Functional Recovery and Structural Repair Following Acoustic Overstimulation of the Guinea Pig Ear

Abstract

Two series of guinea pigs were used in this study in order to investigate the nature of repair or further deterioration of the structures of the Scala Media following acoustic overstimulation. Each animal was first tested for sensitivity by determining the pressure level of sound necessary at each of 12 frequencies to produce 1 micro-volt of electrical response as recorded by a platinum foil electrode at the round window, and for linear range of the ear's response by recording the level at which the recorded potentials began to become nonlinear. Following this the ear of the animal was stimulated with a uniform spectrum of noise: the first series at 150 db above 0.0002 dyne/cm2 and the second at 138 db, measured at the drum membrane by a probe-microphone calibration system. Sensitivity and linear range were again measured, the operative wound closed, and the animal allowed to recover for 56 days, at which time it was reoperated and the above measures made again. The animal was then sacrificed by intra-vital perfusion of Maximow's fixative and the temporal bones removed for histologic processing. All specimens were eventually cut and mounted serially so that graphic reconstructions could be made for comparison of the anatomical condition of the structures of the Scala Media with the functional changes as measured electrically. It appears that individual susceptibility to damage is more a function of the ear's ability to recover than it is of structural weakness. One cannot predict on the basis of initial loss alone, regardless of level of stimulation, whether the ear will, with time, recover some sensitivity or lose it. The determining factors do not show up histologically in a normal ear because they are probably related to the nutrition of the sensory cell. These conclusions and others will be discussed in the light of the experimental evidence.