Chapter Summary

Internal Affairs (104.0K)

Introduction to the Autonomic Nervous System

1. The autonomic nervous system (ANS) is a functional division of the nervous system; it is composed of portions of the central nervous system (CNS) and portions of the peripheral nervous system (PNS).
2. Preganglionic autonomic neurons originate in the brain or spinal cord; postganglionic neurons originate in ganglia outside the CNS.
3. Smooth muscle, cardiac muscle, and glands receive autonomic innervation.
1. The involuntary effectors are somewhat independent of their innervation and become hypersensitive when their innervation is removed.
2. Myocardial cells are interconnected by electrical synapses, or gap junctions, to form a functional syncytium with independent pacemaker activity.
3. Single-unit smooth muscles are characterized by gap junctions and pacemaker activity; multiunit smooth muscles have few, if any, gap junctions, and thus their individual cells must be stimulated separately by neurons.

Structure of the Autonomic Nervous System

1. Preganglionic neurons of the sympathetic (thoracolumbar) division originate in the spinal cord (T1-L2).
1. Many of these neurons synapse with postganglionic neurons, whose cell bodies are located in a trunk of sympathetic ganglia outside the spinal cord.
2. Some preganglionic neurons synapse in peripheral ganglia; included in these are the celiac, superior mesenteric, and the inferior mesenteric ganglia.
3. Some preganglionic neurons innervate the adrenal medulla, which secretes epinephrine (and some norepinephrine) into the blood in response to this stimulation.

2. Preganglionic parasympathetic neurons originate in the brain and in the sacral levels of the spinal cord.
1. Preganglionic parasympathetic neurons contribute to the oculomotor, facial, glossopharyngeal, and vagus cranial nerves.
2. Preganglionic neurons of the vagus nerve are very long and synapse in terminal ganglia located next to or within the innervated organ; short postganglionic neurons then innervate the effector cells.
3. The vagus nerves provide parasympathetic innervation to the heart, lungs, esophagus, stomach, liver, small intestine, and upper half of the large intestine.
4. Parasympathetic outflow from the sacral levels of the spinal cord innervates terminal ganglia in the lower half of the large intestine, the rectum, and the urinary and reproductive systems.

Functions of the Autonomic Nervous System

1. The effects of sympathetic and parasympathetic activity, together with those of hormones, help to maintain homeostasis. The sympathetic division activates the body to "fight or flight" through adrenergic effects; the parasympathetic division conserves and restores the body's energy through cholinergic effects.
2. All preganglionic autonomic neurons are cholinergic (use acetylcholine as a neurotransmitter).
1. All postganglionic parasympathetic neurons are cholinergic.
2. Most postganglionic sympathetic neurons are adrenergic (use norepinephrine at their synapses).
3. Sympathetic neurons that innervate sweat glands and those that innervate blood vessels in skeletal muscles are cholinergic.

3. Adrenergic effects include stimulation of the heart, vasoconstriction in the viscera and skin, bronchodilation, and glycogenolysis in the liver.
4. Cholinergic effects of parasympathetic nerves are promoted by the drug muscarine and inhibited by atropine.
5. In organs with dual innervation, the effects of the sympathetic and parasympathetic divisions can be antagonistic, complementary, or cooperative.
1. The effects are antagonistic in the heart and pupils.
2. The effects are complementary in the regulation of salivary gland secretion; they are cooperative in the regulation of the reproductive and urinary systems.

6. In organs without dual innervation (such as most blood vessels), regulation is achieved by increases or decreases in sympathetic nerve activity.

Control of the Autonomic Nervous System by Higher Brain Centers

1. Visceral sensory input to the brain may result in the activity of the descending pathways to the preganglionic autonomic neurons. The centers in the brain that control autonomic activity are influenced by higher brain areas, as well as by sensory input.
2. The medulla oblongata of the brain stem is the structure that most directly controls the activity of the ANS.
1. The medulla oblongata is in turn influenced by sensory input and by input from the hypothalamus.
2. The hypothalamus orchestrates somatic, autonomic, and endocrine responses during various behavioral states.

3. The activity of the hypothalamus is influenced by input from the limbic system, cerebellum, and cerebrum; these interconnections provide an autonomic component to changes in body position, emotion, and various expressions of personality.