Electron microscopic studies of the pressoreceptor fields of the carotid sinus of the dog.

In the dog, pressosensitive endings of the sinus nerve extend along the border between the adventitia and media of the carotid sinus wall. The axon endings, containing a great number of mitochondria, can be divided into small (600-2,000 nm) and large (6,000-8,000 nm) end swellings. In the terminal region the pressosensitive fibers are surrounded by ramified and highly structured Schwann "terminal cells". The topographic location in relation to elastic and collagenous tissue indicates a functional connection between receptors and efferent nerve endings in the immediate surroundings has been discussed in this report. Several axon endings contain variable amounts of glycogen which is regarded as an indication for the inactive metabolic state of the ending. Axonal swellings demonstrate considerable modification in structure, such as loss of structural integrity in mitochondria, the formation of lamellar fields, vesicular irregularities and disintegration of axoplasm, all of which are considered as the morphological expression of "wearing out", degeneration and possibly regeneration.