Differences Between the Sympathetic and Parasympathetic Divisions

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Chapter: Anatomy and Physiology for Health Professionals: Autonomic Nervous System (ANS)

Autonomic Nervous System (ANS) : Differences Between the Sympathetic and Parasympathetic Divisions

Divisions of the ANS

The ANS is divided into the sympathetic and parasympathetic­ divisions. Certain visceral organs have fibers from both divisions, controlling the acti-vation or inhibition of their actions. The sympathetic division prepares the body for stressful or emergency ­situations and is part of the fight-or-flight respons. This division can change tissue and organ activities by releasing NE at peripheral synapses and by ­distributing epinephrine and NE throughout the body. ­Sympathetic activation, controlled by the hypothalamus, occurs when the entire sympathetic division responds to a crisis situation. When this happens, a person feels extremely alert, energized, and euphoric. Blood pres-sure, breathing, and heart rate increase; muscle tone is elevated; and energy reserves are mobilized for action.

The parasympathetic division functions in an opposite manner and is part of the rest-and-digest response. When stress occurs, the sympathetic divi-sion increases heart and breathing rates. As the stress subsides, the parasympathetic division decreases these activities. Dual innervation is applied so the two divi-sions counterbalance the effects of each other. This utilizes cardiac, pulmonary, esophageal, celiac, inferior mesenteric, and hypogastric plexuses. Sympathetic and parasympathetic fibers that reach the heart and lungs pass through the cardiac plexus. Parasympathetic activation is signified by constricted pupils for better focusing, increased glandular secretions, raised nutri-ent absorption, and changes in blood flow that are associated with sexual arousal. In the digestive tract, smooth muscle activity increases, defecation is stim-ulated, the urinary bladder contracts, and respiration and heart rate are reduced.

A little-known third division of the ANS is known as the enteric nervous system (ENS). It is a network of neurons and nerve networks in the digestive tract and is influenced by both the sympathetic and para-sympathetic divisions. The ENS is primarily related to the visceral reflexes, and has about 100 million neurons and uses all of the neurotransmitters found in the brain.

Most of the time, the sympathetic and parasympa-thetic divisions of the ANS have opposite effects such as excitation versus inhibition. However, these divi-sions may also be independent, with only one of them innervating certain body structures. They also may work together and each may control just one stage of a complicated series of actions. Basically, the sympa-thetic division activates during emergencies, stress, or exertion, while the parasympathetic division activates when the body is at rest.

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