Adrenal Medullae
The adrenal glands are paired structures that lie on top of each kidney. This places them in a retroperitoneal position in the body. Just like the kidneys, the adrenal glands lie outside the peritoneum and near the body wall in close contact with the spine and supporting muscles. The adrenal glands are actually two glands in one. The outside, or adrenal cortex, is part of the endocrine system and secretes steroid-type hormones. The inner cells, the adrenal medulla, are often discussed in anatomy texts as part of the central nervous system even though they secrete hormonal-type chemicals.
The adrenal medulla, or inner core of the gland, is actually made up of highly modified nervous cells (cromaffin cells) that are part of the sympathetic, or autonomic nervous system. The cells are rounded, but slightly columnar and tightly packed. They originate from ectodermal cells. As a unit they function as part of the sympathetic nervous system and are often described as a modified sympathetic ganglion. A ganglion is a cluster of nerve cells that form a lump or swelling in the peripheral nervous system. These lumps will process signals from the nervous system and integrate specific information to specifically targeted areas of the body. The neuronal message is sent by means of chemicals
The position of the adrenal medullae puts them in proximity to the spinal cord. Preganglionic fibers from the sympathetic nervous system leave the thoracic spinal cord (mid-back region) and go through the adrenal cortex until they reach the cells of the adrenal medullae. When these cells are excited by the splanchnic nervous system (a route of nerve fibers that travels from the spinal cord through the ganglia without synapsing) they release a group of important chemicals called catecholamines. They are better known as epinephrine (adrenalin) and norepinephrine (noradrenalin). The concentrations of these chemicals when produced and released are about 85% and 15% respectively. The two chemicals function as neurotransmitters and include dopamine as well. The adrenal medullae and the sympathetic nervous system function so closely together they are given the title of the sympathoadrenal system.
The chemicals they produce are part of the "fight or flight" reaction the body experiences during periods of high stress. Epinephrine causes the heart rate to accelerate. Norepinephrine is a vasoconstrictor that causes the internal flow size of arteries and veins to become smaller. The result of these two actions is a greater ability of the body to respond to physical activity. This condition is only temporary, lasting only a few seconds to minutes.
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