionic fibers from the ganglia activated by the vagus nerve are often incorporated into the structure of the organ, such as the mesenteric plexus of the digestive tract organs and the intramural ganglia.
Neurons from brain-stem nuclei, or from the lateral horn of the sacral spinal cord, project to terminal ganglia near or within the various organs of the body. Somatic neurons send axons to skeletal muscle, which is usually under conscious control. Visceral efferent neurons- motor neurons whose job it is to conduct impulses to cardiac muscle, smooth muscles and glands.
They may originate in the brain or spinal cord CNS. Two visceral efferent neurons are required to conduct an impulse from the brain or spinal cord to the target tissue.
Preganglionic presynaptic neurons- the cell body of the neuron is located in the grey matter of the spinal cord or brain. It ends in a sympathetic or parasympathetic ganglion. Preganglionic autonomic fibers- may begin in the hindbrain, midbrain, upper thoracic spinal cord, or fourth sacral level of the spinal cord.
Autonomic ganglia may be found in the head, neck or abdomen. Chains of autonomic ganglia also run parallel to each side of the spinal cord.
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Postganglionic postsynaptic neurons- cell body is located in the autonomic ganglion sympathetic or parasympathetic. The neuron ends in a visceral structure the target tissue Where preganglionic fibers originate and autonomic ganglia are found helps in differentiating between the sympathetic nervous system and the parasympathetic nervous system.
They are located at approximately the level of the 12th thoracic vertebrae. The adrenal gland has two parts, an outer cortex and an inner medulla. Both parts produce hormones: The medulla releases epinephrine and norepinephrine when the body responds to a stressor i. The cells of the adrenal medulla are derived from the same embryonic tissue as sympathetic postganglionic neurons; therefore the medulla is akin to a modified sympathetic ganglion.
The cells of the medulla are innervated by sympathetic preganglionic fibers. In response to neural stimulation, the medulla secretes epinephrine into the bloodstream.
Epinephrine effects are similar to norepinephrine. The hormones produced by the adrenal glands are crucial to normal healthy functioning of the body. Cortisol released as a response to chronic stress or increased sympathetic tone can be damaging to the body i. If the body is stressed for a prolonged period of time, cortisol levels may be insufficient adrenal fatiguecausing low blood sugar, excessive tiredness and muscle pain.
Parasympathetic Craniosacral Division The parasympathetic division of the autonomic nervous system is often referred to as the craniosacral division. This is due to the fact that cell bodies of preganglionic neurons are located in the brain stem nuclei, and also in the lateral grey horns of the 2nd through the 4th sacral segments of the spinal cord; hence, the term craniosacral is often used to refer to the parasympathetic division.
When faced with situations of intense fear, anxiety or stress, the body reacts by speeding up the heart rate, increasing blood flow to vital organs and muscles, slowing digestion, making changes to our vision to allow us to see better and numerous other changes that allow us to react quickly in dangerous or stressful situations. These reactions have allowed us to survive as a species for thousands of years. These ganglia are located about halfway between the CNS and the effector tissue.
Finally, the preganglionic neuron may travel to the adrenal medulla and synapse directly with this glandular tissue. The cells of the adrenal medulla have the same embryonic origin as neural tissue and, in fact, function as modified postganglionic neurons. Instead of the release of neurotransmitter directly at the synapse with an effector tissue, the secretory products of the adrenal medulla are picked up by the blood and travel throughout the body to all of the effector tissues of the sympathetic system.
An important feature of this system, which is quite distinct from the parasympathetic system, is that the postganglionic neurons of the sympathetic system travel within each of the 31 pairs of spinal nerves.
This allows for the distribution of sympathetic nerve fibers to the effectors of the skin including blood vessels and sweat glands.Sympathetic Nervous System: Crash Course A&P #14
In fact, most innervated blood vessels in the entire body, primarily arterioles and veins, receive only sympathetic nerve fibers. Therefore, vascular smooth muscle tone and sweating are regulated by the sympathetic system only. In addition, the sympathetic system innervates structures of the head eye, salivary glands, mucus membranes of the nasal cavitythoracic viscera heart, lungs and viscera of the abdominal and pelvic cavities eg, stomach, intestines, pancreas, spleen, adrenal medulla, urinary bladder.
Parasympathetic Division The preganglionic neurons of the parasympathetic system arise from several nuclei of the brainstem and from the sacral region of the spinal cord segments S2-S4.
The axons of the preganglionic neurons are quite long compared to those of the sympathetic system and synapse with postganglionic neurons within terminal ganglia which are close to or embedded within the effector tissues. The axons of the postganglionic neurons, which are very short, then provide input to the cells of that effector tissue. The preganglionic neurons that arise from the brainstem exit the CNS through the cranial nerves. The occulomotor nerve III innervates the eyes; the facial nerve VII innervates the lacrimal gland, the salivary glands and the mucus membranes of the nasal cavity; the glossopharyngeal nerve IX innervates the parotid salivary gland; and the vagus nerve X innervates the viscera of the thorax and the abdomen eg, heart, lungs, stomach, pancreas, small intestine, upper half of the large intestine, and liver.
The preganglionic neurons that arise from the sacral region of the spinal cord exit the CNS and join together to form the pelvic nerves. These nerves innervate the viscera of the pelvic cavity eg, lower half of the large intestine and organs of the renal and reproductive systems.
Because the terminal ganglia are located within the innervated tissue, there is typically little divergence in the parasympathetic system compared to the sympathetic system. In many organs, there is a 1: Therefore, the effects of the parasympathetic system tend to be more discrete and localized, with only specific tissues being stimulated at any given moment, compared to the sympathetic system where a more diffuse discharge is possible. Neurotransmitters of the Autonomic Nervous System The 2 most common neurotransmitters released by neurons of the ANS are acetylcholine and norepinephrine.
Neurotransmitters are synthesized in the axon varicosities and stored in vesicles for subsequent release. Nerve fibers that release acetylcholine are referred to as cholinergic fibers. These include all preganglionic fibers of the ANS, both sympathetic and parasympathetic systems; all postganglionic fibers of the parasympathetic system; and sympathetic postganglionic fibers innervating sweat glands.
Nerve fibers that release norepinephrine are referred to as adrenergic fibers.
Most sympathetic postganglionic fibers release norepinephrine. Table 2 Open in a separate window As previously mentioned, the cells of the adrenal medulla are considered modified sympathetic postganglionic neurons. Instead of a neurotransmitter, these cells release hormones into the blood. Unlike true postganglionic neurons in the sympathetic system, the adrenal medulla contains an enzyme that methylates norepinephrine to form epinephrine.
The synthesis of epinephrine, also known as adrenaline, is enhanced under conditions of stress. These 2 hormones released by the adrenal medulla are collectively referred to as the catecholamines. Termination of Neurotransmitter Activity For any substance to serve effectively as a neurotransmitter, it must be rapidly inactivated or removed from the synapse or, in this case, the neuroeffector junction.
This is necessary in order to allow new signals to get through and influence effector tissue function. The primary mechanism used by cholinergic synapses is enzymatic degradation. Acetylcholinesterase hydrolyzes acetylcholine to its component choline and acetate. It is one of the fastest acting enzymes in the body and acetylcholine removal occurs in less than 1 msec. The most important mechanism for the removal of norepinephrine from the neuroeffector junction is the reuptake of this neurotransmitter into the sympathetic nerve that released it.
Norepinephrine may then be metabolized intraneuronally by monoamine oxidase MAO. The circulating catecholamines, epinephrine and norepinephrine, are inactivated by catechol-O-methyltransferase COMT in the liver. Receptors for Autonomic Neurotransmitters As discussed in the previous section, all of the effects of the ANS in tissues and organs throughout the body, including smooth muscle contraction or relaxation, alteration of myocardial activity, and increased or decreased glandular secretion, are carried out by only 3 substances, acetylcholine, norepinephrine, and epinephrine.
Furthermore, each of these substances may stimulate activity in some tissues and inhibit activity in others. How can this wide variety of effects on many different tissues be carried out by so few neurotransmitters or hormones?
The effect caused by any of these substances is determined by the receptor distribution in a particular tissue and the biochemical properties of the cells in that tissue, specifically, the second messenger and enzyme systems present within the cell.
The neurotransmitters of the ANS and the circulating catecholamines bind to specific receptors on the cell membranes of the effector tissue. All adrenergic receptors and muscarinic receptors are coupled to G proteins which are also embedded within the plasma membrane.
Receptor stimulation causes activation of the G protein and the formation of an intracellular chemical, the second messenger.
The neurotransmitter molecule, which cannot enter the cell itself, is the first messenger. The function of the intracellular second messenger molecules is to elicit tissue-specific biochemical events within the cell which alter the cell's activity.
In this way, a given neurotransmitter may stimulate the same type of receptor on 2 different types of tissue and cause 2 different responses due to the presence of different biochemical pathways within each tissue.
Acetylcholine binds to 2 types of cholinergic receptors. Nicotinic receptors are found on the cell bodies of all postganglionic neurons, both sympathetic and parasympathetic, in the ganglia of the ANS. The resulting influx of these 2 cations causes depolarization and excitation of the postganglionic neurons the ANS pathways. Muscarinic receptors are found on the cell membranes of the effector tissues and are linked to G proteins and second messenger systems which carry out the intracellular effects.
Acetylcholine released from all parasympathetic postganglionic neurons and some sympathetic postganglionic neurons traveling to sweat glands binds to these receptors. Muscarinic receptors may be either inhibitory or excitatory, depending on the tissue upon which they are found. For example, muscarinic receptor stimulation in the myocardium is inhibitory and decreases heart rate while stimulation of these receptors in the lungs is excitatory, causing contraction of airway smooth muscle and bronchoconstriction.
Furthermore, there are at least 2 subtypes of receptors in each class: All of these receptors are linked to G proteins and second messenger systems which carry out the intracellular effects. Alpha receptors are the more abundant of the adrenergic receptors.
Alpha one receptor stimulation leads to an increase in intracellular calcium. As a result, these receptors tend to be excitatory.
Alpha One Adrenergic Receptor Antagonists. Hypertension, or a chronic elevation in blood pressure, is a major risk factor for coronary artery disease, congestive heart failure, stroke, kidney failure, and retinopathy. An important cause of hypertension is excessive vascular smooth muscle tone or vasoconstriction.
Alpha 2 receptor stimulation causes a decrease in cAMP and, therefore, inhibitory effects such as smooth muscle relaxation and decreased glandular secretion. In this way, norepinephrine inhibits its own release from the sympathetic postganglionic neuron and controls its own activity. Whether this results in an excitatory or an inhibitory response depends upon the specific cell type. Beta 2 receptors tend to be inhibitory.
Beta 2 receptors have a significantly greater affinity for epinephrine than for norepinephrine. Furthermore, terminations of sympathetic pathways are not found near these receptors. Beta 1 receptors are also found on certain cells in the kidney. Stimulation of these receptors, which have a stronger affinity for norepinephrine, causes lipolysis. Sympathomimetic drugs are those that produce effects in a tissue resembling those caused from stimulation by the sympathetic nervous system.
An important use for these drugs is in the treatment of bronchial asthma which is characterized by bronchospasm. Therefore, in patients with bronchospasm, an undesirable side effect of treatment with these non-selective agents is an increase in heart rate. They are equally effective in causing bronchodilation with a much lower risk of adverse cardiovascular effects.
As such, the physiological effects caused by each system are quite predictable. In other words, all of the changes in organ and tissue function induced by the sympathetic system work together to support strenuous physical activity and the changes induced by the parasympathetic system are appropriate for when the body is resting.
Changes in organ and tissue function throughout the body are coordinated so that there is an increase in the delivery of well-oxygenated, nutrient-rich blood to the working skeletal muscles. Both heart rate and myocardial contractility are increased so that the heart pumps more blood per minute. Sympathetic stimulation of vascular smooth muscle causes widespread vasoconstriction, particularly in the organs of the gastrointestinal system and in the kidneys. Bronchodilation in the lungs facilitates the movement of air in and out of the lungs so that the uptake of oxygen from the atmosphere and the elimination of carbon dioxide from the body are maximized.
An enhanced rate of glycogenolysis breakdown of glycogen into its component glucose molecules and gluconeogenesis formation of new glucose from noncarbohydrate sources in the liver increases the concentration of glucose molecules in the blood. This is necessary for the brain as glucose is the only nutrient molecule that it can utilize to form metabolic energy.
An enhanced rate of lipolysis in adipose tissue increases the concentration of fatty acid molecules in the blood.
Skeletal muscles then utilize these fatty acids to form metabolic energy for contraction. Generalized sweating elicited by the sympathetic system enables the individual to thermoregulate during these conditions of increased physical activity and heat production. Finally, the eye is adjusted such that the pupil dilates letting more light in toward the retina mydriasis and the lens adapts for distance vision.
Physiology of the Autonomic Nervous System
The parasympathetic system decreases heart rate which helps to conserve energy under resting conditions. Salivary secretion is enhanced to facilitate the swallowing of food. Gastric motility and secretion are stimulated to begin the processing of ingested food. Intestinal motility and secretion are also stimulated to continue the processing and to facilitate the absorption of these nutrients. Both exocrine and endocrine secretion from the pancreas is promoted.
AUTONOMIC NERVOUS SYSTEM
Enzymes released from the exocrine glands of the pancreas contribute to the chemical breakdown of the food in the intestine and insulin released from the pancreatic islets promotes the storage of nutrient molecules within the tissues once they are absorbed into the body.
Another bodily maintenance type of function caused by the parasympathetic system is contraction of the urinary bladder which results in urination. Finally, the eye is adjusted such that the pupil contracts miosis and the lens adapts for near vision.