The Autonomic Nervous System Participates in Many Homeostatic Reflexes
Many of the body's visceral functions are regulated by autonomic reflexes. As with reflex arcs in the somatic nervous system (see Chapter 7), autonomic reflex arcs also include a sensory side to the arc, including a visceral receptor; a sensory neuron, often called a visceral afferent neuron; and one or more synapses in the CNS.
I he ANS is usually defined as the peripheral motor preganglionic and postganglionic neurons. Visceral afferent neurons are often not included in this definition but generally are essential parts of the autonomic reflex arc.Autonomic reflexes are extremely common and are described in detail for each body system in later chapters. A few are described briefly here as examples.
Table 13-1
Responses of Effector Organs to Autonomic Nerve Impulses and Circulating Catecholamines
| Effector organ | Cholinergic impulses: response | Noradrenergic impulses | |||
| Receptor | Response | ||||
| Eye | |||||
| Radial muscle of iris | veins | — | a β2 | Constriction Dilation | |
| Lung | |||||
| Bronchial muscle | Contraction | β2 | Relaxation | ||
| Bronchial glands | Stimulation | «1 β2 | Decrease in secretion Increase in secretion | ||
| Stomach (monogastric) | |||||
| Motility and tone | Increase | a, β | Decrease (usually) | ||
| Sphincters | Relaxation (usually) | «1 | Contraction (usually) | ||
| Secretion | Stimulation | a2 | Inhibition | ||
| Intestine | |||||
| Motility and tone | Increase | a, ∣i | Decrease | ||
| Sphincters | Relaxation (usually) | «1 | Contraction | ||
| Secretion | Stimulation | a2 | Inhibition | ||
| Gallbladder and ducts | Contraction | β2 | Relaxation | ||
| Urinary bladder | |||||
| Detrusor | Contraction | β2 | Relaxation | ||
| Trigone and sphincter | Relaxation | a1 | Contraction | ||
| Ureter | |||||
| Motility and tone | Increase (?) | a1 | Increase | ||
Continued
Table 13-1
Responses of Effector Organs to Autonomic Nerve Impulses and Circulating Catecholamines—cont'd
| Effector organ | Cholinergic impulses: response | Noradrenergic impulses | |
| Receptor | Response | ||
| Reproductive system | |||
| Uterus | Variable* | «1. β2 | Variable |
| Male se? organs | Erection | «1 | Ejaculation |
| Skin | |||
| Pilomotor muscles | — | «1 | Contraction |
| Sweat glands | Generalized secretion | «1 | Localized secretion1 |
| Upper abdominal structures | |||
| Spleen capsule | — | «1 | Contraction |
| β2 | Relaxation | ||
| Adrenal medulla | Secretion of epinephrine and norepinephrine | — | — |
| Liver | — | «V β2 | Glycogenolysis and |
| gluconeogenesis | |||
| Pancreas | |||
| Acini | Increased secretion | α | Decreased secretion |
| Islets | — | a2 | Decreased insulin and |
| glucagon secretion | |||
| β2 | Increased insulin and glucagon | ||
| secretion | |||
| Other glands | |||
| Salivary glands | Profuse, watery secretion | a | Scant, viscous secretion |
| β2 | Amylase secretion | ||
| Lacrimal glands | Secretion | a | Secretion |
| Juxtaglomerular cells | — | βl | Increased in renin secretion |
| Pineal gland | — | β | Increase in melatonin |
| synthesis and secretion | |||
Modified from Weiner N. Taylor P: Neurohumoral transmission: the autonomic and somatic motor nervous systems.
In Gilman AG, Goodman LS, RaIITW, Murad F: Goodman and Gilman's the pharmacological basis of therapeutics, ed 7, NewYork, 1985, Macmillan. Copyright 1985, reproduced with permission of the McGraw-HiII Companies.•Depends on stage of estrous cycle, amount of circulating estrogen and progesterone, pregnancy, and other factors. tOn palms of human hands and in some other locations (adrenergic sweating).
Control of Blood Pressure
Stretch receptors in the internal carotid artery and the aorta detect systemic blood pressure. As blood pressure rises above normal limits in animals, sympathetic adrenergic vasoconstrictor nerves are inhibited, and blood pressure falls back to within normal limits. (It is not clear why this does not happen in hypertensive humans.)
Pupillary Light Reflex
When a flashlight is shone into an animal’s eye, light stimulates photoreceptors in the retina (see Chapter 14). Sensory action potentials arc then transmitted to the brainstem along the optic nerve, where, through several interneurons, parasympathetic cholinergic neurons stimulate the constrictor smooth muscle of the iris. This causes the pupillary diameter to become smaller.
Gastric secretion of digestive fluids in anticipation of food and emptying of the rectum and bladder in response to filling are but a few of the many other autonomic reflexes described in more detail throughout this book.