Reflections on the Carotid Nerve Sensory Discharge and Coupling Between Glomus Cells

Abstract

In most sensory receptors of vertebrates, the action potentials of myelinated fibers originate from the first node of Ranvier, closest to the nerve terminals (Loewenstein & Rathkamp, 1958). The spike origin is not established in receptors innervated by unmyelinated (C) fibers, but probably occurs in the fiber at some distance from the endings (Edwards & Ottoson, 1958; Ringham, 1971). The carotid body glomus cells are innervated by myelinated (A) and unmyelinated (C) fibers from the carotid nerve (Fidone & Sato, 1969), although it is not known where the action potentials originate in either type of fiber. These uncertainties are due to a large extent to the complex morphology of the carotid body innervation. A single carotid nerve fiber divides extensively to innervate up to 20 glomus cells (Eyzaguirre & Gallego, 1975; Kondo, 1976) forming what is, presumably, the sensory unit. Moreover, glomus cells may be innervated more than once by the same fiber, and some cells are innervated by more than one carotid nerve fiber (De Castro, 1940; 1951). Figure 1 is a composite drawing from De Castro’s original microscope slides examined by Eyzaguirre & Gallego (1975). Axon 1 innervates glomeruli A and C. Cells marked by dots in glomerulus A are innervated by branches from axons 1 and 2. As shown below, this complex innervation pattern has to be considered when analyzing the origin of the sensory discharge.