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First published online January 3, 2006
Journal of Experimental Biology 209, 302-313 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.01989

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Middle ear dynamics in response to seismic stimuli in the Cape golden mole (Chrysochloris asiatica)

U. B. Willi¹, G. N. Bronner² and P. M. Narins^1,3,*

¹ Department of Physiological Science, UCLA, Los Angeles, CA 90095, USA
² Small Mammal Research Unit, Department Zoology, University of Cape Town, Cape Town, Republic of South Africa
³ Department of Ecology and Evolutionary Biology, UCLA, Los Angeles, CA 90095, USA

View larger version (11K): [in a new window]   Fig. 1. (A) Lateral view of skull (C. asiatica). The dimensions in the sketch are to scale and illustrate the enormous relative size of the hypertrophied malleus (black shading) as well as its position and orientation within the skull. (B) In order to attain optimal coupling between the skull and the vibration exciter, the skull was divided into two sections by a frontal plane cut and (C) the posterior half was mounted on a stainless steel disc with acrylic resin. The disc was secured to a mounting block, which could be driven by a vibration exciter. Seismic stimuli were applied in two different directions. (D) For lateral stimuli, the malleus heads were within the horizontal plane and the excitation was coplanar but perpendicular to the long axes of the mallei. (E) For vertical stimuli, the skull remained in place but the excitation direction was changed.

View larger version (18K): [in a new window]   Fig. 2. Setup for vertical excitation. The entire setup is placed on a high-performance laminar-flow isolation table (1). The scanning laser Doppler vibrometer (SLDV) (2) is suspended and positioned in three dimensions by a telescopic lifting column (3) and a XY-translation stage (4). A solid extension arm (5), only used during vertical excitation, allowed positioning of the first-surface mirror (6), tilted at an angle of 45° in order to deflect the laser beam downwards by 90°. At the distal end of the extension arm, an accelerometer (7) monitored the motion of the arm during the experiment. The specimen was tightly mounted to a metal block (8), which was driven by the vibration exciter (9). In order to avoid transmission of vibrations to the SLDV via the supporting table, a rubber mat (10) on top of an open-cell foam mat (11) separated the table from the vibration exciter.

View larger version (44K): [in a new window]   Fig. 3. (A) Lateral view of the ossicular chain of C. asiatica obtained by microcomputed tomography (µCT). The coordinate system for the measurement points (black squares) was aligned to anatomical landmarks. The x-, y- and z-axes and the rotations about them (x, y and z) describe the six possible degrees of freedom of the ossicular chain. The anterior process of the malleus (APM) and the manubrium (MA) were added manually and were not rendered in 3-D from the µCT data set. (B) Reconstruction of lenticular process of incus (LPI) motion based on three motion components for lateral and (C) vertical stimulation. The indicated motion directions (x, y, z, vyt and vzt) in B and C are adjusted in order to point towards the LPI, whereas in A they indicate the mathematically correct polarity of the vectors. SPI, short process of incus.

View larger version (56K): [in a new window]   Fig. 4. Two control experiments. (A) Verification of induced vibrations on the scanning laser Doppler vibrometer (SLDV) and the extension arm. The noise level was measured with the accelerometer at the tip of the extension arm (dotted line). During application of a vertical stimulus (frequency, 10-600 Hz, velocity, 1x10-4 m s-1), the skull velocity was measured with the SLDV (dashed line) while the accelerometer simultaneously monitored the motion of the extension arm (solid line). (B) Relative plot of the mounting block response in three orthogonal directions for lateral and (C) vertical stimulation. The colors refer to those in Fig. 3A. The driving direction (intended excitation direction) of the vibration exciter is represented by a flat line at 0 dB (green line for lateral, and blue line for vertical stimulation, respectively).

View larger version (13K): [in a new window]   Fig. 5. Velocities measured at 15 points on a line along the ossicular chain are plotted in the Gaussian plane (real-imaginary). The real and the imaginary parts of the velocity measured from the 15 points; three on the incus (open circles) and 12 on the malleus, are plotted for three different frequencies (150, 350 and 545 Hz). The points on both malleus and incus represent a reasonably straight line, which indicates that the measured points are located on structures (malleus and incus) that dynamically function as one unit.

View larger version (55K): [in a new window]   Fig. 6. Four views of the ossicular chain of C. asiatica derived from the three-dimensional data obtained by means of microcomputed tomography (µCT). The manubrium (MA) and the anterior process of the malleus (APM) were lost in the 3-D rendering process and were manually inserted. IMJ, incudo-mallear joint; LPI, lenticular process of the incus; SPI, short process of the incus; U, umbo.

View larger version (47K): [in a new window]   Fig. 7. Two examples of original µCT-reconstructed cross sections through one middle ear of C. asiatica. The sections are therefore in parallel and 1.8 mm apart. (A) Cross-section through the tympanic membrane (TM) and the manubrium (MA). (B) Cross section 1.8 mm towards the tip of the malleus (M) head, showing the thin structure of the anterior process of the malleus (APM). The asterisks represent the stapedial artery (it bifurcates before it passes the stapes and fuses again after it passes it). I, incus; S, stapes; CO, cochlea; EAM, external auditory meatus; DO, dome (bony shell of external auditory canal near the TM).

View larger version (42K): [in a new window]   Fig. 8. Relative ossicular response at the distal tip on the malleus head in line with the stimulation direction of the same five right ears for lateral (black) and vertical (gray) stimulation. Each frequency response was shifted along the logarithmic frequency scale to bring the resonant frequencies into alignment at an arbitrary frequency of 150 Hz. Over the frequency band tested, malleus velocity amplitudes in response to lateral stimulation exceeded those in response to vertical stimulation.

View larger version (78K): [in a new window]   Fig. 9. Iso-velocity amplitude map for lateral (A) and vertical (B) stimulation based on the SLDV measurements of the malleus head. The two graphs represent the motion pattern at the resonant frequencies 178 Hz and 182 Hz, respectively, measured from the left ear of animal #17. Velocity amplitudes were calculated by applying Eqn 2 at each point of a grid at a spatial resolution of 0.5 mm. The map and an outline of the ossicular chain are superimposed to indicate the motion pattern with respect to the ossicular anatomy. The axes of minimal amplitude are indicated by the y'- and z'-axis, respectively. For the LPI-motion reconstruction in response to vertical stimulation, the coordinate system was aligned with respect to the z'-axis. APM, anterior process of malleus; LPI, lenticular process of the incus; MA, manubrium; SPI, short process of the incus.

View larger version (46K): [in a new window]   Fig. 10. (A) For lateral stimulation, the LPI motion in the medio-lateral direction was reconstructed based on the three motion components: x, y and vzt, and the results are given for the same five right ears whose response is depicted in Fig. 8. Upper graph, amplitude responses; lower graph, phase responses. (B) For vertical stimulation, the LPI motion was reconstructed separately for the three motion components x', z' and vyt, resulting in the three orthogonally oriented LPI motion components LPIx', LPIz' and LPIy. Consistent and reproducible results were observed only for the z'-component (LPIz').