Autonomic Nervous System
Parasympathetic Cranial Nerves
Although four cranial nerves carry parasympathetic fibers, the main parasympathetic nerves are the two vagus nerves (CN X), which together provide about 90% of parasympathetic innervation to the body. Branches of the vagus nerve contribute to several groups of nerves, or plexuses, that innervate specific organs. For example, the cardiac plexus innervates the heart; the pulmonary plexus, the lungs and airway passages; and the esophageal plexus, the esophagus. These preganglionic fibers typically synapse on terminal ganglia in the walls of the organ being innervated. Other cranial nerves also carry parasympathetic fibers. The parasympathetic preganglionic neurons of the oculomotor nerves (CN III) synapse on terminal ganglia called the ciliary ganglia (SIL-ee-ehr-ee). Several terminal ganglia, including the submandibular ganglia and the pterygopalatine ganglia (tehr′-uh-goh-PAL-uh-tyn), house the cell bodies of sensory neurons and are the sites where preganglionic parasympathetic neurons of the facial nerves (CN VII) synapse on postganglionic parasympathetic neurons. The preganglionic parasympathetic neurons of the glossopharyngeal nerves (CN IX) synapse on postganglionic parasympathetic neurons in small terminal ganglia called the otic ganglia (OH-tik).
Interactions of Autonomic Divisions
As you have discovered, the sympathetic and parasympathetic nervous systems generally work antagonistically, each having the opposite effect on a particular body function or organ. Together the two maintain a balance that ensures the body's needs are met appropriately at all times. They can accomplish this because most organs are innervated by neurons from both systems, a phenomenon referred to as dual innervation. This allows the sympathetic nervous system to become dominant and trigger effects that maintain homeostasis during exercise or emergency, and the parasympathetic nervous system to regulate these same organs and preserve homeostasis when the emergency or exercise has finished.
Parasympathetic Neurotransmitters and Receptors
Both preganglionic and postganglionic parasympathetic neurons release ACh at their synapses, and the effect is generally excitatory. As we saw in the sympathetic nervous system, there are two types of cholinergic receptors: nicotinic and muscarinic. Nicotinic receptors are located in the membranes of all postganglionic parasympathetic neurons, whereas muscarinic receptors are found in the membranes of all parasympathetic target cells. Next we examine what happens when ACh binds to these muscarinic receptors.
parasympathetic effects on target cells
Effects on Cardiac Muscle Cells The sympathetic nervous system increases heart rate and blood pressure, so it follows that the parasympathetic nervous system decreases them. Preganglionic parasympathetic neurons travel to the heart via the vagus nerve (CN X) and stimulate postganglionic neurons. These neurons in turn act in the heart to reduce its rate of contraction, which lowers blood pressure. Effects on Smooth Muscle Cells Postganglionic parasympathetic neurons innervate the smooth muscle cells in many different organs, and trigger the following effects: Constriction of the pupil. Fibers of the oculomotor nerve in the ciliary ganglion innervate the sphincter pupillae muscle, which constricts the pupil and so allows less light into the eye. Accommodation of the lens for near vision. Fibers of the oculomotor nerve also innervate the smooth muscle cells of the ciliary muscle. When this muscle contracts, the lens becomes rounder, allowing accommodation for near vision. Constriction of the bronchioles. When vagal parasympathetic neurons stimulate contraction of the smooth muscle cells lining the airways, the airways narrow, a response called bronchoconstriction (brong′-koh-kun-STRIK-shun). This effect is mild, except with very strong stimulation. Contraction of the smooth muscle lining the digestive tract. Parasympathetic neurons of the vagus nerve also trigger contraction of the smooth muscle cells lining the digestive tract. This produces rhythmic contractions known as peristalsis that help to propel ingested food from one part of the digestive tract to the next. Relaxation of digestive and urinary sphincters. Parasympathetic neurons from the vagus and pelvic splanchnic nerves stimulate relaxation of the smooth muscle cells of the urinary and digestive sphincters. These actions promote urination, as well as the movement of digesting food from one area of the digestive tract to the next, ultimately resulting in defecation. Engorgement of the penis or clitoris. In men, the blood vessels in the penis receive parasympathetic innervation, as do the clitoral blood vessels in women. When stimulated by parasympathetic pelvic splanchnic neurons, the smooth muscle cells in these vessels relax, which leads to vasodilation. This engorges the penis or clitoris with blood. Unlike the sympathetic nervous system, the parasympathetic nervous system innervates virtually no blood vessels except in specific areas of the body such as the penis. Yet many blood vessels do dilate when the parasympathetic nervous system is active, particularly those in the digestive and urinary systems. However, this dilation is due to a decrease in sympathetic nervous system activity and removal of epinephrine from the bloodstream rather than any direct action by parasympathetic neurons. Effects on Glandular Epithelial Cells Recall that the sympathetic nervous system promotes sweating while decreasing secretion from other exocrine glands. So it probably won't be surprising that the parasympathetic nervous system has little to no effect on sweat glands while increasing secretion from other exocrine glands. For example, parasympathetic neurons of the facial nerve stimulate tear production from the lacrimal (tear) glands and mucus production from glands in the nasal mucosa. Fibers from the both the facial and glossopharyngeal nerves stimulate secretion of saliva from the salivary glands. Finally, fibers from the vagus nerve stimulate secretion of enzymes and other products from the cells of the digestive tract. Effects on Other Cells Unlike the sympathetic nervous system, the parasympathetic nervous system has no direct effect on the metabolic rate, mental alertness, the force generated by skeletal muscle contractions, blood clotting, adipocytes, or most endocrine secretions. Instead, each of these factors returns to a normal, or "resting," state during times of parasympathetic activity simply because the sympathetic nervous system is no longer dominant. This return to the resting state is an important time for the body to store glucose and other fuels in preparation for the next round of sympathetic activity. Some drugs block cholinergic synapses, and so many also block the effects of the parasympathetic nervous system as a side effect. For more inform
Effects on the Cellular Metabolic Rate
Effects on the Cellular Metabolic Rate During times of sympathetic nervous system activation, nearly all cells, and in particular skeletal muscle cells, consume dramatically more ATP as their metabolic rates increase. To ensure that skeletal muscle cells have a steady supply of fuels with which to make ATP, norepinephrine brings about the following three main effects: (1) It binds to β3 receptors on adipocytes and triggers the breakdown of lipids, which releases free fatty acids into the bloodstream; (2) it binds to β2 receptors on cells of the liver and triggers the release of glucose from glycogen and also the synthesis of glucose from other precursors; and (3) it binds to β2 receptors on cells of the pancreas and triggers the release of a hormone called glucagon that increases the concentration of glucose in the blood. Be careful here not to confuse the metabolic rate with digestion. Digestion is the breakdown of foods into nutrients that can be absorbed into the blood, and these processes decrease when sympathetic activity increases. The metabolic rate refers to the rate at which cells consume ATP, and this increases when sympathetic activity rises. Given all the effects of the sympathetic nervous system on the metabolic rate, it's not surprising that sympathetic receptors are purported targets for weight loss supplement
Summary of Nervous System Control of Homeostasis
Maintenance of homeostasis is the body's most essential function, and the two divisions of the ANS figure importantly in how the nervous system overall fulfills this role (Figure 14.12). Now that we know more about the ANS, we can revisit what you learned in the CNS section on homeostasis of vital functions (see Chapter 12). Recall that regulated physiological variables are largely controlled centrally by the hypothalamus and the brainstem reticular formation. Many actions of these structures are mediated peripherally via the sympathetic and parasympathetic nervous systems. Note in Figure 14.12 that many signals sent from the hypothalamus are directed toward areas of the reticular formation called autonomic centers. These centers contain neurons that control the activity of preganglionic sympathetic and parasympathetic neurons. Although the hypothalamus does exert some degree of control over these centers, the reticular formation can function even if the circuits connecting it with the hypothalamus are severed. This indicates that the reticular formation is capable of controlling many of our most critical autonomic functions, such as heart rate and blood pressure, on its own. Figure 14.12 also shows that both the hypothalamus and reticular formation receive input from higher centers in the brain, including the amygdala and multiple areas of the cerebral cortex. This in part explains why emotion has such a profound effect on our visceral functions. A state of excitement, mediated by the cerebral cortex and amygdala, will be passed on to the hypothalamus, reticular formation, and finally to the ANS, after which we notice an increased heart rate, elevated blood pressure, dilated pupils, and other signs of sympathetic activity. Although the nervous system plays an important role in maintaining homeostasis, it is only part of the story. We explore the other part of it when we discuss the endocrine system (in Chapter 16).
Chapter summary
Module 14.1: Overview of the Autonomic Nervous System The autonomic nervous system (ANS) performs its functions via a series of visceral reflex arcs. Autonomic motor neurons consist of preganglionic neurons that synapse on autonomic ganglia, and then postganglionic neurons that contact the target cells. The autonomic nervous system is made up of the sympathetic and parasympathetic nervous systems. Module 14.2: The Sympathetic Nervous System The sympathetic nervous system is often called the "fight or flight" division of the ANS. The sympathetic nervous system is called the thoracolumbar division. Most ganglia are part of the sympathetic chain ganglia. Preganglionic sympathetic axons release acetylcholine (ACh) at their synapses with postganglionic neurons. Postganglionic axons release norepinephrine, epinephrine, or ACh at their synapses with target cells. Adrenergic receptors bind to epinephrine and norepinephrine. There are five classes of adrenergic receptors: α1, α2, β1, β2, and β3. Cholinergic receptors bind ACh. The two types of cholinergic receptors are muscarinic receptors and nicotinic receptors. The sympathetic nervous system increases blood pressure by raising the rate and force of contraction of the heart and also by causing vasoconstriction. Sympathetic neurons cause bronchodilation. Norepinephrine causes vasodilation of the blood vessels serving skeletal muscles. The adrenal medulla is made up of modified sympathetic postganglionic neurons. When stimulated, the adrenal medulla releases epinephrine and norepinephrine into the blood. Sympathetic activity decreases urinary and digestive functions and the secretion from most glands, dilates the pupil, increases the levels of metabolic fuels in the blood, and increases the secretion from sweat glands. Module 14.3: The Parasympathetic Nervous System The parasympathetic nervous system is the "rest and digest" division of the ANS that performs the body's maintenance functions such as digestion and urine formation. The parasympathetic nervous system is called the craniosacral division. Preganglionic axons synapse with the cell bodies of postganglionic neurons in ganglia near the target cells. Both preganglionic and postganglionic parasympathetic neurons release ACh at their synapses. The parasympathetic nervous system lowers blood pressure by decreasing the rate of contraction of the heart. Parasympathetic activity causes constriction of the pupil, accommodation of the lens for near vision, bronchoconstriction, contraction of digestive tract smooth muscle, relaxation of digestive and urinary sphincters, engorgement of the penis or clitoris, and an increase in secretion from most glands except sweat glands. Module 14.4: Homeostasis Part II: PNS Maintenance of Homeostasis The sympathetic and parasympathetic nervous systems generally work antagonistically. Autonomic tone refers to the constant amount of activity present in each system most of the time. Homeostasis is controlled centrally by the hypothalamus and brainstem reticular formation. Many signals sent by the hypothalamus are directed toward autonomic centers in the reticular formation.
Autonomic Tone
Neither the sympathetic nor the parasympathetic nervous system is ever completely silent, as both are active to some degree most of the time. This constant amount of activity from each system is known as Autonomic tone; it can be divided into sympathetic tone and parasympathetic tone. Interestingly, different degrees of sympathetic and parasympathetic tone exist in different organs. For example, the sympathetic nervous system is normally dominant in blood vessels and keeps them partially constricted at all times, which is important for maintaining blood pressure at rest. The parasympathetic nervous system is normally dominant in the heart and keeps the heart rate at an average of 72 beats per minute. Parasympathetic tone in the heart is often even stronger in athletes, who have been conditioned to recover more quickly from the intense bursts of sympathetic activity that accompany exercise. This causes the resting heart rate of an athlete to be lower than that of someone who is not physically conditioned. The parasympathetic nervous system also dominates in the digestive and urinary systems, and keeps the activity of these systems at normal levels. This is why drugs with anticholinergic action
The Balance between the Parasympathetic and Sympathetic Nervous Systems
The actions of the parasympathetic nervous system directly antagonize those of the sympathetic nervous system. For example, parasympathetic neurons decrease the rate of the heart's contractions, whereas sympathetic neurons increase their rate. Together, these two divisions maintain a delicate balance to ensure that homeostasis is maintained at all times.
Effects on Cells of the Adrenal Medulla
The adrenal medulla (uh-DREE-nul muh-DOOL-uh) is the internal part of the adrenal glands, which sit atop each kidney. It's unique in that it is contacted directly by preganglionic sympathetic neurons. This is because the adrenal medulla is not like the remainder of the adrenal gland—instead of glandular epithelium, it actually consists of modified sympathetic postganglionic neurons. In other words, each adrenal medulla is functionally a ganglion. As shown in Figure 14.8, ❶❶ when the preganglionic neuron releases ACh, ❷❷ it binds to the nicotinic receptors of adrenal medulla cells. ❸❸ ACh then stimulates these cells to release additional epinephrine and norepinephrine into the bloodstream. Although norepinephrine and epinephrine are usually considered neurotransmitters, they can both also function as hormones, long-distance chemical messengers. This means that the adrenal medulla effectively acts as an interface between the sympathetic nervous system and the endocrine system The ratio of chemicals the adrenal medulla produces is generally opposite that of the ANS, with about 80% epinephrine and 20% norepinephrine. This additional epinephrine and norepinephrine is useful in many ways. For one, it prolongs the duration of the sympathetic nervous system's effects. The effects of neurotransmitters released by postganglionic neurons last only a few seconds, whereas in the blood the effects of these chemicals continue for several minutes. A second benefit is that the epinephrine and norepinephrine in the blood can reach cells that are not innervated by sympathetic neurons. This allows the sympathetic nervous system to affect the entire body without innervating every body cell. Finally, the adrenal medulla acts as a "backup" for the sympathetic nervous system—even if the pathways connecting this system with many of its target organs are disrupted, it can still affect these organs indirectly via the adrenal medulla.
Functions of the ANS and Visceral Reflex arcs
The autonomic nervous system oversees such vital functions as heart rate, blood pressure, and digestive and urinary processes. The ANS is named for the fact that it can operate autonomously, without conscious control. It performs these functions via a series of visceral reflex arcs. As you have learned, a reflex arc is a series of events in which a sensory stimulus leads to a predictable motor response
Parasympathetic Sacral Neurons
The cranial nerve parasympathetic fibers supply most viscera of the thoracic and abdominopelvic cavities. The remaining organs, including the last segment of the large intestine, the urinary bladder, and the reproductive organs, are supplied by parasympathetic sacral neurons. Branches from the sacral spinal cord form the pelvic splanchnic nerves, which in turn form plexuses in the pelvic floor. Some preganglionic neurons synapse in terminal ganglia in these plexuses, but most synapse in terminal ganglia in the walls of the organs.
Gross and Microscopic Anatomy of the Parasympathetic Nervous System
The cranial nerves that house parasympathetic preganglionic neurons are the oculomotor (III), facial (VII), glossopharyngeal (IX), and vagus (X) nerves. The sacral nerves that house parasympathetic preganglionic neurons include S2-S4. Figure 14.9 shows the organization of this division. As with preganglionic sympathetic neurons, the axons of preganglionic parasympathetic neurons synapse with postganglionic neurons located within ganglia. The ganglia, known as terminal ganglia, are typically found near the postganglionic neurons' target cells, and the postganglionic axons are in general fairly short.
Classes of Sympathetic Receptors
The neurotransmitter receptors that bind to norepinephrine or epinephrine are called adrenergic receptors (ad-ren-ER-jik; the name comes from their ability to bind adrenalin). Those that bind to ACh are cholinergic receptors (kohl-in-ER-jik).
Effects of the Parasympathetic Nervous System on Target Cells
The parasympathetic nervous system's effects on its target cells are easily understood if you keep in mind its most basic function: to maintain homeostasis when the body is at rest. Consider what happens after you eat dinner and sit down to read your anatomy and physiology textbook—your heart rate slows, your blood pressure decreases, you digest your food, and your eyes adjust for near vision in order to read the book. Each effect results from a decrease in sympathetic activity and the release of ACh from pre- and postganglionic parasympathetic neurons.
Effects on Smooth Muscle Cells
The smooth muscle cells of many organs also have receptors for norepinephrine. When norepinephrine binds, it mediates the following changes: Constriction of blood vessels serving the digestive, urinary, and integumentary systems. Norepinephrine binds to α1 receptors on smooth muscle cells of blood vessels serving the organs of the digestive, urinary, and integumentary systems and causes them to contract. This narrows the blood vessels, an action called vasoconstriction (vay′-zoh-kun-STRIK-shun; vaso- = "vessel"). Vasoconstriction decreases the blood flow to those organs and diverts blood to tissues that are temporarily "more important," such as skeletal and cardiac muscle. Decreasing blood flow to the urinary and digestive organs slows the rate of urine formation and digestion, respectively (if you are running from a bear, digesting the lunch you just ate can wait). This wide-scale vasoconstriction also increases the overall blood pressure. Reduced blood flow to the skin explains why people with light-colored skin "turn white as a sheet" when they are scared—as the blood vessels serving the skin constrict, the pinkish color of the blood under the skin is less visible and the skin appears paler. Dilation of the bronchioles. During times of stress more oxygen needs to be delivered to the body's cells, especially the skeletal muscle cells, to keep up with their metabolic demands. The sympathetic nervous system accomplishes this through norepinephrine's effect on the smooth muscle cells lining the smaller airway passages (the bronchioles). When norepinephrine binds to the β2 receptors on these cells, they relax. This action, bronchodilation (brong′-koh-dy-LAY-shun), increases the diameter of the bronchioles and so the amount of oxygen that can be inhaled with each breath. Although the sympathetic nervous system does increase oxygen intake, note that this system has no effect on the rate of ventilation. The rate does tend to increase during exercise and emergency situations, but this is due to a mechanism independent of the sympathetic nervous system (see Chapter 21). Dilation of blood vessels serving skeletal and cardiac muscle. Although norepinephrine constricts most of the body's blood vessels, when it binds to β2 receptors on the smooth muscle cells of blood vessels serving skeletal and cardiac muscle, it causes them to relax. This opens the blood vessels, an action called vasodilation (vay′-zoh-dy-LAY-shun), which increases the blood flow and the delivery of oxygen and nutrients to these cells. If this vasodilation occurred alone, the blood pressure would decrease. However, it occurs simultaneously with the vasoconstriction of nearly every other blood vessel in the body, so the effect of skeletal and cardiac muscle blood vessel vasodilation on blood pressure is minimal. Contraction of urinary and digestive sphincters. The binding of norepinephrine to the smooth muscle cells of the sphincters of the urinary and digestive systems causes them to contract. This makes emptying the bowel and bladder more difficult during times of exercise, stress, or emergency. Relaxation of the smooth muscle of the digestive tract. Although stimulation by the sympathetic nervous system causes the smooth muscle of the digestive tract sphincters to contract, when norepinephrine binds to β3 receptors it leads to relaxation of the remainder of the smooth muscle in the digestive tract. This decreases the movement of food through the digestive system and slows digestive processes overall. Dilation of the pupils. The dilator pupillae muscles are innervated by sympathetic nervous system neurons. When norepinephrine binds to the α1 receptors on these smooth muscle cells, it causes them to contract, which dilates the pupils and allows more light to enter the eyes. Constriction of blood vessels serving most exocrine glands. The sympathetic nervous system almost universally decreases secretion from exocrine glands—apart from sweat glands—via norepinephrine binding to β2 receptors. This affects most exocrine cells of the digestive system, including the salivary glands around your mouth and the cells that secrete digestive products. This effect on salivary glands is why your mouth is often dry when you're nervous. The decreased secretion is generally due to reduced blood flow to these glands rather than a direct effect on the gland itself.
Effects on Other Cells
The sympathetic nervous system influences many of the body's other processes via its effects on target cells. For instance, this system enhances mental alertness, increasing neuron activity in the association areas of the cerebral cortex. Stimulation by this system also increases the blood's tendency to clot, which can be helpful if a person is injured during a "fight" or "flight" situation. In addition, the sympathetic nervous system can temporarily elevate the tension generated by skeletal muscle cells during a muscle contraction, which is why some people have been known to perform unusual feats of strength under the influence of an "adrenaline (epinephrine) rush." Furthermore, sympathetic neurons trigger contraction of the arrector pili muscles, which produces "goose bumps." Finally, these neurons also cause ejaculation of semen via effects on smooth muscle cells of the male reproductive ducts.
Classes of Sympathetic Neurotransmitters
The sympathetic preganglionic axon communicates with the postganglionic neuron by way of an excitatory synapse using the neurotransmitter acetylcholine (ACh). This is the same neurotransmitter released by somatic motor neurons, as we discussed in the muscle tissue chapter (see Chapter 10). The unmyelinated axons of the postganglionic neurons then carry the message in the form of an action potential to their target cells. At the synapses with their target cells, postganglionic axons release one of three neurotransmitters: norepinephrine, epinephrine, or ACh. Norepinephrine and epinephrine are also known as noradrenalin and adrenalin, respectively; these names derive from the adrenal gland, where these chemicals were first discovered. Approximately 80% of postganglionic sympathetic neurons release norepinephrine, and the remainder release epinephrine or ACh.
Adrenergic Receptors
There are two major types of adrenergic receptors: alpha (α) receptors and beta β receptors. These receptors are further classified according to the structure of their proteins. The subtypes include the following: Alpha-1 (α1) receptors are located in the plasma membranes of smooth muscle cells of many different organs, including the blood vessels that supply the skin, organs of the gastrointestinal system, and the kidneys. These receptors are also found on arrector pili muscles in the dermis, the uterus (during pregnancy), and certain organs of the genitourinary system. Alpha-2 (α2) receptors are different from the other adrenergic receptors—we find most of them in the membrane of preganglionic sympathetic neurons rather than in peripheral target cells. Recall that normally an action potential in a preganglionic neuron leads to ACh release, which stimulates the postganglionic neuron (Figure 14.6a). However, notice in Figure 14.6b that when norepinephrine binds to α2 receptors, the axon terminal is hyperpolarized, slowing or even canceling the action potential. As a result, the preganglionic neuron stops stimulating the postganglionic neuron, which dampens or even shuts off the sympathetic response. This is part of a negative feedback loop that prevents excessive sympathetic activity, another example of the Feedback Loops Core Principle (Module 1.5.5). We also find α2 receptors in the plasma membranes of certain sympathetic target cells, including cells in the pancreas and adipose tissue. Beta-1 (β1) receptors are located in the plasma membranes of cardiac muscle cells as well as certain cells of the kidney and adipose tissue. Beta-2 (β2) receptors are found in the plasma membranes of smooth muscle cells lining the airway passages in the lungs (the bronchioles). β2 receptors are also found on skeletal muscle fibers, smooth muscle cells of the urinary bladder, smooth muscle cells lining the blood vessels serving skeletal muscles, cells of the liver and pancreas, and cells of the salivary glands. Beta-3 (β3) receptors are located predominantly on the cells of adipose tissue and smooth muscle cells in the wall of the digestive tract.
cholinergic receptors
There are two types of cholinergic receptors: the muscarinic receptor (muss-kuh-RIN-ik; so named because it binds the poison muscarine) and the nicotinic receptor (nik-oh-TIN-ik; so named because it binds the poison found in tobacco, nicotine). Muscarinic receptors are found on sweat glands in the skin. Nicotinic receptors are located in the membranes of all postganglionic neurons and the cells of the adrenal medulla (which consists of modified postganglionic neurons).
Effects on Secretion from Sweat Glands
When you're nervous or exercising, you sweat. This useful (but often annoying) attempt to help the body maintain temperature homeostasis is due to postganglionic sympathetic neurons that release ACh onto the cells of sweat glands in the skin. ACh binds to these cells' muscarinic receptors and increases their secretions. This occurs as part of a negative feedback loop that controls body temperature, which reminds us of the Feedback Loops Core Principle
preganglionic neurons synapse
a preganglionic neuron may synapse with a postganglionic neuron in one of three ways. the axon descends or ascends, and synapses with a postganglionic neuron in a different chain ganglion; or the axon synapses with a postganglionic neuron in the sympathetic chain ganglion; the axon passes through the chain ganglion and synapses with a postganglionic neuron in a collateral ganglion.
Pharmacology and Sympathetic Nervous System Receptors
has allowed researchers to design drugs that are fairly specific for one type of receptor and so certain organs. This is advantageous because it helps to minimize potential side effects. Most drugs targeting the sympathetic nervous system work in one of two ways: Either they are antagonists that block the receptor and prevent norepinephrine from binding to it, or they are agonists that bind the receptor and mimic the effects of norepinephrine. Some of the more common drugs that bind to sympathetic receptors include the following: α1 Blockers (antagonists) bind to α1 receptors, particularly those on the smooth muscle cells lining blood vessels. They block the action of norepinephrine and prevent the blood vessels from constricting, an effect that lowers blood pressure and is useful in treating hypertension (high blood pressure). Certain α1 blockers also cause relaxation of the smooth muscle in the prostate gland; these are used to treat benign prostatic hyperplasia (see Chapter 26). α2 Agonists bind to the presynaptic α2 receptors and activate them, which decreases the output of both preganglionic and postganglionic sympathetic neurons. These drugs may be used in the treatment of hypertension and other conditions such as opiate withdrawal. β Blockers are antagonists that bind to β1 receptors on the heart and decrease its rate and force of contraction. These drugs are widely used in the treatment of hypertension and other diseases of the cardiovascular system. β2 Agonists bind to β2 receptors on the smooth muscle of the bronchioles and cause bronchodilation. These drugs are commonly used to treat asthma.
organization of the sympathetic nervous system and its preganglionic and postganglionic neurons
hat most cell bodies of the postganglionic neurons are found in a series along the vertebral column. Their chainlike appearance has given them the name Sympathetic chain ganglia. These ganglia extend beyond the thoracic and lumbar spinal cord, from the superior cervical ganglion down to the inferior sacral ganglion. Preganglionic neurons begin in the lateral horns of the thoracic and lumbar spinal cord. The axons of preganglionic neurons exit the spinal cord with the axons of lower motor neurons via the anterior root. They then travel with the spinal nerve and the anterior ramus for a short distance before branching off to form small nerves called white rami communicantes (kuh-myoo′-nih-KAN-teez; singular, communicans; called white because they are myelinated), as shown here: The axons in the white rami communicantes then enter the sympathetic chain ganglia that house the cell bodies of the postganglionic sympathetic neurons. Some preganglionic neurons pass through the sympathetic chain ganglia without synapsing. Instead, they may synapse on cell bodies in different chain ganglia, or on collateral ganglia, which are located near the target organ. Many collateral ganglia are located near the aorta, and for this reason are called preaortic ganglia. Other collateral ganglia are located near the organs of the abdominopelvic cavity. The preganglionic axons that synapse here are part of nerves called the splanchnic nerves (SPLANK-nik; splanchn- = "viscera") that synapse on ganglia including the celiac ganglion, the superior mesenteric ganglion, and the inferior mesenteric ganglion. The postganglionic neuron then innervates its target cells. Notice in Figure 14.5 that postganglionic axons that take the first two pathways exit the ganglia as small gray rami communicantes (called gray because they are unmyelinated), which rejoin and travel with spinal nerves to reach their target cells. In contrast, postganglionic axons that take the third pathway travel to their target cells directly. The largest population of postganglionic axons that take this third path are the splanchnic nerves.
The Parasympathetic Nervous System intro
he cell bodies of the preganglionic parasympathetic neurons are located within the nuclei of several cranial nerves in the brainstem and in the sacral region of the spinal cord. For this reason, the parasympathetic nervous system is often called the craniosacral division (kray′-nee-oh-SAY-krul) of the ANS. The cranial nerves innervate structures of the head and neck, the thoracic viscera, and most abdominal viscera. The sacral nerves innervate structures within the pelvic cavity. As in the sympathetic division, the preganglionic neurons synapse first in parasympathetic ganglia, and the postganglionic neurons then innervate the target organs. However, parasympathetic ganglia are typically located near or within the target organs, as opposed to near the spinal cord. The parasympathetic nervous system is often described as the "rest and digest" division of the ANS. This reflects its role in digestion and in maintaining the body's homeostasis when at rest.
neurotransmitters
r interacts with a target cell by binding to a protein-based receptor for that neurotransmitter embedded in the plasma membrane of the target cell. The following subsections discuss the neurotransmitters released by sympathetic neurons and the receptors to which these neurotransmitters bind on their target cells.
Division of ANS
sympathetic and parasympathetic
The Sympathetic Nervous System intro
the cell bodies of the preganglionic neurons originate in the thoracic and upper lumbar spinal cord. For this reason, the sympathetic nervous system is often called the thoracolumbar division (thor′-aeh-koh-LUM-bar) of the ANS. These neurons synapse first in sympathetic ganglia, which are generally located near the spinal cord (with a few exceptions). The postganglionic neurons innervate their target cells. Figure 14.3 The sympathetic nervous system is usually described as the "fight or flight" division of the ANS—this reflects its role in preparing the body for emergency situations in which one would need to fight off an attacker or flee from danger. The sympathetic nervous system's role is broader than this, however, as it maintains homeostasis when the body is engaged in any type of physical work and mediates the body's visceral responses to emotion.
effect of cardiac muscle cells
when you are nervous or exercising, your heart rate and blood pressure rise. This is due to the effect of norepinephrine binding to β1 receptors on cardiac muscle cells, which opens ion channels that increase both the rate and force of contractions. This heightens not only the amount of blood delivered to the tissues but also blood pressure, which maintains homeostasis during periods of increased activity.
visceral reflex arcs of the ANS, steps
❶❶ sensory signals from the viscera and skin are sent by afferent sensory neurons to the brain or spinal cord. ❷❷ The stimuli are then integrated by the CNS. ❸❸ Next, motor impulses from the CNS are sent out via efferent motor neurons in cranial and spinal nerves. These nerves usually lead to ganglia in the PNS, called Autonomic ganglion. ❹❹ Finally, the autonomic ganglia send the impulses via other efferent motor neurons to various target organs, where they trigger a motor response in the target cells. Note that visceral reflex arcs provide another example of the Cell-Cell Communication Core Principle
