Vibrometric studies of the middle ear of the bullfrog Rana catesbeiana I. The extrastapes
Department of Physiological Science, UCLA, 405 Hilgard Avenue, Los Angeles, CA 90095, USA
* Author for correspondence at present address: Department of Zoology, Downing Street, Cambridge CB2 3EJ, UK (e-mail: mjm68{at}hermes.cam.ac.uk)
Accepted 15 July 2002
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Summary |
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Key words: middle ear, bullfrog, Rana catesbeiana, lever ratio, stapes, ear evolution, hearing
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Introduction |
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Non-mammalian vertebrates lack a malleus and an incus. In typical frogs,
such as the American bullfrog Rana catesbeiana, the vibrations of the
tympanic membrane are conveyed to the oval window by means of the extrastapes
and stapes (also known as the extracolumella and columella)
(Fig. 1). The extrastapes, or
pars externa plectri, is a cartilaginous element articulated distally with the
inside of the tympanic membrane and proximally with the pars media plectri,
the bony shaft of the stapes proper. The extrastapes is also attached to the
skull by means of a strap-like cartilaginous process, the ascending process
(processus ascendens plectri), which is articulated with the ventral side of
the parotic crest (Wever,
1985). The pars media expands medially, where it is continuous
with the thick, cartilaginous pars interna. This proximal portion of the
stapes is known as the footplate and is contained within the rostral half of
the oval window. Just ventrolateral to the pars interna, a ridge of the pars
media articulates with the otic capsule
(Bolt and Lombard, 1985
;
Jaslow et al., 1988
;
Hetherington, 1992
). The caudal
half of the oval window is occupied by the operculum, a cartilaginous or bony
element unique to amphibians.
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Jørgensen and Kanneworff
(1998) used laser vibrometry
to study the middle ear lever mechanism of the grass frog (Rana
temporaria). The stapes footplate was found to rock about the
articulation between the pars media and otic capsule (referred to henceforth
as the `footplate axis'; Fig.
1), resulting in a phase difference of 180° between the
tympanic membrane and footplate at low vibration frequencies. The
vibrometrically measured velocity ratio of the grass frog was more than 10 dB
higher than the value expected if the extrastapes and stapes were to vibrate
as a stiff unit firmly tethered to the tympanic membrane. Jørgensen and
Kanneworff (1998
) argued that
the discrepancy between vibrometric and anatomically predicted values could be
accounted for by a drop in velocity amplitude between the tympanic membrane
and extrastapes due to the angle between them, together with bending of the
extrastapes. Moffat and Capranica
(1978
), apparently by analogy
with Manley's (1972a
) model of
the reptile and bird ossicular system, suggested that the extrastapes in frogs
might be hinged at its connection to the pars media, which could also increase
the velocity ratio, but Wilczynski and Capranica
(1984
) asserted that this
articulation is stiff. It would appear that the motion of the extrastapes of
frogs has never been directly investigated.
In the present study, the movement of the extrastapes was experimentally examined with regard to elucidating the function of this structure. Vibrometric measurements of the extrastapes shaft were made, apparently for the first time in frogs. These measurements allowed direct examination of the hypotheses that the vibration velocity ratio is increased by means of the connection between tympanic membrane and extrastapes, by bending of the extrastapes or by a hinge-like motion between the extrastapes and pars media. The role of the ascending process of the extrastapes, almost totally ignored in previous studies of middle ear function, was also considered.
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Materials and methods |
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Vibrometric measurements were made on the left ear only of each frog. The animal was anaesthetised with intramuscular injections of pentobarbital sodium solution (Nembutal; Abbott Laboratories) (50 mg ml-1, 1.2µl g-1 body mass) and ketamine (Ketaject; Phoenix Scientific) (100 mg ml-1, 1.2µl g-1 body mass). Smaller supplementary doses were given as necessary to maintain a state of areflexia. The mouth cavity was examined to ensure that the Eustachian tubes were free of mucus. Throughout surgery and the experimentation procedure, the frog was sprayed regularly with water to facilitate cutaneous respiration.
The surgical approach to the stapes footplate involved removing a small, square piece of skin from just rostral to the left suprascapular cartilage. Part of the m. depressor mandibulae was cut away, and the operculum was exposed by clearing away the connective tissue around the m. opercularis. Some of the cranial nerves passing over the operculum were also usually removed, as was part of the jaw adductor musculature. Care was taken to avoid damage to the vena capitis lateralis, which crosses the stapes and lateral part of the operculum. The caudal part of the shelf formed by the prootic bone and parotic crest, which overlies much of the pars media and pars interna of the stapes, was shaved away with a sharp scalpel. By these means, the caudomedial part of the stapes footplate was exposed. Since the middle ear cavity extends medially only approximately half-way along the length of the pars media of the stapes, the cavity was not breached in this surgical approach.
A small square (approximately 0.1-0.25 mm2) of reflective glass beads mounted on adhesive backing material (Polytec) was positioned on the centre of the left tympanic membrane. One or two glass beads, 40-60 µm in diameter, were positioned on the exposed caudomedial part of the footplate near its articulation with the operculum. Later dissection (following experimentation) showed that the beads were situated approximately three-quarters of the length of the footplate away from its rotatory axis, nearly always on the pars interna rather than the pars media. Beads were also placed on the parotic crest, an exposed region of the skull just rostral and dorsal to the stapes footplate. In some experiments, the extrastapes was later exposed by removing a small flap of tympanic membrane dorsal to its centre and reflection or excision of the connective tissue and blood vessels running between the extrastapes and the tympanic membrane. Reflective beads were placed in up to three positions on the dorsal surface of the extrastapes.
The anaesthetised frog was propped in what approximated an upright, sitting position by means of a piece of foam positioned under the pectoral girdle. The head was inclined, snout tilted slightly downwards, to an angle of approximately 20° below the horizontal to give access to the footplate from a vertical approach. In some experiments, a second piece of foam was positioned under the tip of the snout to support the head. The animal was placed on a vibration-isolated table (Backer-Loring Micro-g) within a double-walled, sound-attenuating chamber (IAC 1202-A). A single-point, He-Ne laser vibrometer sensor head (Polytec OFV-303) was positioned approximately 60 cm from the animal's head, and a binocular light microscope (Zeiss OP-1), its objective lens 22 cm above the head of the frog, was also resting on the table. The laser beam, if inclined at an angle of approximately 25° to the horizontal, could be aimed at the tympanic membrane. The velocities of movement of all other structures were recorded using a vertical beam. A prism was mounted in the microscope such that the beam, directed into the side-port of the barrel, could be reflected downwards onto the frog. The angle of the reflected laser beam deviated by up to 4° from the vertical according to the position of the laser point within the field of view of the microscope, but any effect of this deviation was neglected in the subsequent calculations for simplicity. The position of the laser point could be monitored through the microscope. A 10 cm diameter speaker (Analog and Digital Systems Inc. 300) was positioned in front of the frog with the centre of its cone 75 cm from the centre of the frog's interaural axis, at an azimuth of 30° to the midline and an elevation of 13°. The speaker was not resting on the same table as the frog. A probe microphone (Knowles Electronics, type EK-3033) was positioned approximately 2 mm from the centre of the tympanic membrane. The rest of the apparatus was located outside the sound-attenuating chamber.
Pure tones (2s duration, from 180 Hz to 3 kHz in 30 Hz steps), were synthesised by a custom-designed program (Acoustic Analyzer 0.20ß: author A. Purgue, 1999) running on an Apple Macintosh iMac computer. The output of the computer's 16-bit D/A board was amplified (Optimus MPA-50) and sent to the speaker; a returning signal from the amplifier was attenuated (Hewlett-Packard 350D or Agilent 355D) and sent back to the computer, where it was used as a reference signal for the phase measurements. The same software was also used to monitor and record the returning signal from either the probe microphone or the laser vibrometer. The sampling frequency was 44.1 kHz. The probe microphone was connected to a custom-made AC amplifier (J. Wang), the output from which was bandpass-filtered (Krohn-Hite 3323 active filter) with 100 Hz and 3500 Hz cut-off frequencies. The filter output was attenuated (Hewlett-Packard 350D or Agilent 355D and 355C array) and sent to the computer. The computer program automatically adjusted the sound pressure level (SPL) measured at the probe microphone at each successive frequency to 90 dB SPL. The sound pressure level produced, having been adjusted for the probe microphone's calibration curve, was flat ±3 dB from 180 to 3000 Hz at the position of measurement. To measure the harmonics produced by this experimental arrangement, a microphone (Brüel and Kjaer 4134) was positioned where the frog would be and was attached via a heterodyne analyser (Brüel and Kjaer 2010) to a fast Fourier transform (FFT) network analyser (Stanford Research SR770). For the velocity measurements, the probe microphone was removed. The laser sensor head was connected to a vibrometer controller processor (Polytec OFV-3001), which produced a voltage output proportional to the vibration velocity of the structure being measured. The output from the vibrometer controller was fed to the computer via the filter and the attenuator.
Comparisons of the tympanic membrane responses were made at the beginning and end of every experiment. In experiments in which the tympanic membrane was intact, the responses remained very consistent. Three consecutive measurement runs were usually made from each structure being examined, and each set was averaged. In experiments in which the tympanic membrane was perforated to give access to the extrastapes, its response often changed slowly over time as a result of drying. In these experiments, single measurement runs of each structure in turn were made, generally in the order tympanic membrane extrastapes footplate parotic crest tympanic membrane. Three sets of runs were obtained in this manner, and data were compared. Velocity ratios obtained in this way were very consistent despite small changes in absolute responses. The velocity amplitudes considered are peak values.
The response that could be measured from the stapes, following the surgical
approach outlined earlier, was the vertical component of the movement of the
footplate relative to the otic capsule, superimposed onto the response of the
otic capsule itself (the `background vibration'). The response of the parotic
crest was taken to be representative of the response of the otic capsule. The
velocity of the footplate within the oval window (VFPVa)
was calculated by subtracting the velocity of the parotic crest from the
velocity of the footplate. The measured responses were both assumed to be pure
sinusoids, and the movements of the stapes footplate were assumed not to
affect the response of the parotic crest. In the calculation below,
VFPV is the vertical component of the velocity of the
stapes footplate at frequency and time t; its amplitude is
a mm s-1. VPCV is the vertical
component of the velocity of the parotic crest at the same frequency, of
amplitude b mm s-1.
1 and
2 are the phase lags of these two structures, respectively,
relative to the signal returning from the speaker. It can be shown that:
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The tympanic membrane of the bullfrog is inclined at an angle of
approximately 50° to the horizontal plane in both sexes (M. J. Mason and
P. M. Narins, personal observation). The experimental apparatus permitted the
measurement of the component of membrane velocity at an angle
(las) of approximately 25° to the horizontal
(Fig. 2). This component of
velocity is here termed VTMA. By deflecting the laser beam
with the prism, the vertical component of tympanic membrane velocity,
VTMV, could also be measured. The two components
VTMA and VTMV can be used to establish
the true tympanic velocity, VTM. VTM
is the maximum velocity of the centre of the tympanic membrane and is expected
to be normal to the plane of the membrane. Let us call the angle between
VTM and the horizontal
TM (see
Fig. 2). In this case, the
angled laser will record a velocity:
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Following each experiment, the frog was euthanized with an overdose of
Nembutal or by double-pithing, and was then decapitated. The head was
positioned under a light microscope (Wild, 12x-100x magnification)
fitted with a grid eyepiece lens, such that the microscope image was of a
caudal view of the left middle ear apparatus. Lightly pressing on the tympanic
membrane resulted in a visible rocking motion of the stapes footplate. The
estimated position of the footplate axis always corresponded with the
articulation between the pars media and otic capsule, just ventrolateral to
the footplate (see Fig. 1).
Angles and lengths were measured from scale diagrams, including the angle
bead subtended between a line joining the bead position on
the footplate to the footplate axis and the horizontal at the axis.
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Results |
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The responses of the tympanic membrane at different sound pressure levels (ranging from 70 to 100 dB SPL) were measured in five frogs, and the responses of the stapes footplate were also examined in one of these animals. No non-linearities were apparent in the tympanic membrane response at these levels. Although the stapes response was not examined at levels above 90 dB SPL, no consistent non-linearities were apparent at 90 dB SPL or below, and the velocity ratios measured at the different sound pressure levels were very similar.
Angle of the tympanic membrane velocity vector
Equation 6 was used to establish the true vibration velocity of the
tympanic membrane (VTM) before surgery in five male and
three female frogs. At most frequencies below 2 kHz, the difference between
VTM and VTMA was very small, generally
less than 2 dB. Above 2 kHz, the difference often rose (to a maximum of
approximately 6 dB), suggesting that the angle TM was
increasing, but the mean differences for both sexes were less than 3 dB at all
frequencies. Measurements obtained from two animals after the stapes had been
surgically exposed were similar. The results of these experiments suggest that
the tympanic membrane velocity component VTMA is a very
good approximation to the true velocity VTM at frequencies
below 2 kHz and is a reasonable approximation at higher frequencies.
VTMA was used as an estimate of VTM in
all further calculations.
The stapes response and the velocity ratio
Tympanic membrane, extrastapes and stapes responses from a representative
male frog are depicted in Fig.
3. The responses measured from both the proximal and distal
extrastapes positions (dark blue and light blue traces, respectively) are
close to the response of the tympanic membrane (black trace) over a broad
frequency range. The response of the stapes footplate (red trace) is smaller
than that of the extrastapes, but is considerably greater in amplitude than
that of the parotic crest (green trace) at most frequencies. At the lowest and
highest frequencies, and between 450 and 480 Hz, the velocity amplitude of the
parotic crest is high relative to that of the footplate. As a result of this,
the measured footplate response does not follow the response of the
extrastapes as faithfully at these frequencies. Such discrepancies can be
greatly reduced or eliminated when the `background' head response is
controlled for (by the application of equation 1). Results from females were
broadly similar, except that females lack the peak in tympanic membrane and
stapes footplate responses at 200 Hz.
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Velocity ratios, calculated as tympanic membrane velocity (VTM) divided by the adjusted vertical component of stapes footplate velocity (VFPVa), are presented in Fig. 4A for 12 male frogs (blue traces) and six female frogs (red traces). For frequencies below 2 kHz, the velocity ratios remain approximately flat. Data become noisy at frequencies higher than this. A mean value of VTM/VFPVa for frequencies below 2 kHz was calculated for each frog. In males, this value averages 24.3 dB (range 17.5-28.8 dB) and in females 17.0 dB (range 13.7-19.7 dB). The variances of these values (converted to absolute units) differ (F-test: F=12.94, d.f.=11.5, P=0.011) so a heteroscedastic t-test was used to examine the difference between (absolute) male and female values. The difference is statistically significant (t=3.84, d.f.=14, P=0.002). The phase lags between the tympanic membrane and footplate are approximately 180° at low frequencies (Fig. 4B), increasing to a mean of approximately 290° in females and 330° in males at 2 kHz.
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The extrastapes
As a result of the perforation made in the tympanic membrane to expose the
extrastapes, the response of the membrane always dropped, especially at low
frequencies. However, the VTM/VFPVa
ratios measured before and after this surgical procedure were found to be
similar (generally within ±3 dB at frequencies under 2 kHz) in all but
one frog (discussed below). This suggests that the middle ear system was not
greatly affected by the surgery.
The vertical component of the velocity of the distal extrastapes (the lateral half of this structure) was measured in three female and seven male frogs. In most frogs, the velocity amplitude measured was within 5 dB of the tympanic membrane response at frequencies, up to 2 kHz (Fig. 5A). At the lowest frequencies, the phase lag between the tympanic membrane and extrastapes is zero, increasing to an average of approximately 45° at 2 kHz (Fig. 5B). At frequencies above 2 kHz, the relative velocity amplitudes are more variable. This variability probably reflects a change in the velocity vector direction of the tympanic membrane, and possibly of the extrastapes, together with the low signal-to-noise ratio at the highest frequencies. There are no consistent differences between female and male frogs.
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In one male, the mean velocity amplitude difference between the tympanic membrane and distal extrastapes, at frequencies up to 2 kHz, was 9.0 dB (Fig. 5A), considerably greater than in the other frogs tested. However, the mean VTM/VFPVa ratio of this frog over the same frequency range had risen by 5.8 dB after exposing the extrastapes, a much greater change than in any other frog, and its tympanic membrane was clearly drying. It seems probable that the increase in lever ratio was because the extrastapes of this frog was becoming uncoupled from the tympanic membrane as a result of the drying. If so, the velocity difference between the membrane and extrastapes in the ear prior to extrastapes surgery would have been approximately 3 dB, in the middle of the normal range.
Measurements were made from more than one position on the extrastapes in three female and six male frogs (Fig. 6). At frequencies below 2kHz, the velocity amplitude of the distal extrastapes is greater than that of more proximal positions, typically by 1.2-5.0 dB (Fig. 6A). The phase lags between these positions are generally negligible, less than 10° in most frogs for frequencies up to 2kHz (Fig. 6B). Data are noisy at the highest frequencies.
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The ascending process
In two female and two male frogs, a small central portion of the ascending
process of the extrastapes was excised. Access to this structure was afforded
by means of the wide Eustachian tube opening into the buccal cavity. Following
the excision, the stapes footplate response
(Fig. 7; red trace) dropped
substantially in all frogs, becoming almost indistinguishable from that of the
parotic crest (representing background skull vibration; green trace) in the
male animals. In the females, the stapes response was still clearly greater
than that of the parotic crest after the process had been severed, but the
mean velocity ratio (for frequencies from 180 to 1980 Hz inclusive) had
increased in both animals by approximately 20 dB
(Fig. 8). In subsequent
dissections, it was verified that the ascending process was indeed severed in
all cases and that the rest of the extrastapes was still intact and formed a
complete connection between the tympanic membrane and pars media.
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In the female frog of Fig. 8, measurements at two different points on the extrastapes were made before and after the ascending process had been severed (Fig. 8A,B). The velocity difference between tympanic membrane (black trace) and distal extrastapes (light blue trace) increased by approximately 5 dB at frequencies below approximately 1.5 kHz, the difference diminishing at higher frequencies. A more dramatic change was seen in the relative velocity amplitudes of the two extrastapes positions. The velocity difference between the distal and proximal positions was approximately 3 dB prior to ascending process surgery: after surgery, this difference increased to approximately 13 dB at low frequencies (300 Hz), dropping to approximately 7 dB at 2 kHz. A sharp drop in the response of the proximal bead (dark blue trace) between 700 and 800 Hz interrupted this general decline (Fig. 8B). This coincided with a rapid change in the phase lag between the distal and proximal extrastapes, from approximately 0° at frequencies of up to 650 Hz, through 180° at approximately 750 Hz, and then back to approximately 0° above approximately 1 kHz.
Upon pressing inwards on the tympanic membrane during dissections, the extrastapes tip moves inwards and downwards, sliding on the internal surface of the tympanic membrane to which it is only loosely attached. The ascending process bends to accommodate this, and there is a small amount of flexion between the extrastapes and pars media in the region of their articulation. Severing the ascending process increases the flexion at the extrastapes/pars media articulation while greatly reducing the movement of the pars media and footplate. These observations are entirely consistent with the experimental data, although the amplitude of movement in response to manually applied pressure is obviously many orders of magnitude greater than that in response to airborne sound.
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Discussion |
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If the stapes and extrastapes together form a stiff unit vibrating around
the footplate axis, and if the tip of the extrastapes has a velocity equal to
that of the tympanic membrane, the measured velocity ratio
VTM/VFPVa can be predicted from the
following equation:
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Phase changes measured in the present study suggest that the coupling
between the tympanic membrane and extrastapes is not perfectly stiff, and this
is supported by anatomical observations
(Jørgensen and Kanneworff,
1998; present study). However, the velocity drop between the
tympanic membrane and distal extrastapes position at frequencies below 2 kHz
is usually small, averaging approximately 2.5 dB. Since the apparatus used in
the present study examines only the vertical component of the velocity of the
extrastapes at each position, and since the distal extrastapes bead could not
be placed on the very tip of this structure, the difference between tympanic
membrane velocity and extrastapes tip velocity will be smaller than the
measured value. The contribution of tympanic membraneextrastapes
coupling to the overall VTM/VFPVa
ratio would appear to be very small.
Experimental measurements show that points on the extrastapes move in phase
across a broad frequency range and that the amplitude difference between these
points varies little at frequencies below 2 kHz. This suggests that the main
body of the extrastapes is stiff. To examine the motion of the extrastapes in
more detail, consider a case in which the velocity amplitude is measured from
two positions (Fig. 10). The
difference between the amplitudes measured at these positions allows the
calculation of an `extrastapes axis', about which these points appear to be
vibrating. Let us call the first and more distal position A, and the
velocity at this position VA. The vertical component of
VA measured by the laser is VAV. The
line between A and the extrastapes axis makes an angle
A with the horizontal, and the distance between A
and this axis is lA. Corresponding values for the second,
more proximal, position B are given the subscript B. In
Fig. 10, the extrastapes axis
is assumed to be coincident with the footplate axis, but this need not
necessarily be so. From the above definitions:
![]() | (8) |
![]() | (9) |
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When the ascending process was severed in the female frog of Fig. 8, the velocity difference between the proximal and distal extrastapes increased dramatically, especially at low frequencies (Fig. 8B). Excluding frequencies from 700 to 800 Hz, where a pronounced dip was seen, the mean velocity ratio VAV/VBV was 3.46 (10.8 dB). Using the same anatomical measurements, the horizontal position of the extrastapes axis is now calculated to be 1.3 mm from the distal extrastapes position. This corresponds with the position of the articulation between the extrastapes and pars media, falling on the dashed line shown in Fig. 11C.
Wever (1985) mentions that
the ascending process `probably adds stability to the tympanic membrane and
protects it against undue forces', but does not go into any further detail.
Moffat and Capranica (1978
)
refer to this structure as the `plectral ligament', which implies a role as a
tether. The results of the present study suggest that the ascending process is
critical to the normal functioning of the ossicular apparatus of the frog.
While most of the extrastapes is sturdy but slightly flattened in the
transverse vertical plane, the ascending process is thin and flattened in the
horizontal plane. This morphology favours bending at the ascending process
when the extrastapes is exposed to forces acting in the vertical plane.
Bending at the ascending process translates inward motion of the tympanic
membrane into roughly vertical motion at the extrastapes/pars media
articulation (Fig. 12),
resulting in rotation of the pars media and footplate about the footplate
axis. The calculated horizontal position of the extrastapes axis, close to the
centre of the ascending process, would be consistent with this interpretation.
Some flexibility at the articulation between the extrastapes and pars media is
required to accommodate this mode of vibration. Severing the ascending process
changes the motion of the extrastapes such that it now pivots on its flexible
articulation with the pars media (Fig.
11C). The vertical component of motion at the articulation is much
reduced, and the velocities of the pars media and footplate are greatly
decreased. This general picture is complicated by the appearance of
frequency-dependent vibration modes when the ascending process is severed.
Between 700 and 800 Hz, the data suggest that the extrastapes rotates about a
point just lateral to the proximal measurement position (which was close to
the articulation with the pars media in this frog), resulting in a `see-saw'
type motion of the extrastapes over this narrow frequency range.
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Although the velocity ratio
VTM/VFPVa differs significantly
between male and female bullfrogs, the above calculations suggest that the
motion of the extrastapes is actually similar in both sexes. The high velocity
ratios measured in some male frogs (Fig.
9) could be due to differences in the coupling between the
extrastapes and pars media: increased flexibility at the articulation would
result in a smaller movement of the pars media and footplate for a given
displacement of the extrastapes. Increased flexibility in the ascending
process might have a similar effect. Alternatively, increased impedance at the
stapes footplate would result in greater flexion and a higher lever ratio even
if the ossicular structures were identical in the two sexes.
Frequency-dependent changes in inner ear impedance have been thought to affect
the measured values of middle ear lever ratios in other vertebrates
(Manley, 1972b;
Gyo et al., 1987
;
Gummer et al., 1989
), although
no sex differences were noted. A higher impedance in the ear of the male
bullfrog could reflect an anatomical difference in the inner ears between the
sexes, a stiffer articulation between the stapes and oval window or perhaps an
effect of the operculum, which is coupled to the stapes footplate
(Mason and Narins, 2002
).
When under water, or when accidentally touched, the externally located
tympanic membrane of a frog will be subject to a static pressure inflexion
much greater than that induced by airborne vibrations. Under these
circumstances, the ascending process will buckle, and the joint between the
extrastapes and pars media will flex. This is observed during dissections when
pressure is exerted on the outside of the tympanic membrane. As a result, the
displacement of the pars media and footplate will be smaller than with a stiff
ossicular system, helping to prevent large static pressure changes from being
transmitted to the inner ear. Conversely, when the tympanic membrane is forced
outwards during middle ear air pressure increases associated with breathing or
vocalising (Hetherington and Lombard,
1983; Narins,
1992
; Purgue,
1997
), the stapes footplate would tend to be pushed into the oval
window (Purgue and Narins,
2000
). A large outward displacement of the extrastapes may be
resisted by tension in the ascending process, reducing the displacement of the
pars media and pars interna.
This system could act in conjunction with a newly proposed function of the
opercularis system (Mason and Narins,
2002), whereby contraction of the opercularis muscle during
breathing (and presumably also vocalisation) may restrain the operculum and
stapes footplate and resist their being pushed into the oval window. In
dissections, pulling on the opercularis muscle can be seen to pull both the
stapes and operculum outwards (Mason and
Narins, 2002
). The tip of the extrastapes is attached only loosely
to the tympanic membrane and it slides ventrally, perhaps explaining why
manipulating this muscle does not have a measurable effect on tympanic
membrane vibration (Hetherington,
1994
). The loose connection between the extrastapes and tympanic
membrane might represent another mechanism to restrict ossicular movement in
response to high-amplitude tympanic displacement. The likely cost of such
flexibility would be a reduction in transmission efficiency at high
frequencies (Manley,
1972a
).
It is interesting to note that reptiles and birds both have a cartilaginous
extrastapes, and there is evidence for flexibility in their ossicular systems
(Manley,
1972b,c
;
Norberg, 1978
;
Rosowski et al., 1985
;
Saunders, 1985
;
Saunders et al., 2000
).
Although mammalian ossicles are largely ossified, the majority of mammalian
species examined have synovial articulations both between the malleus and
incus and between the incus and stapes (see
Mason, 1999
). With the small
fluctuations in pressure associated with normal sound transmission, mammalian
ossicular articulations are often seen as being effectively rigid, although
relative movement between the ossicles may increase at high frequencies
(Møller, 1963
;
Guinan and Peake, 1967
;
Gyo et al., 1987
). However,
much greater relative motion is seen with increased pressures, when the
flexible articulations between the ossicles help to decouple the excessively
displaced tympanic membrane from the stapes footplate, thus protecting the
inner ear (Cancura, 1980
;
Marquet, 1981
;
Hüttenbrink, 1988
). The
flexibility of the extrastapes relative to the pars media in frogs may have an
analogous function.
Norberg (1978) functionally
equates the extrastapes of the owl to the (fused) malleus and incus of
mammals. It is suggested in the present study that the extrastapes and stapes
of frogs are functionally two `ossicles', rather than one, and that the middle
ear mechanics of `columellar' and tri-ossicular ears may be more similar than
is commonly assumed.
In summary, the following conclusions are reached: (i) relative movement between the tympanic membrane and extrastapes, and bending of the extrastapes itself, makes only a small contribution to the discrepancy between anatomically predicted and experimentally measured velocity ratios; (ii) most of this discrepancy, especially in male frogs, probably arises as a result of flexion between the extrastapes and pars media; (iii) the ascending process of the extrastapes plays a vital role in limiting the amount of flexion at this articulation and in controlling the movement of the extrastapes; and (iv) flexibility within the middle ear apparatus of the bullfrog probably acts as a protective device.
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