THE DIAMETERS OF GUINEA-PIG AUDITORY-NERVE FIBERS - DISTRIBUTION AND CORRELATION WITH SPONTANEOUS RATE

In the mammalian auditory nerve physiological recordings revealed that the spontaneous discharge rate of single auditory fibres correlates w ith the diversity of input-output functions which may be important for intensity discrimination (e.g., Sachs and Abbas, 1974, Liberman, 1978 ; Winter et al., 1990). In this study we determined if the spontaneous discharge rate of auditory nerve fibres in the guinea pig is correlat ed with an anatomical feature, namely the diameter of the respective f ibres. The diameter of myelinated (Type I) guinea pig auditory nerve f ibres was measured after staining with different techniques. Measureme nts were made on semithin sections using a video image analysis system . The diameters of fibres stained with toluidine blue from the portion of the auditory nerve containing fibres from the basal turn of the co chlea were found to have a normal distribution. Fibres were also label led with horseradish peroxidase by bulk injection into the spiral gang lion. It was found that the presence of horseradish peroxidase within the fibres reduced the measured diameter in comparison to adjacent unl abelled fibres. A number of fibres were physiologically characterized with respect to spontaneous discharge rate and subsequently intracellu larly labelled with horseradish peroxidase. Fibre diameter of a select ed sample of intracellularly labelled fibres was measured over a dista nce of 800 mu m within the internal auditory meatus. At the positions nearest to the spiral ganglion fibres possessing low spontaneous rates were found to have smaller diameters than high spontaneous rate fibre s. No difference in fibre diameter was found for the positions near th e cochlear nucleus.