Divisions of the Autonomic Nervous System (ANS)

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.

Sympathetic Division

The sympathetic division’s functions include increased heart rate and respiration, reduced salivation, clammy skin, and dilation of eye pupils. Mental alertness is dramatically increased as is the metabolic rate. During physical activity, it constricts visceral blood vessels and sometimes constricts cutaneous blood vessels. Blood is moved to active skeletal muscles and the heart, and blood pressure increases. The sweat glands are activated. Lung bronchioles are dilated, resulting in increases in ventilation, moving more oxygen to body cells. The liver releases more glucose into the blood to supply energy to the body. Simultaneously, it slows down nonessential activities such as motility in the gastrointestinal tract and the urinary system. The sympathetic division may be referred to as the “E division” (signifying exercise, emergency, and excitement).

Parasympathetic Division

The parasympathetic division keeps energy use by the body to its lowest possible amounts. It controls digestion of food and elimination of feces and urine. This division stimulates visceral activity, decreases the metabolic rate, increases salivary and digestive secre-tions, and increases digestive motility and blood flow. During digestion, blood pressure and heart rate are lowered while the gastrointestinal system is active. Also, the pupils of the eyes become constricted and the lenses are accommodated for close vision. The parasympathetic division may be referred to as the “D division” (signifying digestion, defecation, and diuresis). Both divisions antagonize each other’s effects greatly to maintain homeostasis.

Parasympathetic fibers are craniosacral, originat-ing in the brain and sacral spinal cord (FIGURE 14-4). They have long preganglionic and short postganglionic fibers. Parasympathetic ganglia are mostly located in the visceral effector organs. Overall, the parasympa-thetic division is simpler than the sympathetic division. It is also known as the craniosacral division because its preganglionic fibers emerge from opposite ends of the CNS (the brain stem and sacral spinal cord). The preganglionic axons run from the CNS, nearly all the way to the innervated target structures. At these points, axons synapse with postganglionic neurons of the terminal ganglia, lying close to or inside the target organs. Extremely short postganglionic axons emerge from the terminal ganglia, synapsing with nearby effec-tor cells. In the cranial portion of the parasympathetic nervous system, preganglionic fibers exist in the facial, oculomotor, glossopharyngeal, and vagus nerves. Cell bodies of these fibers lie in related motor cranial nerve nuclei of the brain stem.

Oculomotor Nerves

The parasympathetic fibers of the oculomotor (III) nerves innervate smooth muscles that cause the pupils of the eyes to constrict and the lenses to bulge, which are both used when focusing. Cell bodies of the ­postganglionic neurons lie in the ciliary ganglia inside the eye orbits.

Facial Nerves

The parasympathetic fibers of the facial (VII) nerves stimulate large glands located in the head. Pregangli-onic fibers synapse with postganglionic neurons in the pterygopalatine ganglia, which are just posterior to the maxillae. Preganglionic neurons stimulating sali-vary glands (submandibular and sublingual) synapse with postganglionic neurons in the submandibular ganglia.

Glossopharyngeal Nerves

The parasympathetic fibers of the glossopharyngeal (IX) nerves begin in the inferior salivary nuclei of the medulla. They synapse in the otic ganglia just infe-rior to the foramen ovale of the skull. The glossopha-ryngeal nerves innervate the parotid salivary glands.

Vagus Nerves

The two vagus (X) nerves make up between 80% and 90% of the parasympathetic outflow fibers in the body. Their preganglionic axons begin primarily in the dor-sal motor nuclei of the medulla. They synapse in termi-nal ganglia that are mostly located in the target organ walls. Branches of the vagus nerves pass to the cardiac­ plexuses, which supply fibers to the heart that slow the heart rate. Other branches supply the pulmonary plexuses serving the lungs and the esophageal plexuses serving the esophagus. Near the esophagus, the main trunks of the vagus nerves join fibers to form anterior and posterior vagal trunks, each with fibers from both vagus nerves. The trunk continues down to the abdominal cavity, sending fibers through the large abdominal aortic plexus, which is made up of the celiac, superior mesenteric, and hypogastric plexuses running along the aorta. The large abdominal aortic plexus then branches off to the abdominal viscera. The celiac ganglion innervates the smooth muscles of the stomach, small intestine, liver, pancreas, and the ascending half of the transverse colon.

The sacral portion of the parasympathetic division serves the pelvic organs and the distal half of the large intestine. It arises from the lateral gray matter neurons of the spinal cord segments S₂ to S₄. Axons of these neurons continue through the ventral roots of the spinal nerves to the ventral rami. They branch to form the pelvic splanchnic nerves that pass through the inferior hypogastric (pelvic) plexus in the floor of the pelvis. Splanchnic nerves carry fibers that synapse in collateral ganglia. TABLE 14-1 compares differences between the sympathetic and parasympathetic divisions.

1. Identify the roles of the sympathetic and parasympathetic nervous systems.

2. Define the terms “terminal ganglia” and “lateral horns.”

3. Explain what may occur with preganglionic and postganglionic neurons when a preganglionic axon reaches a trunk ganglion.