What are the functions of the mastoid air cells?

Answer 1

The mastoid air cells of the temporal bones are an interconnected network of small sinus cavities which communicate with the middle ear via the aditus ad antrum superiorly. The middle ear, which does not communicate with the external auditory canal due to presence of a (normally) intact tympanic membrane (the "ear drum"), depends on the additional compliance of the air-filled mastoid air cells in order to allow the tympanic membrane to move in response to sound waves (= changes in air pressure). The tympanic membrane needs to be able to move freely in order to convert external pressure changes into movement of the ossicular chain of the middle ear, which in turn conveys that motion to the membrane-covered oval window of the fluid-filled cochlea. The cochlea is responsible for converting that movement into nerve signals to send to the brain for interpretation.

Interestingly, the cochlea has a second small membrane-covered opening, the round window, to enable the fluid to move in the cochlea secondary to action of the ossicles on the oval window, similar to the function of the mastoid air cells in allowing the tympanic membrane to move in response to changes in external air pressure.

If the mastoid air cells are absent or fluid filled, there is less compliance in the system, decreasing the ability for the tympanic membrane to move and transmit sound, decreasing auditory acuity.

In addition, the mastoid air cells provide secretions that slowly travel from the mastoid air cells through the middle ear and into the posterior nasopharynx by way of the eustachian tubes in order to keep the middle ear clear of debris. The eustachian tubes are the structures that allow you to "pop" your ears in response to pressure changes, by equalizing pressures in the middle ear with the external environment through transmission of air from your nose and mouth. This is also the primary reason that people (and especially children, with their smaller eustachian tubes) almost always have fluid in their middle ears when they are suffering from upper respiratory tract infections ... the upper respiratory tract is contiguous with the middle ear by way of the eustachian tubes.

N.B: The mastoid air cells are not "air bags" or "crumple zones" to protect the brain from injury; severe temporal bone injury carries significant morbidity (hearing loss, facial nerve paralysis, chronic infections, vascular injury of the carotid and internal jugular veins) and mortality, and the mastoid air cells themselves are highly protected with thick overlying cortical bone.

Answer 2

Depending on the location, mastoid air cells are divided into

1. Zygomatic cells (in the root of zygoma)
2. Tegmen cells (extending in tegmen tympani)
3. Perisinus cells (overlying the sinus plate)
4. Retrofacial cells (round the facial nerve)
5. Perilabyrinthine cells (located above, below and behind the labyrinth, some of them passing through the arch of superior semicircular canal. These cells may communicate with the petrous apex)
6. Peritubal (around the Eustachian tube. Along with hypotympanic cells they also communicate with the petrous apex)
7. Tip cells which are quite large and lie medial and lateral to the digastric ridge in the tip of mastoid
8. Marginal cells (lying behing the sinus plate and may extend into the occipital bone)
9. Squamosal cells (lying in the squamous part of temporal bones).

These cells are found in the mastoid process of the temporal bone (behind the ear).