Autonomic Nervous System page 436

The autonomic nervous system functions without conscious effort. It is concerned primarily with regulating visceral activities that maintain homeostasis.

1. General characteristics a. Autonomic functions are reflexes controlled from centers in the hypothalamus, brain stem, and spinal cord.

b. Autonomic nerve fibers are associated with ganglia where impulses are integrated before distribution to effectors.

c. The integrative function of the ganglia provides a degree of independence from the central nervous system.

d. The autonomic nervous system consists of the visceral efferent fibers associated with these ganglia.

e. The autonomic nervous system is subdivided into two divisions—sympathetic and parasympathetic.

f. The sympathetic division prepares the body for stressful and emergency conditions.

g. The parasympathetic division is most active under ordinary conditions.

2. Autonomic nerve fibers a. The autonomic fibers are efferent, or motor.

b. Sympathetic fibers leave the spinal cord and synapse in ganglia.

(1) Preganglionic fibers pass through white rami to reach paravertebral ganglia.

(2) Paravertebral ganglia and interconnecting fibers comprise the sympathetic trunks.

(3) Preganglionic fibers synapse within paravertebral or collateral ganglia.

(4) Postganglionic fibers usually pass through gray rami to reach spinal nerves before passing to effectors.

(5) A special set of sympathetic preganglionic fibers passes through ganglia and extends to the adrenal medulla.

c. Parasympathetic fibers begin in the brain stem and sacral region of the spinal cord and synapse in ganglia near various organs or in the organs themselves.

3. Autonomic neurotransmitters a. Sympathetic and parasympathetic preganglionic fibers secrete acetylcholine.

b. Most sympathetic postganglionic fibers secrete norepinephrine and are adrenergic; postganglionic parasympathetic fibers secrete acetylcholine and are cholinergic.

c. The different effects of the autonomic divisions are due to the different neurotransmitters the postganglionic fibers release.

4. Actions of autonomic neurotransmitters a. Neurotransmitters combine with receptors and alter cell membranes.

b. There are two types of cholinergic receptors and two types of adrenergic receptors.

c. How cells respond to neurotransmitters depends upon the number and type of receptors in their membranes.

d. Acetylcholine acts very briefly; norepinephrine and epinephrine may have more prolonged effects.

5. Control of autonomic activity a. The central nervous system largely controls the autonomic nervous system.

b. The medulla oblongata uses autonomic fibers to regulate cardiac, vasomotor, and respiratory activities.

c. The hypothalamus uses autonomic fibers in regulating visceral functions.

d. The limbic system and cerebral cortex control emotional responses through the autonomic nervous system.

Life-Span Changes (page 446)

Aging of the nervous system is a gradual elimination of cells and, eventually, slowed functioning.

1. Apoptosis of brain neurons begins before birth.

2. Neuron loss among brain regions is uneven.

3. In adulthood, numbers of dendrites in the cerebral cortex fall, as more generally neurotransmission slows.

4. Nervous system changes in older persons increase the risk of falling.

5. Sleep problems are common in the later years.

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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