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Norepinephrine - Anatomy
The major site of NE cell bodies is the medulla and pons. The NE cells consist of three main groups shown in Figure 12.4:
Figure 12.4 |
- locus coeruleus complex (purple and red in Figure 12.4),
- lateral tegmental system (blue in Figure 12.4), and
- dorsal medullary system (green in Figure 12.4).
In all three cases the neurons project diffusely to broad regions of the brain where their nerve terminals lack conventional synaptic junctions. Release of transmitter from these cells is described as volume transmission, because NE, once released, is thought to diffuse and influence a number of adjacent cells.
Locus Coeruleus System - The locus coeruleus (LC-purple and red) is considered the most influential of the cell groups even though it consists of less than 2,000 cells on either side of the midline. This importance is because LC axons project rostrally via the dorsal noradrenergic bundle to innervate nearly the entire telencephalon and diencephalon, as well as dorsally to innervate the cerebellum and caudally to innervate the spinal cord. The nerve fibers are so highly ramified in the terminal fields such that each axon may branch as many as 100,000 times. This pattern of innervation enables the LC to synchronously modulate cellular activity across wide expanses of the cortex.
Lateral Tegmental System - The axons of the lateral tegmental system (blue) project caudally to the intermediolateral cell column of the spinal cord where they inhibit sympathetic preganglionic cells, and ventrally to the hypothalamus. The joint innervation of the hypothalamus and the intermediolateral column cells is believed to be the basis for NE integration of central and peripheral sympathetic autonomic function.
Dorsal Medullary System - As a complement to the lateral tegmental system, the dorsal medullary system (green) projects to the nucleus solitarius, as well as to the brain stem nuclei that control cranial parasympathetic function (glossopharyngeal, facial, and trigeminal- nuclei and the dorsal vagal nuclear complex). These NE systems are believed to provide control of the cranial parasympathetic system in a manner analogous to the lateral tegmental system's control of the sympathetic system.
Epinephrine - Anatomy
(Note: there is no figure for epinephrine anatomy)Two clusters of epinephrine (E) cells are located in the medullary reticular formation. One cluster of cells in the ventrolateral medulla sends ascending projections to innervate the periaqueductal gray and several hypothalamic and olfactory nuclei. This cluster also sends a descending projection to innervate the sympathetic preganglionic cells of the intermediolateral column in a manner analogous to the NE lateral tegmental system. The second group of cells, located in the dorsomedial medulla near the floor of the fourth ventricle, project to several parasympathetic cranial nerve nuclei (similar to the dorsal medullary NE system, described above). These adrenergic cells are believed to coordinate eating and various visceral functions including the regulation of blood pressure.
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