Intra-axonal labeling of saccular afferents in the goldfish, Carassius auratus: correlations between morphological and physiological characteristics.

Lucifer yellow was used to label axons of 32 physiologically defined S1 afferent fibers and 23 physiologically defined S2 afferent fibers of the goldfish sacculus. Analysis of these labeled fibers allowed us to study the relationship between electrical activity in these primary neurons and the morphology of their peripheral arborizations in the sensory macula. Morphological characteristics were highly indicative of response type: The peripheral arborizations of individual S1 fibers occupied a relatively small region (approximately 40 microns in diameter) in the rostral one-fourth of the saccular macula, whereas those of individual S2 fibers covered a larger area (roughly 80 microns across) in the caudal part of the macula. In addition to this rostrocaudal dimension, a ventral projection was related to a rarefaction response, a dorsal projection was related to a compression response, and a two-sided innervation was related to fibers having both responses. S1 fibers had either large or small terminals; the S2 fibers had only small terminals. On average, S1 fibers gave rise to approximately four terminals (e.g., seven small terminals or one to two large terminals) and S2 fibers approximately ten terminals. Spontaneous discharges were absent in all S1 fibers but present in some S2 fibers. Such S2 fibers showed spontaneous activity of either an irregular type or a burst type. In these, there was a tendency for fibers having more extensive arborizations to exhibit a burst type of spontaneous discharge. We conclude that structure-function relationships can be determined for these primary neurons.