This response is the classic example of the sympathetic nervous system's ability to produce widespread activation of its effectors,- it is activated when an organism's survival is in jeopardy and the animal may have to fight or flee. Some components of the response result from the direct effects of sympathetic activation, while the secretion of epinephrine by the adrenal medulla also contributes.
Sympathetic stimulation of the heart and blood vessels results in a rise in blood pressure because of increased cardiac output and increased total peripheral resistance. There is also a redistribution of the blood flow so that the muscles and heart receive more blood, while the splanchnic territory and the skin receive less. The need for an increased exchange of blood gases is met by acceleration of the respiratory rate and dilation of the bronchiolar tree. The volume of salivary secretion is reduced but the relative proportion of mucus increases, permitting lubrication of the mouth despite increased ventilation. The potential demand for an enhanced supply of metabolic substrates, like glucose and fatty acids, is met by the actions of the sympathetic nerves and circulating epinephrine on hepato-cytes and adipose cells. Glycogenolysis mobilizes stored liver glycogen, increasing plasma levels of glucose. Lipol-ysis in fat cells converts stored triglycerides to free fatty acids that enter the bloodstream.
The skin plays an important role in maintaining body temperature in the face of increased heat production from contracting muscles. The sympathetic innervation of the skin vasculature can adjust blood flow and heat exchange by vasodilation to dissipate heat or by vasoconstriction to protect blood volume. The eccrine sweat glands are important structures that also can be activated to enhance heat loss. Sympathetic nerve stimulation of the sweat glands results in the secretion of a watery fluid, and evaporation then dissipates body heat. Constriction of the skin vasculature, concurrent with sweat gland activation, produces the cold, clammy skin of a frightened individual. Hair-standing-on-end sensations result from activation of the piloerector muscles associated with hair follicles. In humans, this action is likely a phylogenetic remnant from animals that use hair erection for body temperature preservation or to enhance the appearance of body size or ferocity.
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