Functions of the autonomic nervous system

The two divisions of the ANS are dominant under different conditions: as stated previously, the sympathetic system is activated during emergency "fight-or-flight" reactions and during exercise, and the parasympathetic system is predominant during quiet resting conditions. As such, the physiological effects caused by each system are quite predictable. In other words, all the changes in organ and tissue function induced by the sympathetic system work together to support strenuous physical activity and changes induced by the parasympathetic system are appropriate when the body is resting. Several of the specific effects elicited by sympathetic and parasympathetic stimulation of various organs and tissues are summarized in Table 9.4.

The "fight-or-flight" reaction elicited by the sympathetic system is essentially a whole-body response. Changes in organ and tissue function throughout the body are coordinated so that delivery of well-oxygenated, nutrient-rich blood to the working skeletal muscles increases. Heart rate and myocardial contractility are increased so that the heart pumps more blood per minute. Sympathetic stimulation of vascular smooth muscle causes widespread vasoconstriction, particularly in organs of the gastrointestinal system and in the kidneys. This vasoconstriction serves to "redirect" or redistribute the blood away from these metabolically inactive tissues and toward the contracting muscles. Bronchodilation in the lungs facilitates the movement

Table 9.4 Effects of Autonomic Nerve Activity on Some Effector Tissues

Tissue Sympatheticreceptor Sympathetic stimulation Parasympathetic stimulation h-1 O

Radial muscle of iris Sphincter muscle of iris Ciliary muscle

Heart

Arterioles

Skin

Abdominal viscera Kidney

Skeletal muscle

Airways Glands

Lungs

Liver

Adipose tissue Sweat glands

Muscarinic a.

Contraction (mydriasis) Relaxation for far vision

Increased heart rate Increased contractility

Strong constriction Strong constriction Strong constriction Weak constriction

Bronchodilation Decreased secretion

Glycogenolysis Gluconeogenesis

Lipolysis

Generalized sweating Localized sweating

Contraction (miosis) Contraction for near vision

Decreased heart rate

Bronchoconstriction Increased secretion tm cn cn TO S

Si oT

Adrenal medulla Nicotinic

Salvivary glands a,, fi

Stomach

Motility (Xj, P2

Sphincters a1 Secretion

Intestine

Motility a1; p2

Sphincters a1 Secretion

Gall Bladder p.,

Pancreas

Exocrine a

Endocrine a (Islet p cells)

Urinary Bladder

Detrusor muscle fi2 (Bladder wall)

Sphincter o^

Kidney px

Increased secretion of epinephrine and norepineprine

Small volume K + and water secretion

Decreased contraction

Decreased contraction

Relaxation

Decreased secretion Decreased secretion

Relaxation Contraction

Large volume K + and water secretion Amylase secretion

Increased relaxation Stimulation

Increased relaxation Stimulation

Contraction

Increased secretion Increased secretion

Contraction Relaxation

Renin secretion of air in and out of the lungs so that uptake of oxygen from the atmosphere and elimination of carbon dioxide from the body are maximized. An enhanced rate of glycogenolysis (breakdown of glycogen into its component glucose molecules) and gluconeogenesis (formation of new glucose from noncarbohydrate sources) in the liver increases the concentration of glucose molecules in the blood. This is necessary for the brain because glucose is the only nutrient molecule that it can utilize to form metabolic energy. An enhanced rate of lipolysis in adipose tissue increases the concentration of fatty acid molecules in the blood. Skeletal muscles then utilize these fatty acids to form metabolic energy for contraction. Generalized sweating elicited by the sympathetic system enables the individual to thermoregulate during conditions of increased physical activity and heat production. Finally, the eye is adjusted so that the pupil dilates, letting more light in toward the retina (mydriasis) and the lens adapts for distance vision.

The parasympathetic system decreases heart rate, which helps to conserve energy under resting conditions. Salivary secretion is enhanced to facilitate swallowing food. Gastric motility and secretion are stimulated to begin processing of ingested food. Intestinal motility and secretion are also stimulated to continue processing and to facilitate absorption of these nutrients. Exocrine and endocrine secretion from the pancreas is promoted. Enzymes released from the exocrine glands of the pancreas contribute to the chemical breakdown of the food in the intestine, and insulin released from the pancreatic islets promotes storage of nutrient molecules within the tissues once they are absorbed into the body. Another bodily maintenance type of function caused by the parasympathetic system is contraction of the urinary bladder, which results in urination. Finally, the eye is adjusted so that the pupil contracts (miosis) and the lens adapts for near vision.

Pharmacy application: cholinomimetic drugs

Cholinomimetic drugs produce effects in a tissue resembling those caused from stimulation by the parasympathetic nervous system. These drugs have many important uses including treatment of gastrointestinal and urinary tract disorders that involve depressed smooth muscle activity without obstruction. For example, postoperative ileus is characterized by a loss of tone or paralysis of the stomach or bowel following surgical manipulation. Urinary retention may also occur postoperatively or may be secondary to spinal cord injury or disease (neurogenic bladder). Normally, parasympathetic stimulation of the smooth muscle in each of these organ systems causes contraction to maintain gastrointestinal motility as well as urination. The pharmacotherapy of these disorders has two different approaches. One type of agent is a muscarinic receptor agonist that mimics the effect of the parasympathetic neurotransmitter, acetylcholine, and stimulates smooth muscle contraction. One of the most widely used agents in this category is bethanechol, which can be given subcutane-ously. Another approach is to increase the concentration, and therefore activity, of endogenously produced acetylcholine in the neuroeffector synapse. Administration of an acetylcholinesterase inhibitor prevents degradation and removal of neuronally released acetylcholine. In this case, neostigmine is the most widely used agent and may be given subcutaneously or orally.

Pharmacy application: muscarinic receptor antagonists

Inspection of the retina during an ophthalmoscopic examination is greatly facilitated by mydriasis, or the dilation of the pupil. Parasympathetic stimulation of the circular muscle layer in the iris causes contraction and a decrease in the diameter of the pupil. Administration of a muscarinic receptor antagonist such as atropine or scopolamine prevents this smooth muscle contraction. As a result, sympathetic stimulation of the radial muscle layer is unopposed, causing an increase in the diameter of the pupil. These agents are given in the form of eye drops that act locally and limit the possibility of systemic side effects.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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