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Vibrometric studies of the middle ear of the bullfrog Rana catesbeiana II. The operculum

Matthew J. Mason^* and Peter M. Narins

Department of Physiological Science, UCLA, 405 Hilgard Avenue, Los Angeles, CA 90095, USA

View larger version (41K): [in a new window]   Fig. 1. The left stapes footplate and operculum of a bullfrog, seen from (A) dorsolateral and (B) ventromedial views. The stapes footplate is composed of the proximally expanded portion of the pars media together with the pars interna. Cartilage is shaded. The operculum in A is drawn as if semi-translucent to reveal the flange of the pars interna.

View larger version (20K): [in a new window]   Fig. 2. (A) Responses of the left tympanic membrane (black), stapes footplate (red), operculum (orange) and parotic crest (green) in a male bullfrog (SVL 123.6 mm). Note that the responses of the footplate and operculum are greater than that of the parotic crest at all but the lowest and highest frequencies. (B) Phase lags between the tympanic membrane and footplate (black) and between the footplate and operculum (red). The footplate vibrates 180° out-of-phase with the tympanic membrane at low frequencies, this phase lag increasing with frequency. However, the footplate and operculum vibrate in phase over a wide frequency range.

View larger version (19K): [in a new window]   Fig. 3. (A) Responses of the same structures as represented in Fig. 2, but after the extrastapes had been severed. The tympanic membrane response is represented in black. Note that the responses of both footplate (red) and operculum (orange) have dropped to match the `background' response of the parotic crest (green). (B) Phase lag between the tympanic membrane and the footplate (black) and between the footplate and operculum (red) in the same frog after the extrastapes had been cut. Although the footplate no longer shows the typical phase relationship with the tympanic membrane (represented in Fig. 2B), the responses of footplate and operculum remain in phase.

View larger version (28K): [in a new window]   Fig. 4. (A) Responses of the tympanic membrane (black), medial footplate (red), lateral footplate (pink), medial operculum (dark brown) and lateral operculum (orange) of a male bullfrog (SVL 123.2 mm). The inset shows a representation of the left stapes and operculum, indicating the approximate positions of the reflective beads that defined the measurement positions during the experiments. There are two beads on the stapes footplate and two on the operculum. The colours of the beads, represented as dots, match the corresponding response curves. The footplate and opercular responses were adjusted for the response of the parotic crest (not shown) and for velocity component angles of 57° (footplate) and 20° (operculum). (B) Phase lag between the medial and the lateral footplate positions (red) and between the lateral and medial opercular positions (black).

View larger version (22K): [in a new window]   Fig. 5. (A) Responses measured from the left tympanic membrane (black) and from three positions, rostral (orange), middle (red) and caudal (dark brown), on the operculum of a female bullfrog (SVL 133.9 mm). Opercular responses have been adjusted for the response of the parotic crest (not shown) and for velocity component angles of 20°. The inset shows approximate measurement positions on the operculum. (B) Phase lags between the rostral and middle operculum (red) and between the rostral and caudal operculum (black).

View larger version (27K): [in a new window]   Fig. 6. Diagrammatic representation of the movements of the stapes and operculum of the bullfrog at low frequencies. The operculum is portrayed as semitransparent to reveal the flange of the pars interna underneath. In this example, when the pars media is pushed down, which happens when the tympanic membrane is inflected, the footplate is moved out of the oval window. As a result of the coupling between the footplate and operculum, the operculum also moves outwards. Note that the stapes is hinged ventrolaterally but the operculum is hinged dorsomedially.