Parasympathetic Division of the Autonomic Nervous System
As we stated earlier, there are two divisions of the ANS, one for fight and flight the other for rest and digest. As you sit here reading this text your body is relaxed, heart rate and ventilations are lowered, and your body is focusing on diverting blood to your digestive system to absorb nutrients that power life's functions. This is in deep contrast to the fight and flight response, which flushes adrenaline into your body and amps up your responses in survival situations. The parasympathetic division calms your body back down because it is not only wasteful to expend energy and resources when there is a lack of activity or demand but leads to unnecessary wear and tear. The overall systemic response of the two divisions is starkly different which leads us to infer that there are great differences in the distribution and neurotransmitters utilized.
A large difference that exists between the two divisions is the extent of information divergence. If we remember back to our classroom example, the sympathetic system passed the information along in a very diffuse manner 1 20 400. The parasympathetic system is much more controlled. Here the teacher, representing the preganglionic fiber, one has a class of roughly 5 students. The students, representing the postganglionic fiber, each go into a different classroom of 1 or 2 students, representing the effector organs, and pass on the information. This means the information diverged significantly less from 1 5 10. Since fewer effectors are activated with a single stimulus, there is a greater selection of which effector we wish to activate and the parasympathetic response is more controlled.
The parasympathetic division of the autonomic nervous system is named because its preganglionic neurons are located on either side (superior and inferior) of the thoracolumbar region of the spinal cord (para- = "near"). The parasympathetic system can also be referred to as the craniosacral system because the somas of the preganglionic neurons are located in nuclei of the brain stem (hence cranio-) and the lateral horn of the sacral region of the spinal cord (hence –sacral). To clarify the sacral region of the spinal cord is located inferior to the lumbar enlargement in vertebral foramen of the lumbar vertebrae L1 and L2.
The connections, or "circuits," of the parasympathetic division are similar to the general layout of the sympathetic division with a few specific differences (Figure). The preganglionic fibers from the cranial region travel in cranial nerves, whereas preganglionic fibers from the sacral region travel in spinal nerves. The targets of these preganglionic fibers are terminal ganglia, which are located near—or even within—the target effector. This means that that preganglionic fiber is rather long and must convey the signal over great distance. The postganglionic fiber projects from the terminal ganglia a short distance to the target effector, or to the specific target tissue within the organ. Comparing the relative lengths of axons in the parasympathetic system, the preganglionic fibers are long and the postganglionic fibers are short because the ganglia are close to—and sometimes within—the target effectors.
The cranial component of the parasympathetic system is based in particular nuclei of the brain stem. In the midbrain, preganglionic parasympathetic fiber travels with the fibers in the oculomotor nerve (cranial nerve III) that innervate the extraocular muscles. The preganglionic parasympathetic fibers within cranial nerve III terminate in the ciliary ganglion, which is located in the posterior orbit. The postganglionic parasympathetic fibers then project to the smooth muscle of the iris to control pupillary size. In the upper medulla, the preganglionic parasympathetic fibers project through the facial and glossopharyngeal nerves to ganglia that control salivary glands. Tear production is influenced by parasympathetic fibers in the facial nerve, which activate a ganglion, and ultimately the lacrimal (tear) gland. Preganglionic parasympathetic fibers project through the vagus nerve (cranial nerve X) to the terminal ganglia of the thoracic and abdominal cavities. Parasympathetic preganglionic fibers primarily influence the heart, bronchi, and esophagus in the thoracic cavity and the stomach, liver, pancreas, gall bladder, and small intestine of the abdominal cavity.
