many instances, almost all portions of the sympathetic nervous system discharge simultaneously as a complete unit, a phenomenon called mass discharge. This frequently occurs when the hypothalamus is activated by fright or fear or severe pain. The result is a widespread reaction throughout the body called the alarm or stress response, which we shall discuss shortly.
At other times, activation occurs in isolated portions of the sympathetic nervous system. The most important of these are the following: (1) During the process of heat regulation, the sympathetics control sweating and blood flow in the skin without affecting other organs innervated by the sympathetics. (2) Many "local reflexes" involving sensory afferent fibers travel centrally in the peripheral nerves to the sympathetic ganglia and spinal cord and cause highly localized reflex responses. For instance, heating a local skin area causes local vasodilation and enhanced local sweating, whereas cooling causes opposite effects. (3) Many of the sympathetic reflexes that control gastrointestinal functions operate by way of nerve pathways that do not even enter the spinal cord, merely passing from the gut mainly to the paravertebral ganglia, and then back to the gut through sympathetic nerves to control motor or secretory activity.
Parasympathetic System Usually Causes Specific Localized Responses. In contrast to the common mass discharge response of the sympathetic system, control functions by the parasympathetic system are much more likely to be highly specific. For instance, parasympathetic cardiovascular reflexes usually act only on the heart to increase or decrease its rate of beating. Likewise, other parasympathetic reflexes cause secretion mainly by the mouth glands, or in other instances secretion is mainly by the stomach glands. Finally, the rectal emptying reflex does not affect other parts of the bowel to a major extent.
Hypoth Adenohypophysis Mamillary body Pons
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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.