Expansion
Ma Married Ivan Stapel
Mnemonic
“MIS”, from tympanic membrane to oval window:
- M - Malleus, the hammer, attached to the tympanic membrane
- I - Incus, the anvil
- S - Stapes, the stirrup, the smallest bone in the body, in the oval window
They amplify sound about 20-fold, by the area ratio of the tympanic membrane to the oval window (about 17:1) and the lever action of the ossicular chain.
Two muscles damp the chain, and their nerves follow the pharyngeal arches:
- Tensor tympani on the malleus, from arch 1, so trigeminal (V3)
- Stapedius on the stapes, from arch 2, so facial (VII)
Loss of stapedius causes hyperacusis, which is why it localises a facial nerve lesion to above the stapedial branch.
Otosclerosis fixes the stapes footplate, giving a conductive loss in a young adult, often with a positive family history and worsening in pregnancy.
Expansion
- Malleus - hammer, attached to the eardrum
- Incus - anvil, the middle link
- Stapes - stirrup, sits in the oval window