Effect of anesthesia on maximal cochlear microphonics

During prolonged periods of pentobarbital anesthesia (6-8 h), cochlear microphonics (CM) in cats became reduced by approximately 6 dB, but only in the region of maximal responses ("truncation"). For the following reasons, this change is held to be neurally mediated, specifically by an impairment of the olivocochlear bundle (OCB): (1) Its onset coincided with the disappearance of efferent action potentials, normally superimposed on low-frequency CM. (2) OCB stimulation augments CM by approximately 6 dB, once more only in the region of maximal responses [4]. (3) Sectioning of the OCB reduces maximal CM responses [8]. (4) Truncation was produced by two neural blocking agents, azide and DFP [10], most likely also resulting from OCB impairments. (5) At the high input levels required to produce maximal responses, CM ought to become slightly elevated by acoustically-evoked OCB activity. This elevation should cease to occur on impairment of OCB under the effect of pro-longed anesthesia; thus there would be an apparent truncation.