Parasympathetic division

Preganglionic neurons of the parasympathetic system arise from several nuclei of the brainstem and from the sacral region of the spinal cord (segments S2 to S4; see Figure 9.1). The axons of the preganglionic neurons are quite long compared to those of the sympathetic system and synapse with postganglionic neurons within terminal ganglia that are close to or embedded within the effector tissues. The very short axons of the postganglionic neurons then provide input to the cells of that effector tissue.

The preganglionic neurons that arise from the brainstem exit the CNS through cranial nerves. The occulomotor nerve (III) innervates the eyes; the facial nerve (VII) innervates the lacrimal gland, salivary glands, and mucus membranes of the nasal cavity; the glossopharyngeal nerve (IX) innervates the parotid (salivary) gland; and the vagus nerve (X) innervates the viscera of the thorax and abdomen (e.g., heart, lungs, stomach, intestines, and pancreas). The physiological significance of this latter nerve in terms of influence of the parasympathetic system is clearly illustrated by its widespread distribution and the fact that 75% of all parasympathetic fibers are in the vagus nerve. Preganglionic neurons that arise from the sacral region of the spinal cord exit the CNS and join together to form the pelvic nerves. These nerves innervate the viscera of the pelvic cavity (e.g., urinary bladder, colon).

Because the terminal ganglia are located within the innervated tissue, there is typically little divergence in the parasympathetic system compared to the sympathetic system. In many organs, the ratio of preganglionic fibers to postganglionic fibers is 1:1. Therefore, the effects of the parasympathetic system tend to be more discrete and localized, with only specific tissues stimulated at any given moment, compared to the sympathetic system in which a more diffuse discharge is possible.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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