Central projections of intracellularly labeled auditory nerve fibers in cats: morphometric correlations with physiological properties.

The central arborizations and endings of type I spiral ganglion neurons were labeled with intracellular injections of horseradish peroxidase (HRP) after their characteristic frequency (CF) and spontaneous discharge rate (SR) were physiologically determined. A fiber-by-fiber analysis was conducted and the morphological data compared with the fiber's response properties. The total number of branch points was correlated with total fiber length, a relationship that remained relatively constant when analyzing the ascending and descending branches together or separately. On the other hand, the ascending branches of four out of five fibers having CFs below 0.5 kHz bifurcated and gave rise to a pair of terminal endbulbs of Held. Low- and medium-SR fibers gave rise to more endings than did high-SR fibers, especially on the ascending branch. This difference was accounted for by small endings, a category composed of terminal boutons, string endings, and small complex endings. The categories of modified endbulbs, and endbulbs of Held did not vary in number with respect to fiber SR. The mean area of each ending type within the small ending category was statistically smaller for low- and medium-SR fibers than for high-SR fibers, whereas the mean area of modified endbulbs and endbulbs of Held was not correlated with fiber SR. Total ending area per fiber appeared independent of either CF or SR. These results are discussed in relation to issues of conservation of axon arborizations and terminals, and convergence of input from the different SR groups.