Cochlear Adaptation In Guinea Pigs

Abstract

(1) In response to stimulus trains, the amplitude, latency and width of the N₁ component of the round-window-recorded action potentials (AP) reach an equilibrium value after about 5 stimuli. This final value is expressed in percent of the first response and depends on the stimulus intensity, stimulus duration and interstimulus interval (ISI). The equilibrium value is generally reached in an exponential way, the time constant of which also depends on the various stimulus parameters.

(2) The adaptation properties of the AP (i.e., those depending on the ISI) measured in this way are identical to those of action potentials recorded in single-nerve-fiber experiments. It is concluded that the AP reflects the properties of the nerve-fiber population by means of a weighted sum of the single-nerve-fiber contributions.

(3) It is possible to describe the N₁ component of the AP as sum of two partly overlappling Gaussian distribution functions. These two functions reflect the properties of two populations of nerve fibers which have a marked difference in their input-output and adaptation properties.

(4) Desynchronizing factor during adaptation, resulting in an N₁ broadening and a longer latency, arise from random fluctuations in both sunapse and nerve-cell membrane. These processes, however, must be identical for the two populations. For intensities ≥ 30 dB, population II dominates; and for intensities ≤ 70 dB, population I accounts for the major part of the compound response. In these two intensity regions, the parameter dependency of N₁ on stimulus intensity and ISI can be derived from single-nerve-fiber properties alone.

(5) In the intermediate intensity range, we observe anomalous behaviour of the N₁ parameters. It is suggested that a spatial desynchronization between the two populations, reflected in a latency difference, accounts for this behaviour.

(6) The two populations of nerve fibers might be identified with the radial and spiral afferent nerve fibers.

Key Words:

• Cochlear electrophysiology
• Compound action potential
• Nerve-fiber populations