Middle ear transmission in the grass frog, Rana temporaria

The anuran middle ear serves to transmit eardrum vibrations to the inner ear. In order to do this efficiently, the eardrum and middle ear must operate as an impedance transformer matching the low impedance of air to the higher impedance of the fluid-filled inner ear. In amniotes, one of the mechanisms used to achieve impedance transformation is to have the middle ear work as a force-amplifying lever system. Here, we present evidence that the grass frog middle ear also implements a lever system. The columellar footplate, which sits in the oval window, is firmly connected to the otic capsule along its ventral edge. Therefore, simple in-out movement of the columella is prevented while a rotational movement around the footplate's ventral edge is possible. The latter movement pattern was confirmed by laser vibrometry measurements of eardrum and footplate vibrations. The results showed that the footplate vibrations were 20-30 dB weaker than those of the eardrum and that the two structures vibrated 180 degrees out of phase (at low frequencies). The lever ratio was approximately 6, i.e. somewhat higher than lever ratios reported for amniotes. Hence, the middle ear lever probably makes a significant contribution to impedance matching in frogs.