The mature adrenal medulla is made up of modified neurons that possess neither axons nor dendrites and are essentially cell bodies that have been adapted to secretory function in response to a neural signal. During fetal development, it is cortisol from the fetal adrenal cortex that inhibits the growth of axons and dendrites on the medullary cells. These cells are referred to as the chromaffin cells of the adrenal medulla because the presence of epinephrine and norepinephrine cause them to stain darkly with dichromate salts. These cells will be discussed in more detail following a discussion of their role in the sympathoadrenomedullary nervous system.
Figure 1A depicts the autonomic nervous system that governs the involuntary workings of many glands and organs in the body. The autonomic centers in the central nervous system that initiate sympathetic responses lie mainly in the hypothalamus and brain stem, with input from the cerebral cortex and limbic system.
Fig1. Relationship of the adrenal medulla to the sympathetic nervous system. A. The autonomic nervous system is depicted with the sympathetic system on the left and the parasympathetic system on the right. The adrenal medulla is part of the sympathetic nervous system. The spinal nerves of this system, Thoracic nerve 1 through Lumbar nerve 2 (blue, T1–L2) receive input from the central nervous system and transmit the information to a synapse with a nerve in the sympathetic chain near the spinal cord or in a collateral ganglion, the celiac ganglion (CG), superior mesenteric ganglion (SMG), or the inferior mesenteric ganglion (IMG). Acetylcholine is the neurotransmitter at these synapses. A post-ganglionic nerve then carries the signal to the appropriate organ, as shown, to generate the response to the signal from the CNS, with norepinephrine as the neurotransmitter. In contrast, the greater and lesser splanchnic nerves (purple) pass through the celiac ganglion, but do not make a synapse until they reach the adrenal medulla, as shown in B. Here the nerve terminal releases acetylcholine (ACh) at the surface of the chromaffin cells of the adrenal medulla. These cells (see text) serve as postganglionic nerve cells, but instead of making a neural connection of their own, they secrete epinephrine into the blood.
The adrenal medulla is part of the sympathetic nervous system and in fact can be thought of as a large and specialized ganglion of it. As shown in Figure 1B, the axons of the greater and lesser splanchnic nerves pass through, without synapsing, the celiac ganglion, one of the collateral ganglia of the sympathetic nervous system, to the adrenal medulla. At the surface of the adrenal medullary chromaffin cell, the neurotransmitter acetylcholine is released. In response to activation of the acetylcholine receptor on the chromaffin cell and as described in more detail in section III.A, epinephrine and norepinephrine are secreted into the bloodstream.
