Ultrastructure of carotid baroreceptors in the goat.

The initial segment of the occipital artery of the goat appears modified under the electron microscope; endothelial cells are low cuboidal, the tunica media contains many elastic lamellae, and there are few smooth muscle cells. Free afferent nerve endings are seen in this modified arterial wall. They closely resemble the presumptive baroreceptor endings reported in other mammals in their mitochondrial content and abnormal organelles and are interpreted as solely adapting baroreceptors. Special topographical relations between the endings and elastic or collagen fibres do not occur, but microfilaments are present in the vicinity of these endings. Presumptive adrenergic efferent endings are not found in the region of the baroreceptors. We suggest that this zone of the occipited artery is homologous to the carotid sinus.