Intramural neural elements in components of the carotid bifurcation. A histological basis for differential function.

The histology of the specialized region of the carotid bifurcation in man was studied with Orcein stain for elastic tissue, Masson's trichrome for muscle and connective tissue, and Bodian's silver method for neural elements. Four distinct regions exist: the common carotid, which appears to be solely a conduit; the carotid sinus, which is thin and very elastic with its baroreceptors in the medial wall; the external carotid, which is highly muscular and presumably active; and the internal carotid, also highly muscular and presumably active. The transition between zones is abrupt. Prominent clusters of up to 30 or 40 multipolar nerve cells, 15 to 25 microns in diameter, were found in the subintimal region of the internal carotid artery, some in the common, and a few in the external carotid. Occasional rounder cells of similar size with only one process were seen, possibly of sensory type. No cell bodies were seen in the adventitia of any vessel. No relation to the perivascular plexus was established. It is speculated that the neurons may be related to the reported local differential response to environment by the internal and external carotid systems.