Endbulbs of held and spherical bushy cells in cats: morphological correlates with physiological properties.

Single auditory nerve fibers of type I spiral ganglion cells in cats were electrophysiologically characterized by recording with micropipettes inserted into the axon and then labeled by intracellular injections of horseradish peroxidase (HRP) through the same pipettes. This method for staining and studying single neurons allowed us to describe structure-function relationships for labeled endbulbs of Held and the somata of their postsynaptic spherical bushy cells. The silhouette areas of terminal endbulbs and the corresponding somata of spherical bushy cells were determined by planimetry from drawings made with a light microscope and drawing tube. On the presynaptic side, endbulb area is related to fiber characteristic frequency (CF, the frequency to which a fiber is most sensitive) such that the largest endbulbs arise from fibers having CFs between 1 and 4 kHz; smaller endbulbs can arise from fibers of any CF. Endbulb area is not correlated with fiber spontaneous discharge rate (SR). Dividing the endbulb's silhouette area by its silhouette perimeter, however, yields a "form factor" that is a reliable indicator of fiber SR: Endbulbs from fibers of low-medium SR (less than or equal to 18 spikes/second) have form factor values less than 0.52, whereas endbulbs of high SR fibers (greater than 18 spikes/second) have values greater than 0.52. This form factor should therefore be predictive of SR groupings in auditory fibers for which physiological data are not available. On the postsynaptic side, the somata of spherical bushy cells receiving endbulbs from low-medium SR fibers are on average smaller than those receiving endbulbs from high SR fibers. In contrast, the nuclei of the spherical bushy cells are the same size regardless of presynaptic fiber SR. Some of the effects of low-medium SR fibers on their postsynaptic targets, when compared to those of high SR fibers, appear to be mimicked by effects of experimentally induced deprivation.