Preganglionic Fibers Physiology

Neural Control of Involuntary Effectors 220

I. Preganglionic autonomic neurons originate in the brain or spinal cord; postganglionic neurons originate in ganglia located outside the CNS.

II. Smooth muscle, cardiac muscle, and glands receive autonomic innervation.

A. The involuntary effectors are somewhat independent of their innervation and become hypersensitive when their innervation is removed.

B. Autonomic nerves can have either excitatory or inhibitory effects on their target organs.

Divisions of the Autonomic Nervous System 222

I. Preganglionic neurons of the sympathetic division originate in the spinal cord, between the thoracic and lumbar levels.

A. Many of these fibers synapse with postganglionic neurons whose cell bodies are located in a double chain of sympathetic (paravertebral) ganglia outside the spinal cord.

B. Some preganglionic fibers synapse in collateral (prevertebral) ganglia. These are the celiac, superior mesenteric, and inferior mesenteric ganglia.

C. Some preganglionic fibers innervate the adrenal medulla, which secretes epinephrine (and some norepinephrine) into the blood in response to stimulation.

II. Preganglionic parasympathetic fibers originate in the brain and in the sacral levels of the spinal cord. A. Preganglionic parasympathetic fibers contribute to cranial nerves III, VII, IX, and X.

B. The long preganglionic fibers of the vagus (X) nerve synapse in terminal ganglia located next to or within the innervated organ. Short postganglionic fibers then innervate the effector cells.

C. The vagus provides parasympathetic innervation to the heart, lungs, esophagus, stomach, liver, small intestine, and upper half of the large intestine.

D. Parasympathetic outflow from the sacral levels of the spinal cord innervates terminal ganglia in the lower half of the large intestine, in the rectum, and in the urinary and reproductive systems.

Functions of the Autonomic Nervous System 227

I. The sympathetic division of the autonomic system activates the body to "fight or flight" through adrenergic effects. The parasympathetic division often exerts antagonistic actions through cholinergic effects.

II. All preganglionic autonomic nerve fibers are cholinergic (use ACh as a neurotransmitter).

A. All postganglionic parasympathetic fibers are cholinergic.

B. Most postganglionic sympathetic fibers are adrenergic (use norepinephrine as a neurotransmitter).

C. Sympathetic fibers that innervate sweat glands and those that innervate blood vessels in skeletal muscles are cholinergic.

III. Adrenergic effects include stimulation of the heart, vasoconstriction in the viscera and skin, bronchodilation, and glycogenolysis in the liver.

A. The two main classes of adrenergic receptor proteins are alpha and beta.

B. Some organs have only alpha or only beta receptors; other organs (such as the heart) have both types of receptors.

C. There are two subtypes of alpha receptors (aj and a2) and two subtypes of beta receptors (Pj and P2). These subtypes can be selectively stimulated or blocked by therapeutic drugs.

IV. Cholinergic effects of parasympathetic nerves are promoted by the drug muscarine and inhibited by atropine.

V. In organs with dual innervation, the effects of the sympathetic and parasympathetic divisions can be antagonistic, complementary, or cooperative.

A. The effects are antagonistic in the heart and pupils of the eyes.

B. The effects are complementary in the regulation of salivary gland secretion and are cooperative in the regulation of the reproductive and urinary systems.

VI. In organs without dual innervation (such as most blood vessels), regulation is achieved by variations in sympathetic nerve activity.

VII. The medulla oblongata of the brain stem is the area that most directly controls the activity of the autonomic system.

A. The medulla oblongata is in turn influenced by sensory input and by input from the hypothalamus.

B. The hypothalamus is influenced by input from the limbic system, cerebellum, and cerebrum. These interconnections provide an autonomic component to some of the visceral responses that accompany emotions.

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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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