Ultrastructural Changes in Sensory Nerve Endings Accompanying Increased Chemoreceptor Activity: A Morphometric Study of the Rat Carotid Body

Abstract

Most nerve endings on glomus cells of the carotid body are part of sensory neurons whose cell bodies are located in the petrosal ganglion of the glossopharyngeal nerve (5,6,9,11,13,15,17). Ultrastructural analyses of the synaptic connections in the carotid body of the rat (11), cat (13,17), duck (16), and chicken (10) have shown that some sensory nerve endings and glomus cells are interconnected by reciprocal synapses. At such synapses a sensory nerve ending is presynaptic to a glomus cell at one synapse and postsynaptic to the same cell at an adjacent synapse. Sensory nerve endings presynaptic to glomus cells exhibit the morphologic characteristics of nerve endings that release a neurotransmitter: small (synaptic) vesicles in nerve terminals are present near presynaptic dense projections, which are part of synaptic junctions. Ultrastructural studies also have shown that nerve endings on glomus cells are remarkably variable in size, shape, and synaptic vesicle content (3,15). In the present study we used morphometric methods to measure changes in the synaptic vesicle content of sensory nerve endings that accompany increased chemoreceptor activity. In addition, using quantitative methods we assessed the variability of sensory nerve ending morphology.