Anatomy & Physiology of the Middle Ear

The structures that make up the conductive pathway and how their mass and stiffness shape admittance.

The middle ear is a mechanical impedance-matching device. Its job is to transfer airborne sound efficiently into the fluid-filled cochlea, overcoming the impedance mismatch between air and perilymph. It does this through the area ratio of the tympanic membrane to the stapes footplate and the lever action of the ossicular chain.

The conductive pathway

• Tympanic membrane — couples ear-canal sound pressure to the malleus.
• Ossicular chain — malleus, incus and stapes, acting as a lever system.
• Stapes footplate and the oval window — the interface with cochlear fluid.
• Middle-ear air space and the Eustachian tube — keep the system aerated and at ambient pressure.
• Stapedius and tensor tympani muscles — modulate stiffness; the stapedius drives the acoustic reflex.

Mass, stiffness and admittance

The ease with which the system accepts sound energy — its admittance — depends on the balance of stiffness and mass. At the low probe-tone frequencies used in conventional tympanometry (commonly 226 Hz), the normal middle ear is stiffness-controlled. Anything that stiffens the system (fluid, fixation, scarring) reduces admittance; anything that adds mass or loosens the system raises it. [Hunter LL 2013]

This is why otosclerosis, which fixes the stapes, tends to produce a shallow trace, while ossicular discontinuity, which removes a mechanical link, produces a deep one. The tympanogram is, in effect, a picture of the system's mechanical state. [Hunter LL 2013][Jerger J 1970]

The middle ear — labelled

Click any structure to see its role in sound transmission and what it means for immittance testing.