The fish inner ear

Swim bladder

Weberian ossicle

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 23-decibel 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 For a 20 KHz tone presented at 20dB presented at 20dB SPL, stapes SPL, stapes displacement would be displacement would be only 0.0001 nm in only 0.0001 nm in magnitude!!!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

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. Self-generated sound protection (frogs, reptiles?, birds, mammals)

2. Protection from external loud sounds (frogs?, reptiles?, NOT birds, mammals)

(Mammals only also have a much faster mechanism – will talk about later re: cochlea.)

tensortympani

stapedius

Locations of middle ear muscles

Stapedius muscle in various species

Middle ear reflex factoids

• 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)