Transcript Stapedius muscle in various species

The fish inner ear
Weberian ossicle
Swim bladder
Up to 10 dB of
pressure gain in
mammals just from
pinna and meatus
acting as “hearing
trumpet” or
acoustic
waveguide
Tympanic membrane to
stapedius/oval window
area ratio provides
force gain.
Middle ear transformer system. Note in the diagram above
that the handle of the malleus (1) compared to the long
process of the incus (2) adds an advantage of 3-to-1, allowing
a gain in sound energy of only 2.5 decibels. However, the
area ratio of the tympanic membrane footplate is much
greater. The effective ratio is 14:1 and corresponds to a 23decibel gain.
Lizard middle ear
Mammals only:
3 middle ear ossicles: malleus, incus, and stapes
Act as a lever system to increase the force of vibration to the oval
window, and particularly improve transmission of high
frequencies.
Ligaments suspend the ossicles, and minisynovial joints link them
together.
Force amplification is partially a function of lever arm ratios
For a given sound pressure, the stapes displacement is less than air
molecule displacement; the trade-off is greater force. Note high frequency
movement for mammals compared with birds, reptiles.
For a 20 KHz tone
presented at 20dB
SPL, stapes
displacement would be
only 0.0001 nm in
magnitude!!!
Competing selection pressures:
Best for low frequencies
Best for high frequencies
Large tympanic membrane with large
displacement range
Small, light ossicles, small middle ear,
small stiff tympanic membrane
Less stiffness
More stiffness, minimal mass
As you make the tympanic membrane/lever system more stiff, it
responds better to low frequencies, but the acoustic impedance to
low frequencies increases. A large percentage of low frequency
sound is reflected away, setting the practical low frequency hearing
limit.
At high frequencies, the mass of the tympanic membrane/lever
system dominates. There is too much mechanical inertia for high
frequency responsiveness. This sets the high frequency limit.
Audiograms for several terrestrial mammals. Note the improvement in low
frequency hearing with larger middle ears (K-rats, Gerbils)
For the range of hearing important to an animal, selection has compromised
to achieve pretty good middle ear coupling efficiency across frequencies.
The middle ear muscles: dynamic range compensation
(Mammals only also have a much faster mechanism – will talk about later
re: cochlea.)
Tensor tympani
- Attached to the malleus near the tympanic membrane
- Contains both fast and slow acting fibers
Stapedius
- Attached to stapes close to incus
- Contains only fast acting fibers
- Smallest striated muscle in the body
1.
2.
Self-generated sound protection (frogs, reptiles?, birds, mammals)
Protection from external loud sounds (frogs?, reptiles?, NOT birds,
mammals)
Locations of middle ear muscles
tensor
tympani
stapedius
Stapedius muscle in various species
Middle ear reflex factoids
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The middle-ear reflex is too slow for protection against sudden noises.
The reflex attenuates all frequencies, but the muscles are best at
attenuating high amplitude low-frequency sounds, and thus reduce masking
of high-frequency sounds by low-frequency sounds.
For self-generated sound, the muscles are activated before sound reaches
the ear (efference copy!)
Best middle ear muscles re: self-generated sound in bats:
– Fastest contraction rates (10 ms, or able to follow at 100/s)
– Very large middle ear muscles that contain lots of mitochondria, S.R.
– Sensitive to frequencies up to 100 kHz
Middle ear muscle
contraction leads to
approximately 5, 15, and
20 dB of attenuation
depending on the
strength of contraction.
Threshold curves:
the minimum
sound level to
initiate muscle
contraction.
Note greater attenuation at lower frequencies (top
plot) and that low frequenicies are more effective at
eliciting response (bottom plot)