Immediate alterations in the impulse noise exposed organ of Corti of the guinea pig.

Cochlear microphonics and morphological changes in the organ of Corti were examined in guinea pigs exposed to 1 or 10 impulses of 164 dB SPL. Immediately after noise exposure the CM amplitudes of frequencies between 500 and 10000 Hz delcined severly with an accelerating tendency during the next 2 hours. The decrease in CM in two groups differed insignificantly. 10 minutes after impulse exposure, whole-mount specimens showed mainly swollen and translocated nuclei of outer hair cells. After 2 hours we found large quantities of pyknotic nuclei, karyorrhexis, and gaps in the normal OHC pattern. Despite minor numerical differences in injured cells of either group, exposure to 10 impulses caused a greater degree of irreversible alteration. Altogether, altered hair cells were only localized in the basal and lower second turn, and their numbers were relatively small. This stands in contrast to severe decrease of CM from low to high frequencies. Hair cell injury and additional mechanical lesion of the reticular membrane caused many sites of leakage, allowing interchange of endo-and peri-lymph. This effect is considered to be the main cause of functional damage indicated by rapid CM decline.