Sympathetic vs. Parasympathetic
The autonomic nervous system (ANS) regulates the functions of our internal The sympathetic nervous system; The parasympathetic nervous system; The. The parasympathetic nervous system (PSNS) is one of the two divisions of the autonomic nervous. The sympathetic nervous system (SNS) is part of the autonomic nervous The sympathetic nervous system activates what is often termed the fight or flight.
The postganglionic parasympathetic fibers leave the pterygopalatine ganglion in several directions. One division leaves on the zygomatic division of CN V2 and travels on a communicating branch to unite with the lacrimal nerve branch of the ophthalmic nerve of CN V1 before synapsing at the lacrimal gland.
These parasympathetic to the lacrimal gland control tear production. A separate group of parasympathetic leaving from the pterygopalatine ganglion are the descending palatine nerves CN V2 branchwhich include the greater and lesser palatine nerves.
The greater palatine parasympathetic synapse on the hard palate and regulate mucus glands located there. The lesser palatine nerve synapses at the soft palate and controls sparse taste receptors and mucus glands.
Yet another set of divisions from the pterygopalatine ganglion are the posterior, superior, and inferior lateral nasal nerves; and the nasopalatine nerves all branches of CN V2, maxillary division of the trigeminal nerve that bring parasympathetic innervation to glands of the nasal mucosa.
The second parasympathetic branch that leaves the facial nerve is the chorda tympani. This nerve carries secretomotor fibers to the submandibular and sublingual glands. The chorda tympani travels through the middle ear and attaches to the lingual nerve mandibular division of trigeminal, CN V3.
After joining the lingual nerve, the preganglionic fibers synapse at the submandibular ganglion and send postganglionic fibers to the sublingual and submandibular salivary glands. The glossopharyngeal nerve has parasympathetic fibers that innervate the parotid salivary gland. The preganglionic fibers depart CN IX as the tympanic nerve and continue to the middle ear where they make up a tympanic plexus on the cochlear promontory of the mesotympanum. The tympanic plexus of nerves rejoin and form the lesser petrosal nerve and exit through the foramen ovale to synapse at the otic ganglion.
From the otic ganglion postganglionic parasympathetic fibers travel with the auriculotemporal nerve mandibular branch of trigeminal, CN V3 to the parotid salivary gland. Vagus nerve[ edit ] The vagus nerve, named after the Latin word vagus because the nerve controls such a broad range of target tissues — vagus in Latin literally means "wandering"has parasympathetic that originate in the dorsal nucleus of the vagus nerve and the nucleus ambiguus in the CNS.
Autonomic nervous system - Wikipedia
The vagus nerve is an unusual cranial parasympathetic in that it doesn't join the trigeminal nerve in order to get to its target tissues. Another peculiarity is that the vagus has an autonomic ganglion associated with it at approximately the level of C1 vertebra. The vagus gives no parasympathetic to the cranium. The vagus nerve is hard to track definitively due to its ubiquitous nature in the thorax and abdomen so the major contributions will be discussed.
Several parasympathetic nerves come off the vagus nerve as it enters the thorax. One nerve is the recurrent laryngeal nervewhich becomes the inferior laryngeal nerve. From the left vagus nerve the recurrent laryngeal nerve hooks around the aorta to travel back up to the larynx and proximal esophagus while, from the right vagus nerve, the recurrent laryngeal nerve hooks around the right subclavian artery to travel back up to the same location as its counterpart.
These different paths are a direct result of embryological development of the circulatory system. Each recurrent laryngeal nerve supplies the trachea and the esophagus with parasympathetic secretomotor innervation for glands associated with them and other fibers that are not PN.
Another nerve that comes off the vagus nerves approximately at the level of entering the thorax are the cardiac nerves. These cardiac nerves go on to form cardiac and pulmonary plexuses around the heart and lungs. As the main vagus nerves continue into the thorax they become intimately linked with the esophagus and sympathetic nerves from the sympathetic trunks to form the esophageal plexus.
This is very efficient as the major function of the vagus nerve from there on will be control of the gut smooth muscles and glands. As the esophageal plexus enter the abdomen through the esophageal hiatus anterior and posterior vagus trunks form. The vagus trunks then join with preaortic sympathetic ganglion around the aorta to disperse with the blood vessels and sympathetic nerves throughout the abdomen. The extent of the parasympathetic in the abdomen include the pancreaskidneyslivergall bladderstomach and gut tube.
The vagus contribution of parasympathetic continues down the gut tube until the end of the midgut. The midgut ends two thirds of the way across the transverse colon near the splenic flexure. Poor Joe stood by his truck wide-eyed and clutching his chest. His heart began racing, his blood pressure increased, his pupils dilated, he began sweating, the hair on his arms and the back of his neck stood on end, and he felt a surge of adrenaline.
These are some of the effects of sympathetic nervous activity in Joe's body. Meanwhile, as we waited for Joe's early morning arrival, the events occurring in my body were quite different.
My heart rate was comparatively slower and my digestive system was processing the cream and sugar in my coffee. These are some of the effects of parasympathetic nervous activity. I tell my students that during the next several class periods they will learn in great detail about the many functions of the sympathetic and parasympathetic nervous systems, the neurotransmitters released by their neurons, the receptors to which they bind, and how it is all regulated.
At this point, the students often look as afraid as Joe did that Halloween morning. I reassure them and remind them repeatedly that it is not necessary to memorize very much at all.
I encourage them to let it make sense. In other words, this system prepares the body for strenuous physical activity. The events that we would expect to occur within the body to allow this to happen do, in fact, occur. In other words, this system controls basic bodily functions while one is sitting quietly reading a book. Specific learning objectives for the discussion of the autonomic nervous system include the following: Explain how various regions of the central nervous system regulate autonomic nervous system function; Explain how autonomic reflexes contribute to homeostasis; Describe how the neuroeffector junction in the autonomic nervous system differs from that of a neuron-to-neuron synapse; Compare and contrast the anatomical features of the sympathetic and parasympathetic systems; For each neurotransmitter in the autonomic nervous system, list the neurons that release them and the type and location of receptors that bind with them; Describe the mechanism by which neurotransmitters are removed; Distinguish between cholinergic and adrenergic receptors; Describe the overall and specific functions of the sympathetic system; Describe the overall and specific functions of the parasympathetic system; and Explain how the effects of the catecholamines differ from those of direct sympathetic stimulation.
In many of these reflexes, sensory information is transmitted to homeostatic control centers, in particular, those located in the hypothalamus and brainstem.
Much of the sensory input from the thoracic and abdominal viscera is transmitted to the brainstem by afferent fibers of cranial nerve X, the vagus nerve.
Other cranial nerves also contribute sensory input to the hypothalamus and the brainstem. This input is integrated and a response is carried out by the transmission of nerve signals that modify the activity of preganglionic autonomic neurons.
Many important variables in the body are monitored and regulated in the hypothalamus and the brainstem including heart rate, blood pressure, gastrointestinal peristalsis and glandular secretion, body temperature, hunger, thirst, plasma volume, and plasma osmolarity.
An example of this type of autonomic reflex is the baroreceptor reflex. Baroreceptors located in some of the major systemic arteries are sensory receptors that monitor blood pressure. If blood pressure decreases, the number of sensory impulses transmitted from the baroreceptors to the vasomotor center in the brainstem also decreases.
As a result of this change in baroreceptor stimulation and sensory input to the brainstem, ANS activity to the heart and blood vessels is adjusted to increase heart rate and vascular resistance so that blood pressure increases to its normal value. These neural control centers in the hypothalamus and the brainstem may also be influenced by higher brain areas.
Specifically, the cerebral cortex and the limbic system influence ANS activities associated with emotional responses by way of hypothalamic-brainstem pathways. For example, blushing during an embarrassing moment, a response most likely originating in the frontal association cortex, involves vasodilation of blood vessels to the face.
Other emotional responses influenced by these higher brain areas include fainting, breaking out in a cold sweat, and a racing heart rate. Some autonomic reflexes may be processed at the level of the spinal cord. These include the micturition reflex urination and the defecation reflex. Although these reflexes are subject to influence from higher nervous centers, they may occur without input from the brain. The preganglionic neuron originates in the CNS with its cell body in the lateral horn of the gray matter of the spinal cord or in the brainstem.
- [Relationship between the sympathetic and parasympathetic nervous systems].
- Sympathetic vs. Parasympathetic
- Multilevel interactions between the sympathetic and parasympathetic nervous systems: a minireview.
The axon of this neuron travels to an autonomic ganglion located outside the CNS, where it synapses with a postganglionic neuron. This neuron innervates the effector tissue. Synapses between the autonomic postganglionic neuron and effector tissue—the neuroeffector junction—differ greatly from neuron-to-neuron synapses. The postganglionic fibers in the ANS do not terminate in a single swelling like the synaptic knob, nor do they synapse directly with the cells of a tissue.
Instead, where the axons of these fibers enter a given tissue, they contain multiple swellings called varicosities. When the neuron is stimulated, these varicosities release neurotransmitters along a significant length of the axon and, therefore, over a large surface area of the effector tissue.
The neurotransmitter diffuses through the interstitial fluid to wherever its receptors are located in the tissue. This diffuse release of the neurotransmitter affects many tissue cells simultaneously. Furthermore, cardiac muscle and most smooth muscle have gap junctions between cells.
These specialized intercellular communications allow for the spread of electrical activity from one cell to the next. As a result, the discharge of a single autonomic nerve fiber to an effector tissue may alter the activity of the entire tissue. Divisions of the Autonomic Nervous System The ANS is composed of 2 anatomically and functionally distinct divisions, the sympathetic system and the parasympathetic system.
Both systems are tonically active. In other words, they provide some degree of nervous input to a given tissue at all times. Therefore, the frequency of discharge of neurons in both systems can either increase or decrease.
As a result, tissue activity may be either enhanced or inhibited. This characteristic of the ANS improves its ability to more precisely regulate a tissue's function. Without tonic activity, nervous input to a tissue could only increase. Many tissues are innervated by both systems. Because the sympathetic system and the parasympathetic system typically have opposing effects on a given tissue, increasing the activity of one system while simultaneously decreasing the activity of the other results in very rapid and precise control of a tissue's function.
Table 1 Open in a separate window Each system is dominant under certain conditions. The overall effect of the sympathetic system under these conditions is to prepare the body for strenuous physical activity.What is the difference between the sympathetic and parasympathetic nervous systems
More specifically, sympathetic nervous activity will increase the flow of blood that is well-oxygenated and rich in nutrients to the tissues that need it, in particular, the working skeletal muscles. The parasympathetic system predominates during quiet, resting conditions.
The overall effect of the parasympathetic system under these conditions is to conserve and store energy and to regulate basic body functions such as digestion and urination. Sympathetic Division The preganglionic neurons of the sympathetic system arise from the thoracic and lumbar regions of the spinal cord segments T1 through L2. Most of these preganglionic axons are short and synapse with postganglionic neurons within ganglia found in the sympathetic ganglion chains.
These ganglion chains, which run parallel immediately along either side of the spinal cord, each consist of 22 ganglia. The preganglionic neuron may exit the spinal cord and synapse with a postganglionic neuron in a ganglion at the same spinal cord level from which it arises. The preganglionic neuron may also travel more rostrally or caudally upward or downward in the ganglion chain to synapse with postganglionic neurons in ganglia at other levels. In fact, a single preganglionic neuron may synapse with several postganglionic neurons in many different ganglia.
Overall, the ratio of preganglionic fibers to postganglionic fibers is about 1: The long postganglionic neurons originating in the ganglion chain then travel outward and terminate on the effector tissues.
This divergence of the preganglionic neuron results in coordinated sympathetic stimulation to tissues throughout the body.
Physiology of the Autonomic Nervous System
The concurrent stimulation of many organs and tissues in the body is referred to as a mass sympathetic discharge. Other preganglionic neurons exit the spinal cord and pass through the ganglion chain without synapsing with a postganglionic neuron.
Instead, the axons of these neurons travel more peripherally and synapse with postganglionic neurons in one of the sympathetic collateral ganglia. 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. 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.
Sympathetic vs Parasympathetic Nervous System | easybiologyclass
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.