A&P Chapter 16-2 PDF

Chapter 16 – Autonomic Nervous System Overview of the Autonomic Nervous System -the autonomic nervous system maintains homeostasis by regulating involuntary activities, including heart rate, breathing rate, body temperature, digestive processes, and urinary functions -the motor division of the nervo...

Chapter 16 – Autonomic Nervous System Overview of the Autonomic Nervous System -the autonomic nervous system maintains homeostasis by regulating involuntary activities, including heart rate, breathing rate, body temperature, digestive processes, and urinary functions -the motor division of the nervous system controls the effectors or the muscles and glands of the body -the motor division consists of two parts: the somatic nervous system and the autonomic nervous system -the somatic nervous system regulates the activities of skeletal muscle -the autonomic nervous system regulates smooth muscle, cardiac muscle, and glands -the autonomic nervous system is divided into sympathetic division (fight or flight), the parasympathetic division (rest and digest), and the enteric nervous system (digestive tract) Contrasting the Somatic and Autonomic Nervous System -the PNS is composed of sensory and motor neurons. Sensory neurons carry action potentials from the many sensory receptors of the body to the CNS, and motor neurons carry action potentials from the CNS to the effectors of the body -motor neurons that innervate skeletal muscle are called somatic motor neurons, and they are part of the somatic nervous system -motor neurons that innervate smooth muscle, cardiac muscle, and glands are called autonomic motor neurons, and they are part of the autonomic nervous system -a nerve is a bundle of axons, or nerve fibers, that connect the CNS to sensory receptors, muscle and glands. a nerve may contain the axons of autonomic neurons, somatic motor neurons, and/or sensory neurons, but the proportions of the specific types of axons vary from nerve to nerve -nerves innervating smooth muscle, cardiac muscle, and glands, such as the vagus nerves, consist primarily of axons of sensory neurons and autonomic motor neurons -nerves innervating skeletal muscles, such as the sciatic nerves, consist primarily of axons of sensory neurons and somatic motor neurons -some cranial nerves, such as the olfactory, optic, and vestibulocochlear nerves, are composed entirely of sensory neuron axons -in the SNS, the cell bodies of the somatic motor neurons are in the CNS. The axons of these somatic motor neurons extend from their location in the CNS to skeletal muscle -in contrast to a single neuron, the ANS has two neurons in a series extending between the CNS and the innervated organs -the first neuron of the series is called the preganglionic neurons. its cell body is located in the CNS within either the brainstem or the lateral horn of the spinal cord gray matter, and its axon extends to an autonomic ganglion located outside the CNS -the autonomic ganglion contains the cell body of the second neuron of the series called the postganglionic neuron -preganglionic neurons synapse with postganglionic neurons in the autonomic ganglia -the axons of postganglionic neuron extend from autonomic ganglia to effector organs and synapse with their target tissues -in addition to the organization of the neural pathways, the somatic nervous system and ANS vary in other structural and functional ways: -the axons of all somatic motor neurons are myelinated. In the ANS, the axons of preganglionic neurons are myelinated, but the axons of postganglionic neurons are unmyelinated -many movements controlled by the somatic nervous system are voluntary, whereas ANS functions are involuntarily controlled -the effect of somatic motor neurons on skeletal muscle is always excitatory, but the effect of autonomic motor neurons on target tissues can be either excitatory or inhibitory. for example, after a meal the ANS can stimulate stomach activities, but during exercise the ANS can inhibit those activities -the major neurotransmitter in the somatic nervous system is acetylcholine or ACh, while ACh, epinephrine (E), and norepinephrine (NE) are used in the ANS -sensory neurons are not classified as somatic or autonomic. These neurons propagate action potentials from sensory receptors to the CNS and can provide information for reflexes mediated through the somatic nervous system or the autonomic nervous system Anatomy of the Autonomic Nervous System -the ANS is subdivided into the sympathetic division, the parasympathetic division, and the enteric nervous system -the sympathetic and parasympathetic divisions differ anatomically in the location of their preganglionic neuron cell bodies within the CNS and the location of their autonomic ganglia in the PNS -the ENS is a complex network of neuron cell bodies and axons within the wall of the digestive tract SYMPATHETIC DIVISION -the sympathetic division of the ANS is sometimes called the fight or flight division because of its physiological influence or the thoracolumbar division because of its anatomical organization -the sympathetic preganglionic neurons are associated with the thoracic and lumbar regions of the spinal cord, thus the name thoracolumbar. specifically, the cell bodies of sympathetic preganglionic neurons are in the lateral horns of the spinal cord gray matter between the first thoracic (T1) segment and the second lumbar (L2) segment. the axons of the preganglionic neurons exit through the ventral roots of spinal nerves T1–L2. these axons course through the spinal nerves for a short distance before they exit the nerves and project to sympathetic ganglia. -there are two types of sympathetic ganglia: sympathetic chain ganglia and collateral ganglia -sympathetic chain ganglia are located along the left and right sides of the vertebral column. these ganglia are connected to each other, forming a chain, thus the name sympathetic chain. the sympathetic chain ganglia are also called paravertebral ganglia because of their location alongside the vertebral column -collateral ganglia are unpaired ganglia located in the abdominopelvic cavity. they are also called prevertebral ganglia because they are anterior to the vertebral column -collateral meaning “accessory” -the axons of autonomic preganglionic neurons are small in diameter and myelinated. the sympathetic preganglionic axons pass through a short connection between a spinal nerve and a sympathetic chain ganglion called a white ramus communicans, so called because of the whitish color of the myelinated axons -sympathetic preganglionic neurons either synapse with postganglionic neurons in the sympathetic chain ganglion or pass through to synapse in another area. The following are the four potential pathways of axons that exit the sympathetic chain ganglia: spinal nerves, sympathetic nerves, splanchnic nerves, and innervation to the adrenal gland. -spinal nerve: preganglionic axons synapse with postganglionic neurons in sympathetic chain ganglia. they can synapse at the same level that the preganglionic axons enter the sympathetic chain, or they can pass superiorly or inferiorly through one or more ganglia and synapse with postganglionic neurons in a sympathetic chain ganglion at a different level. axons of the postganglionic neurons pass through a gray ramus communicans and reenter a spinal nerve. postganglionic axons are unmyelinated, thereby giving the gray ramus communicans its grayish color. all spinal nerves receive postganglionic axons from a gray ramus communicans. the postganglionic axons then project through the spinal nerve to the skin and blood vessels of skeletal muscles. -sympathetic nerves: preganglionic axons enter the sympathetic chain and synapse in a sympathetic chain ganglion at the same or a different level with postganglionic neurons. the postganglionic axons leaving the sympathetic chain ganglion form sympathetic nerves, which supply organs in the thoracic cavity, such as the heart. -splanchnic nerves: some preganglionic axons enter sympathetic chain ganglia and, without synapsing, exit at the same or a different level to form splanchnic nerves. those preganglionic axons extend to collateral ganglia, where they synapse with postganglionic neurons. axons of the  postganglionic neurons leave the collateral ganglia through small nerves that extend to effectors in the abdominopelvic cavity. -innervation to the adrenal gland: the sympathetic innervation to the adrenal gland is through a splanchnic nerve, but it is different from other ANS nerves. in the case of the adrenal glands, the axons of the preganglionic neurons do not synapse in sympathetic chain ganglia or in collateral ganglia. instead, the axons pass through those ganglia and synapse with cells in the medulla of the adrenal gland. the adrenal medulla is the inner portion of the adrenal gland and consists of specialized cells derived from neural crest cells during embryonic development. neural crest cells are the same cells that give rise to the postganglionic cells of the ANS. adrenal medullary cells are round, have no axons or dendrites, and are divided into two groups, depending on what substance they secrete. about 80% of the cells secrete epinephrine, also called adrenaline, and about 20% secrete norepinephrine, also called noradrenaline. stimulation of these cells by preganglionic axons causes the release of epinephrine and norepinephrine into the blood. these substances circulate in the blood and affect all tissues having receptors to which they can bind. the general response to epinephrine and norepinephrine released from the adrenal medulla is to prepare the individual for physical activity. secretions of the adrenal medulla are considered hormones because they are released into the bloodstream and travel some distance to the effectors. PARASYMPATHETIC DIVISION -the parasympathetic division is called the rest and digest division because of it physiological influence and the craniosacral division because of its anatomical organization -cell bodies of parasympathetic preganglionic neurons are located either within cranial nerve nuclei in the brainstem or within the lateral horns of the gray matter in the sacral region of the spinal cord from S2 to S4 -axons of the parasympathetic preganglionic neurons from the brain are in cranial nerves III, VII, IX, and X -the axons associated with the sacral region are in pelvic splanchnic nerves -the preganglionic axons course through the nerves to terminal ganglia, where they synapse with postganglionic neurons -the axons of the postganglionic neurons extend relatively short distances from the terminal ganglia to the effectors -the terminal ganglia are either near or embedded within the walls of the organs innervated by the parasympathetic neurons AUTONOMIC NERVE PLEXUSES AND DISTRIBUTION OF AUTONOMIC NERVE FIBERS -in some cases, the postganglionic axons extend directly through nerves to the target organ. in other cases, the postganglionic axons become part of the autonomic nerve plexuses -autonomic nerve plexuses are complex, interconnected neural networks formed by neurons of the sympathetic and parasympathetic divisions -axons of sensory neurons contribute to the autonomic nerve plexuses -the autonomic nerve plexuses are typically named according to the organs they supply or the blood vessels along which they are found -sympathetic outflow is through spinal nerves, sympathetic nerves, and splanchnic nerves. branches of these nerves either extend directly to effectors or join autonomic nerve plexuses to be distributed to effectors. from all levels of the sympathetic chain, some postganglionic axons project through gray rami communicants to spinal nerves. the axons extend to a specific region innervated by each pair of spinal nerves, regulating the activity of sweat glands in the skin, the smooth muscle in the blood vessels of the skin, and the smooth muscle of the arrector pili. The major means by which sympathetic postganglionic axons reach effectors via autonomic plexuses include head and neck nerve plexuses, thoracic nerve plexuses, and abdominopelvic nerve plexuses -head and neck nerve plexuses: most of the sympathetic nerve supply to the head and neck is derived from the superior cervical ganglion. postganglionic axons of sympathetic nerves form plexuses that extend superiorly to the head and inferiorly to the neck. the plexuses associated with the head and neck give off branches to regulate the activity of structures of the skin such as sweat glands, the smooth muscle in the blood vessels of the skin, and the smooth muscle of the arrector pili associated with hair follicles. axons from the plexuses also join branches of the trigeminal nerves to regulate the activity of the skin of the face, the salivary glands, as well as the ciliary muscles and the iris of the eye. -thoracic nerve plexuses: the sympathetic innervation for organs of the thorax is mainly derived from the cervical and upper five thoracic sympathetic chain ganglia. postganglionic axons in sympathetic nerves contribute to the cardiac plexus, regulating the heart; the pulmonary plexus, regulating the lungs; and other thoracic plexuses. -abdominopelvic nerve plexuses: sympathetic chain ganglia from T5 and below mainly innervate the abdominopelvic organs. the preganglionic axons of splanchnic nerves synapse with postganglionic neurons in the collateral ganglia of abdominopelvic nerve plexuses. postganglionic axons from the collateral ganglia innervate smooth muscle and glands in the abdominopelvic organs. the following are the abdominopelvic nerve plexuses: celiac plexus, superior mesenteric plexus, inferior mesenteric plexus, and hypogastric plexuses -the celiac plexus has two large celiac ganglia and other, smaller ganglia. the celiac plexus innervates the diaphragm, stomach, spleen, liver, gallbladder, adrenal glands, kidneys, testes, and ovaries. -the superior mesenteric plexus includes the superior mesenteric ganglion and innervates the pancreas, small intestine, ascending colon, and transverse colon. -the inferior mesenteric plexus includes the inferior mesenteric ganglion and innervates the transverse colon to the rectum. -the hypogastric plexuses innervate the descending colon to the rectum, urinary bladder, and reproductive organs in the pelvis -parasympathetic preganglionic fibers extend from the CNS through cranial and pelvic splanchnic nerves. branches of these nerves either directly innervate organs or join nerve plexuses to be distributed to organs. the major means by which parasympathetic postganglionic axons reach effectors include cranial nerves innervating the head and neck, the vagus nerve and thoracic nerve plexus, the vagus nerve and abdominal nerve plexuses, and pelvic splanchnic nerves and pelvic nerve plexuses -cranial nerves innervating the head and neck: three pairs of cranial nerves have parasympathetic preganglionic axons that extend to terminal ganglia in the head. postganglionic neurons from the terminal ganglia innervate and thereby regulate nearby structures -the oculomotor nerve (CN III), through the ciliary ganglion, innervates the ciliary muscles and the iris of the eye -the facial nerve (CN VII), through the pterygopalatine ganglion, innervates the lacrimal gland and the mucosal glands of the nasal cavity and palate. the facial nerve, through the submandibular ganglion, also innervates the submandibular and sublingual salivary glands -the glossopharyngeal nerve (CN IX), through the otic ganglion, innervates the parotid salivary gland -the vagus nerve and thoracic nerve plexuses: although the vagus nerve (CN X) has somatic motor and sensory functions in the head and neck, its parasympathetic distribution is to the thorax and  abdomen. preganglionic axons extend through the vagus nerves to the thorax. within the thorax, the axons pass through branches of the vagus nerves to contribute to the cardiac plexus, which innervates the heart, and the pulmonary plexus, which innervates the lungs. the vagus nerves continue down the esophagus and give off branches to form the esophageal plexus. -the vagus nerve and abdominal nerve plexuses: after the esophageal plexus passes through the diaphragm, some of the vagal preganglionic axons innervate terminal ganglia in the wall of the stomach, whereas others contribute to the celiac and superior mesenteric plexuses. through these plexuses, the preganglionic axons synapse in terminal ganglia in the walls of the gallbladder, biliary ducts, pancreas, small intestine, ascending colon, and transverse colon. -pelvic splanchnic nerves and pelvic nerve plexuses: the parasympathetic preganglionic axons whose cell bodies are in the S2–S4 region of the spinal cord pass to the ventral rami of spinal nerves and enter the pelvic splanchnic nerves. the pelvic splanchnic nerves innervate terminal ganglia in the transverse colon to the rectum, and they contribute to the hypogastric plexus. the hypogastric plexus and its derivatives innervate the lower colon, rectum, urinary bladder, and the reproductive system organs in the pelvis. -the axons of sensory neurons run alongside ANS axons within ANS nerves and plexuses. These sensory neurons are not strictly part of the ANS, but they play important roles in monitoring the activity of structures regulated by the ANS -some sensory neurons are part of the reflex arcs regulating organ activities ENTERIC NERVOUS SYSTEM -the enteric nervous system consists of nerve plexuses within the wall if the digestive tract. The plexuses have contributions from three sources: sensory neurons that connect the digestive tract to the CNS, ANS motor neurons that connect the CNS to the digestive tract, and enteric neurons, which are confined to the enteric plexuses -the CNS is capable of monitoring the digestive tract and controlling its smooth muscle and glands through autonomic reflexes. for example, sensory neurons detect stretch of the digestive tract, and action potentials are transmitted to the CNS. in response, the CNS sends action potentials to glands in the digestive tract, causing them to secrete. -there are three major types of enteric neurons: enteric sensory neurons, enteric motor neurons, and enteric interneurons -enteric sensory neurons detect changes in the chemical composition of the contents of the digestive tracts or detect stretch of the digestive tract wall -enteric motor neurons stimulate or inhibit smooth muscle contraction and gland secretions -enteric interneurons connect enteric sensory and motor neurons to each other -a unique feature of enteric neurons is that they are capable of monitoring and controlling the digestive tract independently of the CNS through local reflexes. for example, stretch of the digestive tract is detected by enteric sensory neurons, which stimulate enteric interneurons. the enteric interneurons stimulate enteric motor neurons, which stimulate glands to secrete. although the enteric nervous system is capable of controlling the activities of the digestive tract completely independently of the CNS, the two systems normally work together. Physiology of the Autonomic Nervous System SYMPATHETIC VS PARASYMPATHETIC ACTIVITY -the sympathetic and parasympathetic divisions of the ANS maintain homeostasis by adjusting body functions to match the level of physical activity. to maintain homeostasis, the ANS must be able to increase as well as decrease the activities of the various organs of the body. -the ANS has the ability to adjust activity in both directions due to dual innervation. dual innervation means that the ANS innervates most organs by both sympathetic and parasympathetic neurons. -examples of dually innervated organs are the gastrointestinal tract, heart, urinary bladder, and reproductive tract -dual innervation of organs is not universal. for example, sweat glands and blood vessels are innervated by sympathetic neurons almost exclusively. -where dual innervation exists, one division may be more predominant than the other division. for example, parasympathetic innervation of the gastrointestinal tract is more extensive and exerts a greater influence than does sympathetic innervation. -in cases of dual innervation of a single organ, the sympathetic division has a major influence under conditions of physical activity or stress, whereas the parasympathetic division has a great influence under resting conditions. However the sympathetic division is not inactive during resting conditions; rather, it plays a major role during rest by maintaining blood pressure and body temperature. -during physical exercise the sympathetic division shunts blood and nutrients to structures that are active and decrease the activity of nonessential organs, this is referred to as the fight or flight response -although neither the sympathetic nor the parasympathetic division is chronically active, or chronically inactive, there are different levels of regulation, depending on the body’s activity level. These differences in regulation of the same organs are the result of the unique neurotransmitters released by the postganglionic fibers and the specific receptors on the target cells NEUROTRANSMITTERS -sympathetic and parasympathetic neurons secrete one of the two transmitters: acetylcholine and norepinephrine -if the neurons secrets acetylcholine, it is called a cholinergic neuron -if the neuron secretes norepinephrine, it is called a adrenergic neuron -adrenergic neurons are so named because at one time they were believed to secrete adrenaline, also called epinephrine -all preganglionic neurons of the sympathetic and parasympathetic division are cholinergic -all postganglionic neurons of the parasympathetic division are cholinergic -almost all postganglionic neurons of the sympathetic division are adrenergic, but a few postganglionic neurons that innervate thermoregulatory swear glands are cholinergic -substances other than the regular neurotransmitters have been extracted from ANS neurons. these substances include nitric oxide; fatty acids, such as eicosanoids peptides, such as gastrin, somatostatin, cholecystokinin, vasoactive intestinal peptide, enkephalins, and substance P; and monoamines, such as dopamine, serotonin, and histamine RECEPTORS -receptors for acetylcholine and norepinephrine are located in the plasma membranes of target cells -receptors are identified as cholinergic and adrenergic based on the type of neurotransmitter that binds to the receptor -the combination of neurotransmitter and receptor functions as a signal to cells, causing them to respond -depending on the type of receptor, the response is excitatory or inhibitory -other substances, such as drugs, can also interact with receptors to alter the activity of the ANS -agonists bind to specific receptors and activate them -antagonists bind to specific receptors and prevent them from being activated -some drugs that affect the ANS have important value in treating certain diseases because they can increase activities or decrease activities normally controlled by the ANS. Chemicals that affect the ANS are also found in medically hazardous substances, such as tobacco and insecticides -both direct and indirect acting drugs influence the ANS. direct acting drugs bond to ANS receptors to produce their effects. for example, agonists, or stimulating agents, bind to specific receptors and activate them, and antagonists, or blocking agents, bind to specific receptors and prevent them from being activated. -some indirect acting drugs produce a stimulatory effect by causing the release of neurotransmitters or by preventing neurotransmitters from breaking down metabolically. Other indirect acting drugs produce an inhibitory effect by preventing the biosynthesis or release of neurotransmitters -drugs that bind to nicotinic receptors and activate them are nicotinic agents. nicotinic agents bind to the nicotinic receptors on all postganglionic neurons within autonomic ganglia and produce stimulation CHOLINERGIC RECEPTORS -cholinergic receptors are receptors to which acetylcholine binds. cholinergic receptors are classified as either nicotinic receptors or muscarinic receptors. -the classification of these receptors is based on laboratory findings that nicotine, an alkaloid in tobacco, can bind to some cholinergic receptors, whereas muscarine, an alkaloid extracted from poisonous mushrooms can bind to other cholinergic receptors -nicotinic receptors are located in the membranes of all postganglionic neurons in autonomic ganglia and the membranes of skeletal muscle cells. muscarinic receptors are located in the membranes of effector cells that respond to acetylcholine released from postganglionic neurons. ADRENERGIC RECEPTORS -adrenergic receptors are receptors to which norepinephrine or epinephrine binds -they are located in the plasma membranes of effectors innervated by the sympathetic division -the response of cells to norepinephrine or epinephrine binding to adrenergic receptors is mediated through G proteins. depending on the effector, the activation of G proteins can result in excitatory or inhibitory responses. -adrenergic receptors are subdivided into two major categories: alpha receptors and beta receptors -epinephrine has a greater effect than norepinephrine on most alpha and beta receptors -adrenergic receptors can be stimulated in two ways: by direct synaptic communication or by epinephrine and norepinephrine released from the adrenal gland. sympathetic postganglionic neurons release norepinephrine, which stimulates adrenergic receptors within synapses. Regulation of the Autonomic Nervous System -an individuals ability to maintain homeostasis is dependent o the regulatory activity of the ANS. much ANS regulation occurs through autonomic reflexes. -autonomic reflexes involve adjustments to smooth muscle, cardiac muscle, and glandular tissue in response to stimuli -baroreceptors or stretch receptors, in the walls of large arteries near the heart detect changes in blood pressure, and sensory neurons transmit information from the baroreceptors through the glossopharyngeal and vagus nerves to the medulla oblongata. -interneurons in the medulla oblongata integrate the information, and action potentials are produced in autonomic neurons that extend to the heart. if baroreceptors detect a change in blood pressure, autonomic reflexes change the heart rate, which returns the blood pressure to normal. -a sudden increase in blood pressure initiates a parasympathetic reflex, which inhibits cardiac muscle cells and reduces the heart rate, thereby bringing blood pressure down toward its normal value. conversely, a sudden decrease in blood pressure initiates a sympathetic reflex, which stimulates the heart to increase its rate and force of contraction, thereby increasing blood pressure. -other autonomic reflexes help regulate blood pressure. for example, numerous sympathetic neurons transmit a low but relatively constant frequency of action potentials that stimulate blood vessels throughout the body, keeping them partially constricted. if the vessels constrict further, blood pressure increases; if they dilate, blood pressure decreases. thus, altering the frequency of action potentials delivered to blood vessels along sympathetic neurons can either raise or lower blood pressure. -the brainstem and the spinal cord contain important autonomic reflex centers responsible for maintaining homeostasis. however, the hypothalamus is in overall control of the ANS. almost any type of autonomic response can be evoked by stimulating a part of the hypothalamus, which in turn stimulates ANS centers in the brainstem or spinal cord. although there is overlap, there is some division localization in the hypothalamus. for the most part, stimulation of the posterior hypothalamus produces sympathetic responses, whereas stimulation of the anterior hypothalamus produces parasympathetic responses.  -the hypothalamus has connections with the cerebrum and is an important part of the limbic system, which plays an important role in emotions. the hypothalamus integrates thoughts and emotions to produce ANS responses. for example, pleasant thoughts of a delicious meal initiate increased secretion by salivary glands and by glands within the stomach, as well as increased smooth muscle contractions within the digestive system. these responses are controlled by parasympathetic neurons. emotions such as anger cause sympathetic stimulation, which increases blood pressure by increasing the heart rate and constricting the blood vessels. -the enteric nervous system is involved with both autonomic reflexes and local reflexes that regulate the activity of the digestive tract. autonomic reflexes help control the digestive tract because sensory neurons of the enteric plexuses supply the CNS with information about intestinal contents, and ANS neurons to the enteric plexuses affect the responses of smooth muscle and glands within the digestive tract wall. for example, sensory neurons detecting stretch of the digestive tract wall send action potentials to the CNS. In response, the CNS sends action potentials through the ANS, causing smooth muscle in the digestive tract wall to contract. -the neurons of the enteric nervous system also operate independently of the CNS to produce local reflexes. a local reflex does not involve the CNS, but produces an involuntary, unconscious, stereotypical response to a stimulus. for example, sensory neurons not connected to the CNS detect stretch of the digestive tract wall. these sensory neurons send action potentials through the enteric plexuses to motor neurons, causing smooth muscle to contract or relax. Functional Generalizations About the Autonomic Nervous System -the sympathetic and parasympathetic divisions work together to modulate the activities of the body to maintain homeostasis, in both times of physical activity as well as times of rest. however, do not confuse the level of activity with stimulation and inhibition. -both divisions of the ANS produce stimulatory and inhibitory effects, but the effect depends on the role the tissue plays in times of physical activity and times of rest. -the sympathetic division, which is more influential during times of increased physical activity, stimulates smooth muscle contraction in blood vessel walls, causing vasoconstriction, but inhibits smooth muscle contractions in the lungs, causing dilation of lung air passageways. thus, it is not true that one division of the ANS is always stimulatory and the other is always inhibitory. -some disorders are the result of abnormal levels of stimulation by one or both of divisions. for example, Hirschsprung disease is caused by ineffective parasympathetic stimulation of the colon. -most of the effectors of the ANS are dually innervated, meaning both the sympathetic division and the parasympathetic division regulate the effector. when a single structure is innervated by both autonomic divisions, the two divisions usually produce opposite effects on the structure. as a consequence, the ANS is capable of both increasing and decreasing the activity of the structure. In the gastrointestinal tract, for example, increased parasympathetic activity increases secretion from glands, whereas increased sympathetic activity decreases secretion. however, in a few instances the effect of the two divisions is not clearly opposite. for example, both divisions of the ANS increase salivary secretion: the parasympathetic division initiates the production of a large volume of thin, watery saliva, and the sympathetic division causes the secretion of a small volume of viscous saliva. -the ANS regulates the activity of many structures that cooperate to carry out a specific function. this may involve activity of just one division or both. one autonomic division can coordinate the activities of different structures. for example, the parasympathetic division stimulates the pancreas to release digestive enzymes into the small intestine and stimulates contractions of the small intestine to mix the digestive enzymes with the food. these responses enhance the digestion and absorption of the food. alternatively, both divisions of the ANS can act together to coordinate the activity of different structures. in the male, the parasympathetic division initiates erection of the penis, whereas the sympathetic division stimulates the release of secretions from male reproductive glands and helps initiate ejaculation. -the effect of the ANS may be general, in that many areas of the body are involved, or the effect may be local, only altering the activity of structures in a specific region of the body. in addition, the extent to which the autonomic divisions affect many areas of the body at the same time is not equal. The sympathetic division has a more general effect than the parasympathetic division because activation of the sympathetic division often causes secretion of both epinephrine and norepinephrine from the adrenal medulla. these hormones circulate in the blood and stimulate effectors throughout the body. because circulating epinephrine and norepinephrine can persist for a few minutes before being broken down, they can also produce an effect for a longer time than the direct stimulation of effectors by postganglionic sympathetic axons. -the sympathetic division diverges more than the parasympathetic division. each sympathetic preganglionic neuron synapses with many postganglionic neurons, whereas each parasympathetic preganglionic neuron synapses with about two postganglionic neurons. consequently, stimulation of sympathetic preganglionic neurons can result in greater stimulation of an effector. -sympathetic stimulation often activates many different kinds of effectors at the same time as a result of CNS stimulation or epinephrine and norepinephrine release from the adrenal medulla. it is possible, however, for the CNS to selectively activate effectors. for example, vasoconstriction of cutaneous blood vessels in a cold hand is not always associated with an increased heart rate or other responses controlled by the sympathetic division.