Protective Mechanism Against Acoustic Insult in the Mouse.

Abstract

Practically all experimental animals except the small rodents show evidence of cochlear degeneration when exposed to prolonged acoustic insult. Experimental degenerations of the cochlea have been employed as a method of determining specific pitch areas in the organ of Corti. There is, however, a difference in opinion on the causal factor of the lesion. The majority hold it is due to an excessive stimulation dependent on the pitch and intensity and is, therefore, a criterion of a specific area of reception. The minority holds the degeneration does not appear until the intrinsic muscles have been fatigued and that it may represent a result of clatter in the ossicular apparatus. This would mean the lesion is not dependent on the pitch. If the mouse really escapes intact under conditions which cause lesions in other experimental animals, then an additional protective mechanism in the middle ear must be sought. Small rodents show a rather remarkable departure from the usual middle ear picture. The stapes is traversed by a large artery which must certainly act as an efficient and tireless damping mechanism. This stapedial artery is phylogenetically responsible for the characteristic stirrup-like ossicle in the mammals as opposed to the mushroom-like columella in birds. A clatter of the ossicles in the mouse may, therefore, be automatically damped out and careful investigation is demanded, employing pure tones of known intensity. The investigation under way may throw light not only on the causal factor of the degenerations of the organ of Corti, but may also solve the problem of the reactions of the intrinsic muscle as a damping mechanism to eliminate clatter, as well as to adjust the ossicles to the variable topography of the middle ear.