Not all axons from the central neurons terminate in the chain ganglia. Additional branches from the ventral nerve root continue through the chain and on to one of the collateral ganglia as the greater splanchnic nerve or lesser splanchnic nerve. For example, the greater splanchnic nerve at the level of T5 synapses with a collateral ganglion outside the chain before making the connection to the postganglionic nerves that innervate the stomach see Figure Collateral ganglia , also called prevertebral ganglia , are situated anterior to the vertebral column and receive inputs from splanchnic nerves as well as central sympathetic neurons.
They are associated with controlling organs in the abdominal cavity, and are also considered part of the enteric nervous system. The three collateral ganglia are the celiac ganglion , the superior mesenteric ganglion , and the inferior mesenteric ganglion see Figure 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 Fig.
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. Figure The adrenal gland sits on top of the kidneys in the abdominal cavity. When the adrenal gland is coronally sectioned, the inner adrenal medulla is visible.
The adrenal medulla contains chromaffin cells that are innervated by preganglionic sympathetic fibers. These cells release epinephrine directly into the bloodstream. 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 Eddinger—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.
Most effector targets of the ANS receive innervation from both the sympathetic and parasympathetic nervous systems. For example, the heart is innervated by both divisions of the ANS, the parasympathetic division slows down heart rate and sympathetic innervation increases heart rate. In Fig. In the table, you can see there are exceptions to rule of dual innervation.
For example, sweat glands are only innervated by the sympathetic division. Another very important exception to dual innervation is the smooth muscle in blood vessels. Most blood vessels receive only sympathetic innervation. The adrenergic receptor present on the blood vessel determines whether the response is vasoconstriction or vasodilation.
The blood vessels that supply blood to the penis and clitoris are the only blood vessels that are innervated by both the parasympathetic and sympathetic divisions. Some organs receive almost constant innervation by one or the other division. One example of this are the blood vessels.
Blood vessels receive constant innervation by the sympathetic system; if activity increases in the sympathetic fibers that innervate a blood vessel, the vessel will vasoconstrict. On the other hand, if activity decreases in the sympathetic fibers, the vessels will vasodilate.
Another example of tonic innervation is the parasympathetic innervation of the heart. Where an autonomic neuron connects with a target, there is a synapse.
The electrical signal of the action potential causes the release of a signaling molecule, which will bind to receptor proteins on the target cell. Synapses of the autonomic system are classified as either cholinergic , meaning that acetylcholine ACh is released, or adrenergic , meaning that norepinephrine is released.
The terms cholinergic and adrenergic refer not only to the signaling molecule that is released but also to the class of receptors that each binds. The cholinergic system includes two classes of receptor: the nicotinic receptor and the muscarinic receptor. Both receptor types bind to ACh and cause changes in the target cell. These neurotransmitters also called catecholamines relay the nerve signals across the gaps synapses created when the nerve connects to other nerves, cells or organs.
The neurotransmitters then attach to either sympathetic receptor sites or parasympathetic receptor sites on the target organ to exert their effect. This is a simplified version of how the autonomic nervous system functions. How is the autonomic nervous system controlled? The ANS is not under conscious control. Limbic system- the limbic system is composed of the hypothalamus, the amydala, the hippocampus, and other nearby areas. These structures lie on both sides of the thalamus, just under the cerebrum.
Hypothalamus- the cells that drive the ANS are located in the lateral medulla. The hypothalamus projects to this area, which includes the parasympathetic vagal nuclei, and also to a group of cells which lead to the sympathetic system in the spinal cord.
By interacting with these systems, the hypothalamus controls digestion, heart rate, sweating and other functions. Brain stem- the brainstem acts as the link between the spinal cord and the cerebrum. Sensory and motor neurons travel through the brainstem, conveying messages between the brain and spinal cord. The brainstem controls many autonomic functions of the PNS, including respiration, heart rate and blood pressure.
Spinal cord- two chains of ganglia are located on either side of the spinal cord. The outer chains form the parasympathetic nervous system, while the chains closest to the spinal cord form the sympathetic element. What are some receptors of the autonomic nervous system? Sensory neuron dendrites are sensory receptors that are highly specialized, receiving specific types of stimuli. We do not consciously sense impulses from these receptors except perhaps pain.
There are numerous sensory receptors: Photoreceptors- respond to light Thermoreceptors- respond to alterations in temperature Mechanoreceptors- respond to stretch and pressure blood pressure or touch Chemoreceptors- respond to changes in internal body chemistry i. In this way, visceral motor neurons can be said to indirectly innervate smooth muscles of arteries and cardiac muscle.
In addition, autonomic motor neurons can continue to function even if their nerve supply is damaged, albeit to a lesser extent. Where are the autonomic nervous system neurons located? The ANS is essentially comprised of two types of neurons connected in a series. The nucleus of the first neuron is located in the central nervous system.
SNS neurons begin at the thoracic and lumbar areas of the spinal cord, PNS neurons begin at the cranial nerves and sacral spinal cord. The first neuron's axons are located in the autonomic ganglia.
In terms of the second neuron, its nucleus is located in the autonomic ganglia, while the axons of the second neuron are located in the target tissue. The two types of giant neurons communicate using acetylcholine. Sympathetic Parasympathetic Function To defend the body against attack Healing, regeneration and nourishing the body Overall Effect Catabolic breaks down the body Anabolic builds up the body Organs and Glands It Activates The brain, muscles, the insulin pancreas, and the thyroid and adrenal glands The liver, kidneys, enzyme pancreas, spleen, stomach, small intestines and colon Hormones and Substances It Increases Insulin, cortisol and the thyroid hormones Parathyroid hormone, pancreatic enzymes, bile and other digestive enzymes Body Functions It Activates Raises blood pressure and blood sugar, and increases heat production Activates digestion, elimination and the immune system Psychological Qualities Fear, guilt, sadness, anger, willfulness, and aggressiveness.
The sympathetic branch mediates this expenditure while the parasympathetic branch serves a restorative function. In general: The sympathetic nervous system causes a speeding up of bodily functions i. The ANS affects changes in the body that are meant to be temporary; in other words, the body should return to its baseline state.
It is natural that there should be brief excursions from the homeostatic baseline, but the return to baseline should occur in a timely manner. When one system is persistently activated increased tone , health may be adversely affected. The branches of the autonomic system are designed to oppose and thus balance each other.
For example, as the sympathetic nervous system begins to work, the parasympathetic nervous system goes into action to return the sympathetic nervous system back to its baseline. Therefore, it is not difficult to understand that persistent action by one branch may cause a persistently decreased tone in the other, which can lead to ill health.
A balance between the two is both necessary and healthy. The parasympathetic nervous system has a quicker ability to respond to change than the sympathetic nervous system.
Why are we designed this way? Imagine if we weren't: exposure to a stressor causes tachycardia; if the parasympathetic system did not immediately begin to counter the increased heart rate, the heart rate could continue to increase until a dangerous rhythm, such as ventricular fibrillation, developed.
Because the parasympathetics are able to respond so quickly, dangerous situations like the one described cannot occur. The parasympathetic nervous system is the first to indicate a change in health condition in the body.
The parasympathetics are the main influencing factor on respiratory activity. As for the heart, parasympathetic nerve fibers synapse deep within the heart muscle, while sympathetic nerve fibers synapse on the surface of the heart. Thus, parasympathetics are more sensitive to heart damage. Transmission of Autonomic Stimuli Neurons generate and propagate action potentials along their axons. They then transmit signals across a synapse through the release of chemicals called neurotransmitters, which stimulate a reaction in another effector cell or neuron.
This process may cause either stimulation or inhibition of the receiving cell, depending which neurotransmitters and receptors are involved. Individual neurons generate the same potential after receiving each stimulus and conduct the axon potential at a fixed rate of velocity along the axon.
Velocity is dependent upon the diameter of the axon and how heavily it is myelinated- speed is faster in myelinated fibers because the axon is exposed at regular intervals nodes of Ranvier. The impulse "jumps" from one node to the next, skipping myelinated sections. Transmission- transmission is chemical, resulting from the release of specific neurotransmitters from the terminal nerve ending. These neurotransmitters diffuse across the cleft of the synapse and bind to specific receptors attached to the effector cell or adjoining neuron.
Response may be excitatory or inhibitory depending on the receptor. Neurotransmitter-receptor interaction must occur and terminate quickly. This allows for repeated and rapid activation of the receptors. Neurotransmitters can be "reused" in one of three ways: Reuptake- neurotransmitters are quickly pumped back into presynaptic nerve terminals Destruction- neurotransmitters are destroyed by enzymes located near the receptors Diffusion- neurotransmitters may diffuse into the surrounding area and eventually be removed Receptors- receptors are protein complexes that cover the membrane of the cell.
Most interact primarily with postsynaptic receptors; some are located on presynaptic neurons, which allows for finer control of the release of the neurotransmitter. There are two major neurotransmitters in the autonomic nervous system: Acetylcholine- the major neurotransmitter of autonomic presynaptic fibers, postsynaptic parasympathetic fibers. Erections are controlled by the parasympathetic system through excitatory pathways.
Excitatory signals originate in the brain, through thought, sight or direct stimulation. Regardless of the origin of the excitatory signal, penile nerves respond by releasing acetylcholine and nitric oxide, which in turn signal the smooth muscles of the arteries of the penis to relax and fill with blood. This cascade of events results in erection. The Sympathetic System "Fight or Flight" response: Stimulation of the sweat glands Constriction of peripheral blood vessels to shunt blood to the core, where it is needed Increased in supply of blood to skeletal muscles that may be needed for activity Dilation of the bronchioles under conditions of low oxygen in the blood Reduction in blood flow to the abdomen; decreased peristalsis and digestive activities Release of glucose stores from the liver to increase glucose in the bloodstream As with the parasympathetic system, it is helpful to look at a real example to understand how the sympathetic nervous system functions: Extreme heat is a stressor that many of us have experienced.
When we are exposed to excessive heat, our bodies respond in the following manner: thermal receptors convey stimuli to sympathetic control centers located in the brain. Inhibitory messages are sent along the sympathetic nerves to the blood vessels in the skin, which dilate in response. This dilation of the blood vessels increases the flow of blood to the body's surface so that heat can be lost through radiation from the body surface. In addition to the dilation of blood vessels in the skin, the body also reacts to excessive heat by sweating.
This occurs through the rise in body temperature, which is sensed by the hypothalamus, which sends a signal via the sympathetic nerves to the sweat glands, which increase the amount of sweat produced. Heat is lost by evaporation of the sweat produced.
Autonomic Neurons Neurons that conduct impulses away from the central nervous system are known as efferent motor neurons. They differ from somatic motor neurons in that Efferent neurons are not under conscious control.
All ganglionic neurons—the targets of these preganglionic fibers—have nicotinic receptors in their cell membranes. The nicotinic receptor is a ligand-gated cation channel that results in depolarization of the postsynaptic membrane.
The postganglionic parasympathetic fibers also release ACh, but the receptors on their targets are muscarinic receptors, which are G protein—coupled receptors and do not exclusively cause depolarization of the postsynaptic membrane. Postganglionic sympathetic fibers release norepinephrine, except for fibers that project to sweat glands and to blood vessels associated with skeletal muscles, which release ACh Table Autonomic System Signaling Molecules. Neurotransmitters are released at synapses, whereas hormones are released into the bloodstream.
These are simplistic definitions, but they can help to clarify this point. Acetylcholine can be considered a neurotransmitter because it is released by axons at synapses. The adrenergic system, however, presents a challenge. Postganglionic sympathetic fibers release norepinephrine, which can be considered a neurotransmitter. But the adrenal medulla releases epinephrine and norepinephrine into circulation, so they should be considered hormones.
What are referred to here as synapses may not fit the strictest definition of synapse. Some sources will refer to the connection between a postganglionic fiber and a target effector as neuroeffector junctions; neurotransmitters, as defined above, would be called neuromodulators. The structure of postganglionic connections are not the typical synaptic end bulb that is found at the neuromuscular junction, but rather are chains of swellings along the length of a postganglionic fiber called a varicosity Figure 4.
Autonomic Varicosities. The concept of homeostasis and the functioning of the sympathetic system had been introduced in France in the previous century. Cannon expanded the idea, and introduced the idea that an animal responds to a threat by preparing to stand and fight or run away. The nature of this response was thoroughly explained in a book on the physiology of pain, hunger, fear, and rage.
When students learn about the sympathetic system and the fight-or-flight response, they often stop and wonder about other responses. If you were faced with a lioness running toward you as pictured at the beginning of this chapter, would you run or would you stand your ground?
Some people would say that they would freeze and not know what to do. What about fear and paralysis in the face of a threat? The sympathetic system is responsible for the physiological responses to emotional states. The primary responsibilities of the autonomic nervous system are to regulate homeostatic mechanisms in the body, which is also part of what the endocrine system does.
The key to understanding the autonomic system is to explore the response pathways—the output of the nervous system. The way we respond to the world around us, to manage the internal environment on the basis of the external environment, is divided between two parts of the autonomic nervous system. The sympathetic division responds to threats and produces a readiness to confront the threat or to run away: the fight-or-flight response.
The parasympathetic division plays the opposite role. When the external environment does not present any immediate danger, a restful mode descends on the body, and the digestive system is more active. The sympathetic output of the nervous system originates out of the lateral horn of the thoracolumbar spinal cord.
An axon from one of these central neurons projects by way of the ventral spinal nerve root and spinal nerve to a sympathetic ganglion, either in the sympathetic chain ganglia or one of the collateral locations, where it synapses on a ganglionic neuron. These preganglionic fibers release ACh, which excites the ganglionic neuron through the nicotinic receptor. The axon from the ganglionic neuron—the postganglionic fiber—then projects to a target effector where it will release norepinephrine to bind to an adrenergic receptor, causing a change in the physiology of that organ in keeping with the broad, divergent sympathetic response.
The postganglionic connections to sweat glands in the skin and blood vessels supplying skeletal muscle are, however, exceptions; those fibers release ACh onto muscarinic receptors. The sympathetic system has a specialized preganglionic connection to the adrenal medulla that causes epinephrine and norepinephrine to be released into the bloodstream rather than exciting a neuron that contacts an organ directly.
This hormonal component means that the sympathetic chemical signal can spread throughout the body very quickly and affect many organ systems at once. The parasympathetic output is based in the brain stem and sacral spinal cord. Neurons from particular nuclei in the brain stem or from the lateral horn of the sacral spinal cord preganglionic neurons project to terminal intramural ganglia located close to or within the wall of target effectors.
These preganglionic fibers also release ACh onto nicotinic receptors to excite the ganglionic neurons. The postganglionic fibers then contact the target tissues within the organ to release ACh, which binds to muscarinic receptors to induce rest-and-digest responses.
Signaling molecules utilized by the autonomic nervous system are released from axons and can be considered as either neurotransmitters when they directly interact with the effector or as hormones when they are released into the bloodstream. The same molecule, such as norepinephrine, could be considered either a neurotransmitter or a hormone on the basis of whether it is released from a postganglionic sympathetic axon or from the adrenal gland.
The synapses in the autonomic system are not always the typical type of connection first described in the neuromuscular junction. Instead of having synaptic end bulbs at the very end of an axonal fiber, they may have swellings—called varicosities—along the length of a fiber so that it makes a network of connections within the target tissue. The autonomic nervous system regulates organ systems through circuits that resemble the reflexes described in the somatic nervous system.
The main difference between the somatic and autonomic systems is in what target tissues are effectors. Somatic responses are solely based on skeletal muscle contraction.
The autonomic system, however, targets cardiac and smooth muscle, as well as glandular tissue. Whereas the basic circuit is a reflex arc , there are differences in the structure of those reflexes for the somatic and autonomic systems. One difference between a somatic reflex , such as the withdrawal reflex, and a visceral reflex , which is an autonomic reflex, is in the efferent branch.
The output of a somatic reflex is the lower motor neuron in the ventral horn of the spinal cord that projects directly to a skeletal muscle to cause its contraction. The output of a visceral reflex is a two-step pathway starting with the preganglionic fiber emerging from a lateral horn neuron in the spinal cord, or a cranial nucleus neuron in the brain stem, to a ganglion—followed by the postganglionic fiber projecting to a target effector.
The other part of a reflex, the afferent branch , is often the same between the two systems. Sensory neurons receiving input from the periphery—with cell bodies in the sensory ganglia, either of a cranial nerve or a dorsal root ganglion adjacent to the spinal cord—project into the CNS to initiate the reflex Figure.
The afferent branch of a reflex arc does differ between somatic and visceral reflexes in some instances. Many of the inputs to visceral reflexes are from special or somatic senses, but particular senses are associated with the viscera that are not part of the conscious perception of the environment through the somatic nervous system.
For example, there is a specific type of mechanoreceptor, called a baroreceptor , in the walls of the aorta and carotid sinuses that senses the stretch of those organs when blood volume or pressure increases. You do not have a conscious perception of having high blood pressure, but that is an important afferent branch of the cardiovascular and, particularly, vasomotor reflexes.
The sensory neuron is essentially the same as any other general sensory neuron. The baroreceptor apparatus is part of the ending of a unipolar neuron that has a cell body in a sensory ganglion. The baroreceptors from the carotid arteries have axons in the glossopharyngeal nerve, and those from the aorta have axons in the vagus nerve.
Though visceral senses are not primarily a part of conscious perception, those sensations sometimes make it to conscious awareness. If a visceral sense is strong enough, it will be perceived. The sensory homunculus—the representation of the body in the primary somatosensory cortex—only has a small region allotted for the perception of internal stimuli.
If you swallow a large bolus of food, for instance, you will probably feel the lump of that food as it pushes through your esophagus, or even if your stomach is distended after a large meal. If you inhale especially cold air, you can feel it as it enters your larynx and trachea. These sensations are not the same as feeling high blood pressure or blood sugar levels.
When particularly strong visceral sensations rise to the level of conscious perception, the sensations are often felt in unexpected places. For example, strong visceral sensations of the heart will be felt as pain in the left shoulder and left arm. This irregular pattern of projection of conscious perception of visceral sensations is called referred pain.
Depending on the organ system affected, the referred pain will project to different areas of the body Figure 2. Referred Pain Chart. The location of referred pain is not random, but a definitive explanation of the mechanism has not been established. The most broadly accepted theory for this phenomenon is that the visceral sensory fibers enter into the same level of the spinal cord as the somatosensory fibers of the referred pain location.
By this explanation, the visceral sensory fibers from the mediastinal region, where the heart is located, would enter the spinal cord at the same level as the spinal nerves from the shoulder and arm, so the brain misinterprets the sensations from the mediastinal region as being from the axillary and brachial regions.
Projections from the medial and inferior divisions of the cervical ganglia do enter the spinal cord at the middle to lower cervical levels, which is where the somatosensory fibers enter. The spleen is in the upper-left abdominopelvic quadrant, but the pain is more in the shoulder and neck. How can this be? The sympathetic fibers connected to the spleen are from the celiac ganglion, which would be from the mid-thoracic to lower thoracic region whereas parasympathetic fibers are found in the vagus nerve, which connects in the medulla of the brain stem.
However, the neck and shoulder would connect to the spinal cord at the mid-cervical level of the spinal cord. These connections do not fit with the expected correspondence of visceral and somatosensory fibers entering at the same level of the spinal cord. The incorrect assumption would be that the visceral sensations are coming from the spleen directly. In fact, the visceral fibers are coming from the diaphragm. The nerve connecting to the diaphragm takes a special route.
The phrenic nerve is connected to the spinal cord at cervical levels 3 to 5. The motor fibers that make up this nerve are responsible for the muscle contractions that drive ventilation. These fibers have left the spinal cord to enter the phrenic nerve, meaning that spinal cord damage below the mid-cervical level is not fatal by making ventilation impossible.
Therefore, the visceral fibers from the diaphragm enter the spinal cord at the same level as the somatosensory fibers from the neck and shoulder. When the spleen ruptures, blood spills into this region. The accumulating hemorrhage then puts pressure on the diaphragm. The visceral sensation is actually in the diaphragm, so the referred pain is in a region of the body that corresponds to the diaphragm, not the spleen.
The efferent branch of the visceral reflex arc begins with the projection from the central neuron along the preganglionic fiber. This fiber then makes a synapse on the ganglionic neuron that projects to the target effector. The effector organs that are the targets of the autonomic system range from the iris and ciliary body of the eye to the urinary bladder and reproductive organs. The thoracolumbar output, through the various sympathetic ganglia, reaches all of these organs.
The cranial component of the parasympathetic system projects from the eye to part of the intestines. The sacral component picks up with the majority of the large intestine and the pelvic organs of the urinary and reproductive systems. Somatic reflexes involve sensory neurons that connect sensory receptors to the CNS and motor neurons that project back out to the skeletal muscles.
Visceral reflexes that involve the thoracolumbar or craniosacral systems share similar connections. However, there are reflexes that do not need to involve any CNS components. A long reflex has afferent branches that enter the spinal cord or brain and involve the efferent branches, as previously explained. A short reflex is completely peripheral and only involves the local integration of sensory input with motor output Figure 3. Short and Long Reflexes.
The difference between short and long reflexes is in the involvement of the CNS. Somatic reflexes always involve the CNS, even in a monosynaptic reflex in which the sensory neuron directly activates the motor neuron. That synapse is in the spinal cord or brain stem, so it has to involve the CNS.
However, in the autonomic system there is the possibility that the CNS is not involved. If a sensory neuron projects directly to the ganglionic neuron and causes it to activate the effector target, then the CNS is not involved. A division of the nervous system that is related to the autonomic nervous system is the enteric nervous system. The word enteric refers to the digestive organs, so this represents the nervous tissue that is part of the digestive system.
There are a few myenteric plexuses in which the nervous tissue in the wall of the digestive tract organs can directly influence digestive function. If stretch receptors in the stomach are activated by the filling and distension of the stomach, a short reflex will directly activate the smooth muscle fibers of the stomach wall to increase motility to digest the excessive food in the stomach. No CNS involvement is needed because the stretch receptor is directly activating a neuron in the wall of the stomach that causes the smooth muscle to contract.
That neuron, connected to the smooth muscle, is a postganglionic parasympathetic neuron that can be controlled by a fiber found in the vagus nerve. The autonomic nervous system is important for homeostasis because its two divisions compete at the target effector. The balance of homeostasis is attributable to the competing inputs from the sympathetic and parasympathetic divisions dual innervation.
At the level of the target effector, the signal of which system is sending the message is strictly chemical. A signaling molecule binds to a receptor that causes changes in the target cell, which in turn causes the tissue or organ to respond to the changing conditions of the body.
The postganglionic fibers of the sympathetic and parasympathetic divisions both release neurotransmitters that bind to receptors on their targets. Postganglionic sympathetic fibers release norepinephrine, with a minor exception, whereas postganglionic parasympathetic fibers release ACh.
For any given target, the difference in which division of the autonomic nervous system is exerting control is just in what chemical binds to its receptors.
The target cells will have adrenergic and muscarinic receptors. If norepinephrine is released, it will bind to the adrenergic receptors present on the target cell, and if ACh is released, it will bind to the muscarinic receptors on the target cell. In the sympathetic system, there are exceptions to this pattern of dual innervation. The postganglionic sympathetic fibers that contact the blood vessels within skeletal muscle and that contact sweat glands do not release norepinephrine, they release ACh.
This does not create any problem because there is no parasympathetic input to the sweat glands. Sweat glands have muscarinic receptors and produce and secrete sweat in response to the presence of ACh.
At most of the other targets of the autonomic system, the effector response is based on which neurotransmitter is released and what receptor is present. For example, regions of the heart that establish heart rate are contacted by postganglionic fibers from both systems.
If norepinephrine is released onto those cells, it binds to an adrenergic receptor that causes the cells to depolarize faster, and the heart rate increases. If ACh is released onto those cells, it binds to a muscarinic receptor that causes the cells to hyperpolarize so that they cannot reach threshold as easily, and the heart rate slows. Without this parasympathetic input, the heart would work at a rate of approximately beats per minute bpm.
The sympathetic system speeds that up, as it would during exercise, to — bpm, for example. The parasympathetic system slows it down to the resting heart rate of 60—80 bpm. Another example is in the control of pupillary size Figure 4. Autonomic Control of Pupillary Size. The afferent branch responds to light hitting the retina. Photoreceptors are activated, and the signal is transferred to the retinal ganglion cells that send an action potential along the optic nerve into the diencephalon.
If light levels are low, the sympathetic system sends a signal out through the upper thoracic spinal cord to the superior cervical ganglion of the sympathetic chain. The postganglionic fiber then projects to the iris, where it releases norepinephrine onto the radial fibers of the iris a smooth muscle. When those fibers contract, the pupil dilates—increasing the amount of light hitting the retina. If light levels are too high, the parasympathetic system sends a signal out from the Eddinger—Westphal nucleus through the oculomotor nerve.
This fiber synapses in the ciliary ganglion in the posterior orbit. The postganglionic fiber then projects to the iris, where it releases ACh onto the circular fibers of the iris—another smooth muscle. When those fibers contract, the pupil constricts to limit the amount of light hitting the retina. In this example, the autonomic system is controlling how much light hits the retina. It is a homeostatic reflex mechanism that keeps the activation of photoreceptors within certain limits.
In the context of avoiding a threat like the lioness on the savannah, the sympathetic response for fight or flight will increase pupillary diameter so that more light hits the retina and more visual information is available for running away. Likewise, the parasympathetic response of rest reduces the amount of light reaching the retina, allowing the photoreceptors to cycle through bleaching and be regenerated for further visual perception; this is what the homeostatic process is attempting to maintain.
Organ systems are balanced between the input from the sympathetic and parasympathetic divisions. When something upsets that balance, the homeostatic mechanisms strive to return it to its regular state. For each organ system, there may be more of a sympathetic or parasympathetic tendency to the resting state, which is known as the autonomic tone of the system.
For example, the heart rate was described above. Because the resting heart rate is the result of the parasympathetic system slowing the heart down from its intrinsic rate of bpm, the heart can be said to be in parasympathetic tone. In a similar fashion, another aspect of the cardiovascular system is primarily under sympathetic control. Blood pressure is partially determined by the contraction of smooth muscle in the walls of blood vessels. These tissues have adrenergic receptors that respond to the release of norepinephrine from postganglionic sympathetic fibers by constricting and increasing blood pressure.
The hormones released from the adrenal medulla—epinephrine and norepinephrine—will also bind to these receptors. Those hormones travel through the bloodstream where they can easily interact with the receptors in the vessel walls.
The parasympathetic system has no significant input to the systemic blood vessels, so the sympathetic system determines their tone. There are a limited number of blood vessels that respond to sympathetic input in a different fashion. Blood vessels in skeletal muscle, particularly those in the lower limbs, are more likely to dilate.
It does not have an overall effect on blood pressure to alter the tone of the vessels, but rather allows for blood flow to increase for those skeletal muscles that will be active in the fight-or-flight response. The blood vessels that have a parasympathetic projection are limited to those in the erectile tissue of the reproductive organs. Acetylcholine released by these postganglionic parasympathetic fibers cause the vessels to dilate, leading to the engorgement of the erectile tissue.
Have you ever stood up quickly and felt dizzy for a moment? This is because, for one reason or another, blood is not getting to your brain so it is briefly deprived of oxygen. When you change position from sitting or lying down to standing, your cardiovascular system has to adjust for a new challenge, keeping blood pumping up into the head while gravity is pulling more and more blood down into the legs.
The reason for this is a sympathetic reflex that maintains the output of the heart in response to postural change. When a person stands up, proprioceptors indicate that the body is changing position. A signal goes to the CNS, which then sends a signal to the upper thoracic spinal cord neurons of the sympathetic division. The sympathetic system then causes the heart to beat faster and the blood vessels to constrict.
Both changes will make it possible for the cardiovascular system to maintain the rate of blood delivery to the brain. Blood is being pumped superiorly through the internal branch of the carotid arteries into the brain, against the force of gravity. Gravity is not increasing while standing, but blood is more likely to flow down into the legs as they are extended for standing.
This sympathetic reflex keeps the brain well oxygenated so that cognitive and other neural processes are not interrupted. Sometimes this does not work properly.
If the sympathetic system cannot increase cardiac output, then blood pressure into the brain will decrease, and a brief neurological loss can be felt. The name for this is orthostatic hypotension, which means that blood pressure goes below the homeostatic set point when standing. There are two basic reasons that orthostatic hypotension can occur.
First, blood volume is too low and the sympathetic reflex is not effective. This hypovolemia may be the result of dehydration or medications that affect fluid balance, such as diuretics or vasodilators.
Both of these medications are meant to lower blood pressure, which may be necessary in the case of systemic hypertension, and regulation of the medications may alleviate the problem. Sometimes increasing fluid intake or water retention through salt intake can improve the situation. The second underlying cause of orthostatic hypotension is autonomic failure. There are several disorders that result in compromised sympathetic functions. The disorders range from diabetes to multiple system atrophy a loss of control over many systems in the body , and addressing the underlying condition can improve the hypotension.
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