The ossicles of the middle ear
Anatomy
Essentially a link between the inner and outer ear.
Used to efficiently transmit vibrations from an air medium to a liquid medium.
Role
The role of the ossicles is that of impedance matching (p/v) between the air and the perilymph (the fluid). Basically, this means that they simplify the energy transfer between these two media. This is needed because the input impedance of the endolymph is around 5000 times that of air, thus a mechanism is required to facilitate the energy transmission. Link to impedances : section 2.3.2.
Without these ossicles, the transmission to the inner ear would be 100 times less efficient.
Remark : This impedance matching depends on the frequency, this signifies that the transmission to the inner ear is not as efficient for every frequency (illustrated in the following schematic).
Remark : transfer function of the middle ear
Movement of the ossicles in the middle ear
Informations[1]Stapedius reflex (protection mechanism)
If a sound is detected by the brain at more than 80 dB SPL, the information is transmitted to the nucleus of the brainstem
A reflex feedback loop orders the contraction of certain muscles, this rigidifies the ossicular chain of the middle ear
Due to this rigidification, the energy transmitted to the inner ear is reduced
This mechanism protects the hair cells of the inner ear from damage by high intensity sounds
Caution : this reflex is limited!
it can tire (rapidly)
it only works for frequencies under 2000 Hz
the vibrations in the the cochlea are only reduced by a factor of 10 (20 dB maximum)
this mechanism only intervenes after around 30 ms because of the reflex latency. Thus, the intensity of impulses is rarely reduced
