Innervation of Adrenal Glands

The preganglionic sympathetic fibers are the axons of cells located in the intermediolateral gray columns of mainly the lower three or four thoracic and upper one or two lumbar segments of the spinal cord. They emerge in the anterior rootlets of the corresponding spinal nerves, pass in white rami communicantes to the sympathetic trunks, and leave them in the thoracic and first lumbar splanchnic nerves that run to the celiac, aorticorenal, and renal ganglia. Some of the fibers conveying impulses for the adrenal vessels may relay in these ganglia, but the majority continue onward to enter the adrenal branches of the celiac plexus.

Some of the parasympathetic fibers reaching the celiac plexus through the vagal trunks may be concerned with adrenal innervation and may relay in small ganglia near or in the glands, but as yet, no definite proof of this hypothesis exists. The adrenal parasympathetic supply may well emerge via posterior spinal nerve root efferents, which enter the thoracic splanchnic nerves and thereafter follow the same routes as the sympathetic preganglionic fibers; however, the existence of such posterior root efferents is still unproven. A proportion of the fibers in the adrenal nerves may be afferent and enter the spinal cord through the ninth to eleventh thoracic spinal nerves.

Adrenal Nerves. Numerous fine nerves pass outward to each gland from the celiac plexus and ganglia. They are joined by contributions from the terminations of the greater and lesser thoracic splanchnic nerves, and they communicate with the ipsilateral phrenic nerve and renal plexus.

Many nerve fibers from the adrenal nerves enter the gland through its hilus and medial margin. Other nerve fibers spread out over the gland to form a delicate subcapsular plexus from which fascicles penetrate the cortex to run alongside arterioles in the trabeculae to the medulla. The majority of nerve fibers entering the gland end in the medulla, where they ramify profusely and give off fibers that mostly terminate in synaptictype contacts with the chromaffin cells. As already stated, these are the homologues of ganglion cells in the sympathetic trunks. Some fibers invaginate the cell membranes deeply but do not penetrate them. A minority of fibers innervate the medullary arterioles and the central vein, which has an unusually thick muscle coat.

Multipolar or bipolar neurons, singly or in small groups, have been noted within the adrenal medullae. Their significance and the destinations of their axons have not yet been determined, although it is assumed that the cells are the final relay stations in the parasympathetic pathways.

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