Functional Properties of the Efferent Cochlear Bundle of the Pigeon Revealed by Stereotaxic Stimulation

The function of the efferent cochlear bundle (ECB) has been studied in over a hundred pigeons with the spinal cord transected at C2–C3, local anesthesia and muscle paralysis. The excellent biological status of the animals was carefully assessed throughout the experiments, which frequently lasted several hours. Electrical responses evoked by click or tone pip were picked up chiefly at the exposed round-window membrane. Bipolar needles were inserted stereotaxically into the brain stem with the ECB as target (histological controls). Repetitive electrical stimulation of the ECB before delivery of the testing sound potentiates markedly the cochlear microphonic receptor potential of the hair cells and inhibits simultaneously the auditory nerve response. This dual effect results in a reduction of acoustic input to the brain. These and other observations suggest that the ECB of the bird is homologous functionally to the olivocochlear bundle of mammals. Important quantitative differences have, nevertheless, been found. The potency of the efferent inhibition is much smaller in the pigeon than it is in the cat, which raises the question of its behavioral significance in the former species. The potentiation of the cochlear microphonic component is relatively more prominent in the pigeon and it dissipates more slowly than the simultaneously recorded inhibition of the neurals. Such different kinetics might be related to the peculiar synaptic organization of the bird's inner ear.