Observations on the fine structure and distribution of presumptive baroreceptor nerves at the carotid sinus

Comparative nerve fiber counts show that the areas of the carotid bifurcation receiving a rich innervation correspond in extent to areas of the vascular wall with a thinned and elastic tunica media. Presumptive baroreceptor nerves within the carotid sinus wall were examined by electron microscopy in normal rabbits and also after superior cervical ganglionectomy and/or Hering's nerve transection. The glossopharyngeal (baroreceptor) nerve terminals are either denuded of a cellular covering or are thinly clothed in Schwann cell cytoplasm, and are intimately related over wide areas to at least three sinus wall components: to elastin at the medio‐adventitial border, to collagen in the inner one‐half of the adventitia, and to the surfaces of isolated adventitial smooth muscle cells. These muscle cells also receive a terminal innervation from the superior cervical ganglion. There is no nervous penetration of the sinus tunica media. The spatial arrangement of the adventitial muscle cells and their associated sensory and motor nerves simulates a smooth muscle spindle, possibly constituting a morphological basis for an efferent control of baroreceptor output. The baroreceptor nerve terminals contain large accumulations of mitochondria, and also myelin figures and small dense bodies resembling glycogen granules. Evidence is presented that baroreceptor impulses may be conveyed from the sinus wall in both myelinated and nonmyelinated nerve fibers.