AUTONOMIC NERVOUS SYSTEM
The preganglionic neurons of the sympathetic nervous system have their cell bodies in the thoracic and lumbar regions of the spinal cord, termed the thoracolumbar di-vision. The preganglionic neurons of the parasympa-thetic division have their cell bodies in the brainstem and in the sacral region of the spinal cord, termed the cran-iosacral division. The cranial part of the parasympathetic nervous system innervates structures in the head, neck, thorax, and abdomen (e.g., the stomach, part of the in-testines, and pancreas). The cranial parasympathetic fibers leave the CNS in the oculomotor, facial, glossopharyngeal, and vagal cranial nerves. The sacral divi-sion of the parasympathetic nervous system innervates the remainder of the intestines and the pelvic viscera.
The sympathetic ganglia consist of two chains of 22 seg-mentally arranged ganglia lateral to the vertebral col-umn. The preganglionic fibers leave the spinal cord in adjacent ventral roots and enter neighboring ganglia, where they make synaptic connections with postgan-glionic neurons. Some preganglionic fibers pass through the vertebral ganglia without making synaptic connec-tions and travel by way of splanchnic nerves to paired prevertebral ganglia in front of the vertebral column, where they make synaptic connections with postgan-glionic neurons. In addition, some sympathetic pregan-glionic fibers pass through the splanchnic nerves into the adrenal glands and make synaptic connections on the chromaffin cells of the adrenal medulla.
Because sympathetic ganglia lie close to the verte-bral column, sympathetic preganglionic fibers are gen-erally short. Postganglionic fibers are generally long, since they arise in vertebral ganglia and must travel to the innervated effector cells. There are exceptions to this generalization. A few sympathetic ganglia lie near the organs innervated (e.g., urinary bladder and rec-tum); thus, these preganglionic fibers are long and the postganglionic fibers are short. In contrast, the parasym-pathetic ganglia lie very close to or actually within the organs innervated by the parasympathetic postgan-glionic neurons.
A single sympathetic preganglionic fiber branches a number of times after entering a ganglion and makes synaptic connection with a number of postganglionic neurons. Furthermore, some branches of this pregan-glionic fiber may ascend or descend to adjacent verte-bral ganglia and terminate on an additional number of postganglionic neurons in these ganglia as well. Therefore, activity in a single sympathetic preganglionic neuron may result in the activation of a number of ef-fector cells in widely separated regions of the body. Anatomically, the sympathetic nervous system is de-signed to produce widespread physiological activity. The sympathetic nervous system prepares the body for strenuous muscular activity, stress, and emergencies.
By contrast, parasympathetic preganglionic neurons are extremely limited in their distribution. In general, a single parasympathetic preganglionic fiber makes a synaptic connection with only one or two postgan-glionic neurons. For this reason, along with the fact that the ganglia are near or are embedded in the organs in-nervated, individual parasympathetic preganglionic neu-rons influence only a small region of the body or affect only specific organs. The parasympathetic nervous sys-tem is involved with the accumulation, storage, and preservation of body resources.
When the sympathetic integrative centers in the brain are activated (by anger, stress, or emergency), the body’s resources are mobilized for combat or for flight. Stimulation of the sympathetic nervous system results in acceleration of the heart rate and an increase in the contractile force of the heart muscle. There is increased blood flow (vasodilation) through skeletal muscle and decreased blood flow (vasoconstriction) through the skin and visceral organs. Activity of the gastrointestinal tract, such as peristaltic and secretory activity, is de-creased, and intestinal sphincters are contracted. The pupils are dilated. The increased breakdown of glyco-gen (glycogenolysis) in the liver produces an increase in blood sugar, while the breakdown of lipids (lipolysis) in adipose tissue produces an increase in blood fatty acids; these biochemical reactions make energy available for active tissues. In addition to generalized activation of the sympathetic system in response to stress, there can be more discrete homeostatic activation of the sympa-thetic system. For example, a selective reflex-associated alteration in the sympathetic outflow to the cardiovas-cular system can occur.
The parasympathetic system is designed to function more or less on an organ system basis, usually under conditions of minimal stress. For example, the activation of the gastrointestinal tract takes place during digestion of a meal; constriction of the pupil and accommodation for near vision are essential for reading.