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autonomic nervous system (ANS)
can be defined as a motor nervous system that controls glands, cardiac muscle, and smooth muscle. It is also called the visceral motor system to distinguish it from the somatic motor system that controls the skeletal muscles. The primary target organs of the ANS are viscera of the thoracic and abdominopelvic cavities and some structures of the body wall, including cutaneous blood vessels, sweat glands, and piloerector muscles.
visceral reflexes
unconscious, automatic, stereotyped responses to stimulation
Receptors, integrating center, and effector
autonomic activity involves a visceral reflex arc that includes receptors (nerve endings that detect stretch, tissue damage, blood chemicals, body temperature, and other internal stimuli), afferent neurons leading to an integrating center in the CNS, interneurons in the CNS, efferent neurons carrying motor signals away from the CNS, and finally an effector that carries out the end response.
sympathetic division
adapts the body in many ways for physical activity—it increases alertness, heart rate, blood pressure, pulmonary airflow, blood glucose concentration, and blood flow to cardiac and skeletal muscle, but at the same time, it reduces blood flow to the skin and digestive tract. Cannon referred to extreme sympathetic responses as the “fight-or-flight” reaction because it comes into play when an animal must attack, defend itself, or flee from danger. In our own lives, this reaction occurs in situations involving arousal, exercise, competition, stress, danger, trauma, anger, or fear.
parasympathetic division
by comparison, has a calming effect on many body functions. It is associated with reduced energy expenditure and normal bodily maintenance, including such functions as digestion and waste elimination. This is often called the “resting-and-digesting” state.
autonomic tone
the balance between sympathetic tone and parasympathetic tone shifts in accordance with the body’s changing needs. Parasympathetic tone, for example, maintains smooth muscle tone in the intestines and holds the resting heart rate down to about 70 to 80 beats/min. If the parasympathetic vagus nerves to the heart are cut, the heart beats at its own intrinsic rate of about 100 beats/min. Sympathetic tone keeps most blood vessels partially constricted and thus maintains blood pressure. A loss of sympathetic tone can cause such a rapid drop in blood pressure that a person goes into shock and may faint.
preganglionic fiber and postganglionic fiber
preganglionic fiber is myelinated and leads from a soma in the brainstem or spinal cord to the autonomic ganglion. It synapses there with a neuron that issues an unmyelinated postganglionic fiber to the target cells. In contrast to somatic motor neurons, postganglionic fibers of the ANS do not end by synapsing with a specific target cell, but with a chain of varicosities that diffusely release neurotransmitter into the tissue and may stimulate many cells simultaneously
white communicating ramus and gray communicating ramus
The preganglionic fibers are small myelinated fibers that travel from the spinal nerve to the ganglion by way of the white communicating ramus, which gets its color and name from the myelin. Unmyelinated postganglionic fibers leave the ganglion by way of the gray communicating ramus, named for its lack of myelin and duller color, and by other routes. This ramus returns to the spinal nerve. Postganglionic fibers extend the rest of the way to the target organ.
The spinal nerve route
Some postganglionic fibers exit a ganglion by way of the gray ramus, return to the spinal nerve or its subdivisions, and travel the rest of the way to the target organ. This is the route to most sweat glands, piloerector muscles, and blood vessels of the skin and skeletal muscles.
The sympathetic nerve route
Other postganglionic fibers leave by way of sympathetic nerves that extend to the heart, lungs, esophagus, and thoracic blood vessels. These nerves form a carotid plexus around each carotid artery of the neck and issue fibers from there to effectors in the head—including sweat, salivary, and nasal glands; piloerector muscles; blood vessels; and dilators of the iris. Some fibers from the superior and middle cervical ganglia form the cardiac nerves to the heart. (The cardiac nerves also contain parasympathetic fibers.)
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