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Vibrometric studies of the middle ear of the bullfrog Rana catesbeiana I. The extrastapes

Matthew J. Mason^* and Peter M. Narins

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

View larger version (51K): [in a new window]   Fig. 1. Diagrammatic transverse section of the left middle ear of a female bullfrog, seen from the caudal direction: lateral is towards the left. Cartilage is stippled. The pars media and pars interna are collectively referred to as the stapes. The hinge-like articulation between the pars media and the otic capsule, along the ventral rim of the oval window, forms the `footplate axis'. Reflective beads, representing points from which laser measurements were made, are represented by small circles on the tympanic membrane, extrastapes and stapes footplate. The tympanic membrane of a male frog would be greater in diameter and would have a large, fatty pad at its centre.

View larger version (17K): [in a new window]   Fig. 2. Laser measurements of tympanic membrane vibration velocity. Two components of the tympanic membrane velocity were measured, the vertical component VTMV and an angled component, VTMA. These components, together with the angle las between VTMA and the horizontal, were used to calculate the true tympanic membrane velocity, VTM. TM, angle between VTM and the horizontal. See text for details.

View larger version (15K): [in a new window]   Fig. 3. Responses of the tympanic membrane (black), distal extrastapes (light blue), proximal extrastapes (dark blue), footplate (red) and parotic crest (green) in a male frog (SVL 131.8 mm). The response of the parotic crest was measured as a control for stapes footplate vibrations.

View larger version (38K): [in a new window]   Fig. 4. (A) Tympanic membrane/stapes footplate velocity ratios in 12 male (blue) and six female (red) bullfrogs. The stapes footplate velocities used to calculate these ratios were adjusted for the responses of the parotic crest. Male values are generally greater than female values. (B) Phase lags between the tympanic membrane and stapes footplate, from the same experiments.

View larger version (31K): [in a new window]   Fig. 5. (A) Velocity amplitude differences between the tympanic membrane and distal extrastapes in seven male (blue) and three female (red) frogs. (B) Phase lags between the tympanic membrane and distal extrastapes, from the same experiments.

View larger version (31K): [in a new window]   Fig. 6. (A) Velocity amplitude differences between the distal and proximal extrastapes positions in six male (blue) and three female (red) frogs. (B) Phase lags between the distal and proximal extrastapes, from the same experiment.

View larger version (20K): [in a new window]   Fig. 7. (A) Responses of the tympanic membrane (black), stapes footplate (red) and parotic crest (green) of a male frog (SVL 131.1 mm) prior to surgery on the ascending process. (B) Responses of the same structures after the ascending process of the extrastapes had been severed. Note that the response measured from the footplate has fallen and now approximates that of the `background' response of the parotic crest.

View larger version (26K): [in a new window]   Fig. 8. (A) Responses of the left tympanic membrane (black), distal extrastapes (light blue), proximal extrastapes (dark blue), footplate (red) and parotic crest (green) of a female frog (SVL 133.9 mm) with the ascending process intact. (B) Responses of the same structures after severing the ascending process.

View larger version (10K): [in a new window]   Fig. 9. A comparison of tympanic membrane/stapes footplate velocity ratios predicted from anatomical measurements if the ossicular apparatus were stiff with those measured experimentally in 12 male (blue squares) and six female (red diamonds) bullfrogs. The line has a gradient of 1, which would be expected if measured values were equal to predicted values. Note that most values fall above the line.

View larger version (14K): [in a new window]   Fig. 10. Diagrammatic representation of the extrastapes and stapes of the bullfrog (see Fig. 11 for labelling of anatomical structures). Measurements of the vertical components of the velocity are made from two positions, A and B, on the extrastapes. The vertical velocity measurements VAV and VBV, at points A and B, respectively, together with the horizontal distance between the beads, dA-dB, are used to calculate the horizontal position of the apparent axis of rotation (see text for details). In the case illustrated, the axis of rotation of the extrastapes coincides with the footplate axis. lA, distance between A and the footplate axis; lB, distance between B and the footplate axis; VA, velocity at A; VB, velocity at B; A, B, angles between lA and lB, respectively, and the horizontal.

View larger version (18K): [in a new window]   Fig. 11. Diagrammatic representation of the ossicular apparatus of the bullfrog showing hypothetical modes of motion. The predicted horizontal position of the `extrastapes axis' is indicated in each case with a dashed line. In A, the extrastapes and pars media are stiffly connected and all rotation is about the footplate axis. In B, there is both rotation around the footplate axis and relative movement between the pars media and extrastapes. The extrastapes axis is predicted to lie somewhere between the footplate axis and the extrastapes/pars media articulation. This is the case in the intact frog ear. In C, movement is entirely restricted to rotation at the extrastapes/pars media articulation. There is no movement of the footplate. The middle ear approaches this condition when the ascending process (not shown) is severed. Amplitudes of motion are greatly exaggerated for clarity.

View larger version (26K): [in a new window]   Fig. 12. Diagrammatic representation of the movement of the left extrastapes when the ascending process is intact. For clarity, surrounding middle ear structures are not shown. When the tympanic membrane is inflected inwards, the distal tip of the extrastapes moves ventromedially, sliding on its loose connection with the internal surface of the membrane. The thin ascending process, connected to the parotic crest dorsomedially, bends. As a result of the bending at the ascending process, the vibration velocity at the region of the extrastapes articulating with the pars media has a strong vertical component. This is necessary for the pars media and footplate to be set into motion. Movement is greatly exaggerated for clarity.