Nerve branching and terminal arborizations in the carotid body of the cat. A light microscopic study following anterograde injury filling of carotid nerve axons with horseradish peroxidase.

The terminal arborizations of carotid nerve axons within the carotid body of the cat were densely filled with horseradish peroxidase and studied under the light microscope. Two types of terminal arborizations were found in contact with glomus (type I) cells. The axons differed principally in the wealth of terminal swellings. The largest and most numerous type of arborization consisted of one to several clusters of terminals of variable size and shape arising from a single fiber and distributed in a rather ellipsoidal domain of about 9,000 microns 3 for each cluster. Thus, these arborizations might be in close relation with 20-60 glomus cells. The second type of arborization had substantially fewer terminal swellings, occupying a smaller volume and probably contacted significantly less glomus cells. Both kinds of axons had small rounded and large calyciform endings. The larger arborizations were derived consistently from larger fibers than those which produced the smaller arborizations. The results suggest that the carotid nerve axons generate two types of arborizations within the carotid body. Thus, glomus cells potentially can contact two classes of afferent fibers. The functional significance of a dual chemoreceptor innervation of the carotid body is discussed.