Physiology 5th Ed.

Chapter 2. Autonomic Nervous System

 

The motor (efferent) nervous system has two components: the somatic and the autonomic. These two systems differ in a number of ways but are chiefly distinguished by the types of effector organs they innervate and the types of functions they control.

The somatic nervous system is a voluntary motor system under conscious control. Each of its pathways consists of a single motoneuron and the skeletal muscle fibers it innervates. The cell body of the motoneuron is located in the central nervous system (CNS), in either the brain stem or spinal cord, and its axon synapses directly on skeletal muscle, the effector organ. The neurotransmitter acetylcholine (ACh) is released from presynaptic terminals of the motoneurons and activates nicotinic receptors located on the motor end plates of the skeletal muscle. An action potential in the motoneuron causes an action potential in the muscle fiber, which causes the muscle to contract. (For a complete discussion of the somatic nervous system, see Chapter 1.)

The autonomic nervous system is an involuntary system that controls and modulates the functions primarily of visceral organs. Each pathway in the autonomic nervous system consists of two neurons: a preganglionic neuron and a postganglionic neuron. The cell body of each preganglionic neuron resides in the CNS. The axons of these preganglionic neurons synapse on the cell bodies of postganglionic neurons in one of several autonomic ganglia located outside the CNS. The axons of the postganglionic neurons then travel to the periphery, where they synapse on visceral effector organs such as the heart, bronchioles, vascular smooth muscle, gastrointestinal tract, bladder, and genitalia. All preganglionic neurons of the autonomic nervous system release ACh. Postganglionic neurons release either ACh or norepinephrine, or, in some cases, neuropeptides.

ORGANIZATION AND GENERAL FEATURES OF THE AUTONOMIC NERVOUS SYSTEM

AUTONOMIC RECEPTORS

SUMMARY

ent The autonomic nervous system is composed of two major divisions, the sympathetic and the parasympathetic, which operate in a coordinated fashion to regulate involuntary functions. The sympathetic division is thoracolumbar, referring to its origin in the spinal cord. The parasympathetic division is craniosacral, referring to its origin in the brain stem and sacral spinal cord.

ent Efferent pathways in the autonomic nervous system consist of a preganglionic and a postganglionic neuron, which synapse in autonomic ganglia. The axons of postganglionic neurons then travel to the periphery to innervate the effector organs. The adrenal medulla is a specialized ganglion of the sympathetic division; when stimulated, it secretes catecholamines into the circulation.

ent Often, the sympathetic and parasympathetic innervations of organs or organ systems have reciprocal effects. These effects are coordinated by autonomic centers in the brain stem. For example, autonomic centers in the brain stem control the heart rate by modulating sympathetic and parasympathetic activity to the SA node.

ent Receptors for neurotransmitters in the autonomic nervous system are either adrenergic (adrenoreceptors) or cholinergic (cholinoreceptors). Adrenoreceptors are activated by the catecholamines norepinephrine and epinephrine. Cholinoreceptors are activated by ACh.

ent Autonomic receptors are coupled to G proteins, which may be stimulatory (Gs) or inhibitory (Gi). The G proteins in turn activate or inhibit enzymes that are responsible for the final physiologic actions.

ent The mechanism of action of the adrenoreceptors can be explained as follows: α1 Receptors act through activation of phospholipase C and generation of IP3. β1 and β2 receptors act through activation of adenylyl cyclase and generation of cAMP. α2 Receptors act through inhibition of adenylyl cyclase.

ent The mechanism of action of cholinoreceptors can be explained as follows: Nicotinic receptors act as ion channels for Na+ and K+. Many muscarinic receptors have the same mechanism of action as α1receptors; some muscarinic receptors act by inhibiting adenylyl cyclase; a few muscarinic receptors involve direct action of a G protein on the physiologic mechanism.


Challenge Yourself

Answer each question with a word, phrase, sentence, or numerical solution. When a list of possible answers is supplied with the question, one, more than one, or none of the choices may be correct. The correct answers are provided at the end of the book.

1 Which of the following actions is/are mediated by β2 receptors: increased heart rate, contraction of gastrointestinal sphincters, contraction of vascular smooth muscle, dilation of airways, relaxation of bladder wall?

2 A woman who is taking atropine for a gastrointestinal disorder notices that her pupils are dilated. This has occurred because atropine blocks ___________ receptors on the __________muscle of the iris.

3 Which of the following is/are characteristic of the parasympathetic nervous system, but not of the sympathetic nervous system: ganglia in or near target tissues, nicotinic receptors on postganglionic neurons, muscarinic receptors on some target tissues, β1 receptors on some target tissues, cholinergic preganglionic neurons?

4 Propranolol causes a decrease in heart rate because it _______________ the _____________ receptors in the sinoatrial node of the heart.

5 Which of the following actions is/are mediated by the adenylyl cyclase mechanism: effect of parasympathetic nervous system to increase gastric acid secretion, effect of epinephrine to increase cardiac contractility, effect of epinephrine to increase heart rate, effect of acetylcholine to decrease heart rate, effect of acetylcholine to constrict airways, constriction of vascular smooth muscle in splanchnic blood vessels?

6 What enzyme is responsible for the fact that the adrenal medulla synthesizes more epinephrine than norepinephrine?

7 A man had a pheochromocytoma that caused severe elevation of his blood pressure. Prior to surgery to remove the tumor, he received the wrong drug, which caused a further elevation in blood pressure. Name two classes of drugs that may have been given in error to cause this further elevation.

8 A man’s bladder is full. When he voids (micturition), __________ receptors cause __________ of the detrusor muscle and __________ receptors cause __________ of the internal sphincter.

9 In the action of α1 receptors, what is the correct order of steps: αq binds to GDP, αq binds to GTP, generation of IP3, release of Ca2+ from intracellular stores, activation of protein kinase, activation of phospholipase C?

10 Which of the following actions are mediated by muscarinic receptors? Slowing of conduction velocity in AV node; gastric acid secretion, mydriasis, contraction of gastrointestinal sphincters, erection, renin secretion, sweating on a hot day.


SELECTED READINGS

Burnstock G, Hoyle CHV: Autonomic Neuroeffector Mechanisms. Newark, NJ, Harwood Academic Publishers, 1992.

Changeux J-P: The acetylcholine receptor: An “allosteric” membrane protein. Harvey Lect 75:85–254, 1981.

Gilman AG: Guanine nucleotide-binding regulatory proteins and dual control of adenylate cyclase. J Clin Invest 73:1–4, 1984.

Houslay MD, Milligan G: G Proteins as Mediators of Cellular Signalling Processes. New York, John Wiley, 1990.

Lefkowitz RJ, Stadel JM, Caron MG: Adenylate cyclase-coupled beta-adrenergic receptors: Structure and mechanisms of activation and desensitization. Annu Rev Biochem 52:159–186, 1983.

Pick J: The Autonomic Nervous System: Morphological, Comparative, Clinical and Surgical Aspects. Philadelphia, JB Lippincott, 1970.