I1

Figure 11.41

Most sympathetic fibers are adrenergic and secrete norepinephrine at the ends of the postganglionic fibers; parasympathetic fibers are cholinergic and secrete acetylcholine at the ends of the postganglionic fibers. The two arrangements of parasympathetic postganglionic fibers are seen in both cranial and sacral portions. Similarly, sympathetic paravertebral and collateral ganglia are seen in both the thoracic and lumbar portions of the nervous system.

relatively slowly. The nicotinic receptors are in the synapses between the preganglionic and postganglionic neurons of the parasympathetic and sympathetic pathways. They produce rapid, excitatory responses (fig. 11.42). (Receptors at neuromuscular junctions of skeletal muscles are nicotinic.)

Epinephrine and norepinephrine are the two chemical mediators of the sympathetic nervous system. The adrenal gland releases both as hormones, but only norepinephrine is released as a neurotransmitter by the sympathetic nervous system. These biochemicals can then combine with adrenergic receptors of effector cells.

The two major types of adrenergic receptors are termed alpha and beta. Exciting them elicits different responses in the effector organs. For example, stimulation of the alpha receptors in vascular smooth muscle causes vasoconstriction, whereas stimulation of the beta receptors in bronchial smooth muscle causes relaxation leading to bronchodilation. Furthermore, although norepinephrine has a somewhat stronger effect on alpha receptors, both of these mediators can stimulate both kinds of receptors. Consequently, the way each of these adrenergic substances influences effector cells depends on the relative numbers of alpha and beta receptors in the cell membranes.

The enzyme acetylcholinesterase rapidly decomposes the acetylcholine that cholinergic fibers release. (Recall that this decomposition also occurs at the neuro-muscular junctions of skeletal muscle.) Thus, acetyl-

choline usually affects the postsynaptic membrane for only a fraction of a second.

Much of the norepinephrine released from adrener-gic fibers is removed from the synapse by active transport back into the nerve endings. The enzyme monoamine oxidase found in mitochondria then inactivates norepinephrine. This may take a few seconds, during which some molecules may diffuse into nearby tissues or the bloodstream, where other enzymes decompose them. On the other hand, some norepinephrine molecules may escape decomposition and remain active for awhile. For these reasons, norepinephrine is likely to produce a more prolonged effect than acetylcholine. In fact, when the adrenal medulla releases norepinephrine and epinephrine into the blood in response to sympathetic stimulation, these substances may trigger sympathetic responses in organs throughout the body that last up to thirty seconds.

Many drugs influence autonomic functions. Some, like ephedrine, enhance sympathetic effects by stimulating release of norepinephrine from postganglionic sympathetic nerve endings. Others, like reserpine, inhibit sympathetic activity by preventing norepinephrine synthesis. Another group of drugs, which includes pilocarpine, produces parasympathetic effects, and some, like atropine, block the action of acetylcholine on visceral effectors.

Effector Location

Effector Location

Response to Sympathetic Stimulation

Response to Parasympathetic Stimulation

Integumentary system Apocrine glands Eccrine glands Special senses Iris of eye Tear gland Endocrine system Adrenal cortex Adrenal medulla Digestive system

Muscle of gallbladder wall Muscle of intestinal wall Muscle of internal anal sphincter Pancreatic glands Salivary glands Respiratory system

Muscles in walls of bronchioles Cardiovascular system

Blood vessels supplying muscles

Blood vessels supplying skin Blood vessels supplying heart

(coronary arteries) Muscles in wall of heart Urinary system

Muscle of bladder wall Muscle of internal urethral sphincter Reproductive systems

Blood vessels to clitoris and penis

Muscles associated with internal reproductive organs

Increased secretion

Increased secretion (cholinergic effect)

Dilation

Slightly increased secretion

Increased secretion Increased secretion

Relaxation

Decreased peristaltic action Contraction Reduced secretion Reduced secretion

Dilation

Constriction (alpha adrenergic) Dilation (beta adrenergic) Constriction

Dilation (beta adrenergic) Constriction (alpha adrenergic) Increased contraction rate

Relaxation Contraction

No action

Male ejaculation, female orgasm

No action No action

Constriction

Greatly increased secretion

No action No action

Contraction

Increased peristaltic action Relaxation

Greatly increased secretion Greatly increased secretion

Constriction

No action

No action No action

Decreased contraction rate

Contraction Relaxation

Dilation leading to erection of clitoris and penis

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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