The function of the middle ear is to transfer air-borne sounds to the fluid-filled spaces of the cochlea. The acoustic impedance of the fluid is about 4000 times greater than that of air, and the middle ear improves the transmission of sound from the air to the fluid by acting as a transformer. To a large extent, the middle ear’s action is accomplished by the mechanical advantage afforded by the difference in the effective areas of the ear drum and the stapes footplate (Wever & Lawrence, 1954). However, the lever action of the ossicles is also thought to contribute towards the overall transformation. The malleus and incus are thought to act as a lever which decreases the displacements of the stapes, while increasing the force applied to it. Anatomically, the lever ratio is given by the length of the malleus (umbo to the point of rotation) divided by the length of the incus (point of rotation to the incudo-stapedial joint). The point of the ossicle’s rotation is difficult to define, however, and it may not even be fixed. Hence, a more appropriate definition of the lever ratio would involve dividing the sound-evoked displacements of the umbo by those of the stapes. The aim of the present experiment was to find out whether this lever ratio changes with the frequency of stimulation.

Thirty guinea-pigs weighing between 300 and 1000 g were used in this study. Most of the animals were used to develop the experimental techniques and only the results from the last five were used for analysis here. The animals were anaesthetised throughout all experiments using neurolept-anaesthesia (25 mg kg^-1 sodium pentobarbitone, I.P., plus 0.6 ml kg^-1 Hypnorm, I.M.; each ml of Hypnorm contains 10 mg fluanisone and 0.315 mg fentanyl citrate), and were killed humanely at the end of the experiments. The pinna and lateral part of the cartilaginous ear canal were retracted to expose the bulla. Small holes were then made in the postero-lateral, dorso-lateral and ventral aspects of the bulla to visualize and measure the ossicular movements. A closed-field sound delivery system was sealed into the stump of the transected external auditory meatus, and short tone pips were generated using a computer. Sound pressure levels used ranged from 80 to 100 dB SPL, and frequencies ranged from 100 Hz to 25.6 kHz. A heterodyne laser interferometer (cf. Cooper, 1999) was used to measure responses from the umbo of the malleus and from the long process of the incus (within 200 µm of the incudo-stapedial joint).In three of the five guinea-pigs considered, the lever ratios increased from values of around 2 below 3200 Hz to values of around 7 above 6400 Hz (see Fig. 1). In the other two guinea-pigs, the lever ratios were almost independent of frequency. The abrupt change in the lever ratio appears to reflect a change in the ossicle’s mode of vibration between low and high frequencies. The findings in three of our experiments are consistent with those reported in other species (e.g. cat: Decraemer et al. 1991; human: Gyo et al. 1987), but findings in the other two experiments may not be, and warrant further investigation.N.P.C. is a Royal Society University Research Fellow.

Figure 1. Frequency-dependent lever ratios in the middle ears of five guinea-pigs.

Cooper, N.P. (1999). J. Neurosci. Meth. 88, 93-102.

Decraemer, W.F., Khanna, S.M. & Funnell, W.R.J. (1991). Hearing Res. 54, 305-318.

Gyo, K., Aritomo, H. & Goode, R.L. (1987). Acta Otolaryngol (Stockh). 103, 87-95.

Wever, E.G. & Lawrence, R.L. (1954). Physiological Accoustics. Princeton University Press, Princeton.