Chapter 15 The Autonomic Nervous System And Visceral Reflexes

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autonomic nervous system (ANS)

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can be defined as a motor nervous system that controls glands, cardiac muscle, and smooth muscle. It is also called the visceral motor system to distinguish it from the somatic motor system that controls the skeletal muscles. The primary target organs of the ANS are viscera of the thoracic and abdominopelvic cavities and some structures of the body wall, including cutaneous blood vessels, sweat glands, and piloerector muscles.

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visceral reflexes

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unconscious, automatic, stereotyped responses to stimulation

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Receptors, integrating center, and effector

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autonomic activity involves a visceral reflex arc that includes receptors (nerve endings that detect stretch, tissue damage, blood chemicals, body temperature, and other internal stimuli), afferent neurons leading to an integrating center in the CNS, interneurons in the CNS, efferent neurons carrying motor signals away from the CNS, and finally an effector that carries out the end response.

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sympathetic division

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adapts the body in many ways for physical activity—it increases alertness, heart rate, blood pressure, pulmonary airflow, blood glucose concentration, and blood flow to cardiac and skeletal muscle, but at the same time, it reduces blood flow to the skin and digestive tract. Cannon referred to extreme sympathetic responses as the “fight-or-flight” reaction because it comes into play when an animal must attack, defend itself, or flee from danger. In our own lives, this reaction occurs in situations involving arousal, exercise, competition, stress, danger, trauma, anger, or fear.

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parasympathetic division

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by comparison, has a calming effect on many body functions. It is associated with reduced energy expenditure and normal bodily maintenance, including such functions as digestion and waste elimination. This is often called the “resting-and-digesting” state.

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autonomic tone

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the balance between sympathetic tone and parasympathetic tone shifts in accordance with the body’s changing needs. Parasympathetic tone, for example, maintains smooth muscle tone in the intestines and holds the resting heart rate down to about 70 to 80 beats/min. If the parasympathetic vagus nerves to the heart are cut, the heart beats at its own intrinsic rate of about 100 beats/min. Sympathetic tone keeps most blood vessels partially constricted and thus maintains blood pressure. A loss of sympathetic tone can cause such a rapid drop in blood pressure that a person goes into shock and may faint.

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preganglionic fiber and postganglionic fiber

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preganglionic fiber is myelinated and leads from a soma in the brainstem or spinal cord to the autonomic ganglion. It synapses there with a neuron that issues an unmyelinated postganglionic fiber to the target cells. In contrast to somatic motor neurons, postganglionic fibers of the ANS do not end by synapsing with a specific target cell, but with a chain of varicosities that diffusely release neurotransmitter into the tissue and may stimulate many cells simultaneously

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white communicating ramus and gray communicating ramus

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The preganglionic fibers are small myelinated fibers that travel from the spinal nerve to the ganglion by way of the white communicating ramus, which gets its color and name from the myelin. Unmyelinated postganglionic fibers leave the ganglion by way of the gray communicating ramus, named for its lack of myelin and duller color, and by other routes. This ramus returns to the spinal nerve. Postganglionic fibers extend the rest of the way to the target organ.

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The spinal nerve route

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Some postganglionic fibers exit a ganglion by way of the gray ramus, return to the spinal nerve or its subdivisions, and travel the rest of the way to the target organ. This is the route to most sweat glands, piloerector muscles, and blood vessels of the skin and skeletal muscles.

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The sympathetic nerve route

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Other postganglionic fibers leave by way of sympathetic nerves that extend to the heart, lungs, esophagus, and thoracic blood vessels. These nerves form a carotid plexus around each carotid artery of the neck and issue fibers from there to effectors in the head—including sweat, salivary, and nasal glands; piloerector muscles; blood vessels; and dilators of the iris. Some fibers from the superior and middle cervical ganglia form the cardiac nerves to the heart. (The cardiac nerves also contain parasympathetic fibers.)

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The splanchnic nerve route.

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Some of the fibers that arise from spinal nerves T5 to T12 pass through the sympathetic ganglia without synapsing. Beyond the ganglia, they continue as splanchnic nerves, which lead to a second set of ganglia called collateral (prevertebral) ganglia. Here the preganglionic fibers synapse with the postganglionics.

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celiac, superior mesenteric, and inferior mesenteric ganglia (abdominal aortic plexus)

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There are three major collateral ganglia in this plexus—the celiac, superior mesenteric, and inferior mesenteric ganglia—located at points where arteries of the same names branch off the aorta. The postganglionic fibers accompany these arteries and their branches to the target organs.

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adrenal (suprarenal) glands

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rest like hats on the superior poles of the kidneys. Each adrenal is actually two glands with different functions and embryonic origins. The outer rind, the adrenal cortex, secretes steroid hormones. The inner core, the adrenal medulla, is essentially a sympathetic ganglion. It consists of modified postganglionic neurons without dendrites or axons. Sympathetic preganglionic fibers penetrate through the cortex and terminate on these cells. The sympathetic nervous system and adrenal medulla are so closely related in development and function that they are referred to collectively as the sympatho-adrenal system.

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Oculomotor nerve (III)

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The oculomotor nerve carries parasympathetic fibers that control the lens and pupil of the eye. The preganglionic fibers enter the orbit and terminate in the ciliary ganglion behind the eyeball. Postganglionic fibers enter the eyeball and innervate the ciliary muscle, which thickens the lens, and the pupillary constrictor, which narrows the pupil.

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Facial nerve (VII)

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The facial nerve carries parasympathetic fibers that regulate the tear glands, salivary glands, and nasal glands. Soon after the facial nerve emerges from the pons, its parasympathetic fibers split away and form two smaller branches. The superior branch ends at the pterygopalatine ganglion near the junction of the maxilla and palatine bone. Postganglionic fibers then continue to the tear glands and glands of the nasal cavity, palate, and other areas of the oral cavity. The inferior branch crosses the middle-ear cavity and ends at the submandibular ganglion near the angle of the mandible. Postganglionic fibers from here supply salivary glands in the floor of the mouth.

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Glossopharyngeal nerve (IX)

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The glossopharyngeal nerve carries parasympathetic fibers concerned with salivation. The preganglionic fibers leave this nerve soon after its origin and form the tympanic nerve. A continuation of this nerve crosses the middle-ear cavity and ends in the otic ganglion near the foramen ovale. The postganglionic fibers then follow the trigeminal nerve to the parotid salivary gland just in front of the earlobe.

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Vagus nerve (X)

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The vagus nerve carries about 90% of all parasympathetic preganglionic fibers. It travels down the neck and forms three networks in the mediastinum of the chest—the cardiac plexus, which supplies fibers to the heart; the pulmonary plexus, whose fibers accompany the bronchi and blood vessels into the lungs; and the esophageal plexus, whose fibers regulate swallowing.

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pelvic splanchnic nerves, inferior hypogastric plexus, and pelvic nerves

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The remaining parasympathetic fibers arise from levels S2 to S4 of the spinal cord. They travel a short distance in the anterior rami of the spinal nerves and then form pelvic splanchnic nerves that lead to the inferior hypogastric plexus. Some parasympathetic fibers synapse here, but most pass through this plexus and travel by way of pelvic nerves to the terminal ganglia in their target organs: the distal half of the colon, the rectum, urinary bladder, and reproductive organs. With few exceptions, the parasympathetic system does not innervate body wall structures (sweat glands, piloerector muscles, or cutaneous blood vessels).

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enteric nervous system

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The digestive tract has a nervous system of its own called the enteric nervous system. Unlike the ANS proper, it does not arise from the brainstem or spinal cord, but like the ANS, it innervates smooth muscle and glands. Thus, opinions differ on whether it should be considered part of the ANS. It consists of about 100 million neurons embedded in the wall of the digestive tract—perhaps more neurons than there are in the spinal cord—and it has its own reflex arcs. The enteric nervous system regulates the motility of the esophagus, stomach, and intestines and the secretion of digestive enzymes and acid.

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Acetylcholine (ACh).

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ACh is secreted by the preganglionic fibers in both divisions and the postganglionic fibers of the parasympathetic division. A few sympathetic postganglionics also secrete ACh—those that innervate sweat glands and some blood vessels. Any nerve fiber that secretes ACh is called a cholinergic (CO-li-NUR-jic) fiber, and any receptor that binds it is called a cholinergic receptor. There are two categories of cholinergic receptors:

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Muscarinic (MUSS-cuh-RIN-ic) receptors

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These are named for muscarine, a mushroom toxin used in their discovery. All cardiac muscle, smooth muscle, and gland cells that receive cholinergic innervation have muscarinic receptors. There are different subclasses of muscarinic receptors with different effects; thus ACh excites intestinal smooth muscle by binding to one type of muscarinic receptor, and inhibits cardiac muscle by binding to a different type. Muscarinic receptors work through a variety of second-messenger systems.

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Nicotinic (NIC-oh-TIN-ic) receptors

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These are named for another botanical toxin helpful to their discovery—nicotine. They occur at all synapses in the autonomic ganglia, where the preganglionic fibers stimulate the postganglionic cells; on cells of the adrenal medulla; and at the neuromuscular junctions of skeletal muscle fibers. The binding of ACh to a nicotinic receptor is always excitatory. Nicotinic receptors work by opening ligand-gated ion channels and producing an excitatory postsynaptic potential in the target cell.

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Norepinephrine (NE)

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This neurotransmitter is secreted by nearly all sympathetic postganglionic fibers. Nerve fibers that secrete it are called adrenergic fibers, and the receptors for it are called adrenergic receptors. (NE is also called noradrenaline, the origin of the term adrenergic.) There are two principal categories of NE receptors:This neurotransmitter is secreted by nearly all sympathetic postganglionic fibers. Nerve fibers that secrete it are called adrenergic fibers, and the receptors for it are called adrenergic receptors. (NE is also called noradrenaline, the origin of the term adrenergic.) There are two principal categories of NE receptors:

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α-adrenergic receptors

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These usually have excitatory effects. For example, the binding of NE to α-adrenergic receptors promotes labor contractions, stimulates piloerection, and constricts dermal blood vessels, yet it inhibits intestinal motility. These contrasting effects result from the different actions of two subclasses of α-adrenergic receptors—α1 and α2. Receptors of the α1 type act through calcium ions as a second messenger, whereas α1 receptors inhibit the synthesis of cyclic AMP (cAMP).

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β-adrenergic receptors

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These are usually inhibitory. For example, NE relaxes and dilates the bronchioles (thus enhancing respiratory airflow) when it binds to 3-adrenergic receptors of the smooth muscle. Yet when it binds to the β-adrenergic receptors of cardiac muscle, it has an excitatory effect. Such contrasting effects—increased pulmonary airflow and a stronger, faster heartbeat—are obviously appropriate to a state of exercise. Here again there are two receptor subclasses, β1 and β2, which mediate different effects. Both types, however, act through cAMP as a second messenger.

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dual innervation

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Most of the viscera receive nerve fibers from both the sympathetic and parasympathetic divisions and thus are said to have dual innervation. In such cases, the two divisions may have either antagonistic or cooperative effects on the same organ.

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vasomotor tone

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The sympathetic fibers to a blood vessel have a baseline sympathetic tone, which keeps the vessels in a state of partial constriction called vasomotor tone. An increase in firing rate constricts a vessel by increasing smooth muscle contraction. A drop in firing frequency dilates a vessel by allowing the smooth muscle to relax. The blood pressure in the vessel, pushing outward on its wall, then dilates the vessel. Thus, the sympathetic division alone exerts opposite effects on the vessels.

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Cerebral cortex

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Even if we usually cannot consciously control the ANS, it is clear that the mind does influence it. Anger raises the blood pressure, fear makes the heart race, thoughts of good food make the stomach rumble, sexual thoughts or images increase blood flow to the genitals, and anxiety inhibits sexual function. The limbic system, an ancient part of the cerebral cortex, is involved in many emotional responses and has extensive connections with the hypothalamus, a site of several nuclei of autonomic control. Thus, the limbic system provides a pathway connecting sensory and mental experiences with the autonomic nervous system.

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Hypothalamus

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Although the major site of CNS control over the somatic motor system is the primary motor cortex, the major control center of the visceral motor system is the hypothalamus. This small but vital region in the floor of the brain contains many nuclei for primitive functions, including hunger, thirst, thermoregulation, emotions, and sexuality. Artificial stimulation of different regions of the hypothalamus can activate the fight-or-flight response typical of the sympathetic nervous system or have the calming effects typical of the parasympathetic. Output from the hypothalamus travels largely to nuclei in more caudal regions of the brainstem, and from there to the cranial nerves and the sympathetic neurons in the spinal cord.

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Midbrain, pons, and medulla oblongata

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These regions of the brainstem house numerous autonomic nuclei: centers for cardiac and vasomotor control, salivation, swallowing, sweating, gastrointestinal secretion, bladder control, pupillary constriction and dilation, and other primitive functions. Many of these nuclei belong to the reticular formation, which extends from the medulla to the hypothalamus. Autonomic output from these nuclei travels by way of the spinal cord and the oculomotor, facial, glossopharyngeal, and vagus nerves.

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Spinal cord.

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Finally, the spinal cord integrates such autonomic reflexes as micturition (urination), defecation, erection, and ejaculation. Fortunately, the brain is able to inhibit defecation and urination consciously, but when injuries sever the spinal cord from the brain, the autonomic spinal reflexes alone control the elimination of urine and feces.