In addition to divergence, the sympathetic division has a hormonal mechanism to activate target tissues endowed with adrenergic receptors, including those innervated by the sympathetic nerves. The hormone is the catecholamine epinephrine, which is secreted with much lesser amounts of norepinephrine by the adrenal medulla during generalized response to stress.
The adrenal medulla, a neuroendocrine gland, forms the inner core of the adrenal gland situated on top of each kidney. Cells of the adrenal medulla are innervated by the lesser splanchnic nerve, which contains preganglionic sympathetic axons originating in the lower thoracic spinal cord (see Fig. 6.4). These axons pass through the paravertebral ganglia and the celiac ganglion without synapsing and terminate on the chromaffin cells of the adrenal medulla (Fig. 6.5). The chromaffin cells are modified ganglion cells that synthesize both epinephrine and norepinephrine in a ratio of about 8:1 and store them in secretory vesicles. Unlike neurons, these cells possess neither axons nor dendrites but function as neuroendocrine cells that release hormone directly into the bloodstream in response to preganglionic axon activation.
Circulating epinephrine mimics the actions of sympathetic nerve stimulation but with greater efficacy because epinephrine is usually more potent than norepinephrine in stimulating both a-adrenergic and ^-adrenergic receptors. Epinephrine can also stimulate adrenergic receptors on
^HNGUREni^fe Sympathetic innervation of the adrenal ^■■■■■■Vmedulla. Preganglionic sympathetic axons terminate on the chromaffin cells. When stimulated, the chromaffin cells release epinephrine into the circulation.
cells that receive little or no direct sympathetic innervation, such as liver and adipose cells for mobilizing glucose and fatty acids, and blood cells which participate in the clotting and immune responses.
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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.