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Anatomy Ch 15 and 16 key concepts

Anatomy Ch 15 and 16 key concepts - Ch 15 General...

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Ch. 15 General Properties of the Autonomic Nervous System -The autonomic nervous system (ANS) carries out many visceral reflexes that are crucial to homeostasis. It is a visceral motor system that acts on cardiac muscle, smooth muscle, and glands. -Functions of the ANS are largely, but not entirely, unconscious and involuntary. -The sympathetic division of the ANS prepares the body for physical activity and is especially active in stressful "fight or flight" situations. -The parasympathetic division has a calming effect on many body functions, but stimulates digestion; it is especially active in "resting and digesting" states. -Although the balance of activity may shift from one division to the other, both divisions are normally active simultaneously. Each maintains a background level of activity called autonomic tone. -The ANS is composed of nuclei in the brainstem, motor neurons in the spinal cord and ganglia, and nerve fibers in the cranial and spinal nerves. -Most autonomic efferent pathways, unlike somatic motor pathways, involve two neurons: a preganglionic neuron whose axon travels to a peripheral ganglion and synapses with a postganglionic neuron , whose axon leads the rest of the way to the target cells. Anatomy of the Autonomic Nervous System -Sympathetic preganglionic neurons arise from thoracic and lumbar segments of the spinal cord and travel through spinal nerves T1 through L2 to a sympathetic chain of ganglia adjacent to the vertebral column. -Most preganglionic fibers synapse with postganglionic neurons in one of the ganglia of this chain, sometimes at a higher or lower level than the ganglion at which they enter. Some fibers pass through the chain without synapsing. -Sympathetic pathways show substantial neuronal divergence, with the average preganglionic neuron synapsing with 17 postganglionic neurons. Sympathetic stimulation therefore tends to have widespread effects on multiple target organs. -Postganglionic fibers leave the sympathetic chain by way of either the spinal nerve route or the sympathetic nerve route. Preganglionic fibers that pass through the chain without synapsing travel by way of splanchnic nerves to various collateral ganglia , and synapse there with postganglionic neurons. -The adrenal medulla is a modified sympathetic ganglion composed of anaxonic neurons. These cells secrete mainly epinephrine and norepinephrine into the blood when stimulated.
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-The parasympathetic division issues relatively long preganglionic fibers through cranial nerves III, VII, IX, and X, and spinal nerves S2 through S4, to their target organs. The vagus nerve carries about 90 % of all parasympathetic preganglionic fibers. -Parasympathetic preganglionic fibers end in terminal ganglia in or near the target organ.
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Anatomy Ch 15 and 16 key concepts - Ch 15 General...

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