The Role of Auditory Ossicles in Hearing

The auditory ossicles are a chain of small bones in the middle ear that transmit sound from the outer ear to the inner ear through mechanical vibration.

The names of the bones that comprise the auditory ossicles are taken from Latin. The malleus translates to the "hammer," the incus is the "anvil," and the stapes is the "stirrup."

While central to hearing, the trio of auditory bones fits into an area no larger than an orange seed.

How the Auditory Ossicles Work

The purpose of the auditory ossicles (also called the ossicular chain) is to transmit sound via a chain reaction of vibrations that connects the eardrum to the inner ear and cochlea. Once the vibrations reach the cochlea, a hollow, snail-like chamber filled with fluid, they are translated into nerve impulses which the brain interprets as sound.

The auditory chain reaction starts when sound reaches the eardrum (tympanic membrane). The vibrational pressure is passed to the malleus, an articulated bone that flexes at one of the two incudomalleolar joints.

The vibration is then passed to the incus which flexes at another incudomalleolar joint before transferring the impulses to the stapes, the bone of which not only looks like a stirrup but is the smallest in the body.

The role of auditory ossicles is complete when the vibrational input is passed to the cochlea through the oval window (the opening between the middle and the inner ear).

The auditory ossicles serve an important role in hearing in that they transition sound waves from the air to the fluid-filled core the cochlea.

Regulation of Sound

In addition to facilitating hearing, the auditory ossicles serve a protective function when exposed to continuous, loud noises.

When this happens, the muscles of the eardrum (called the stapedius and tensor tympani muscles) will contract. The tightening reduces the eardrum's ability to vibrate, thereby limiting the movement of auditory ossicles and the impact of the sound.

It's important to note, however, that this is not an instantaneous response. The loop between the eardrum and brain and back must first be stimulated by a loud noise before the constriction occurs. As such, the eardrum and hearing may not be protected from a sudden, loud burst of noise.

Age can also slow the speed of the loop, making older people more vulnerable to hearing damage.

Disorders of the Auditory Ossicles

The auditory ossicles can occasionally be damaged, both directly and indirectly. Injury can cause hearing loss, sometimes severe, and may include:

• Otosclerosis, a condition by which the auditory bones become fused and sometimes require surgery to be fixed
• Dislocation of the auditory bones due to a fracture, trauma, or a chronic infection
• Cholesteatoma, a cyst that develops behind the eardrum and impedes the movement of the auditory bones
• Eardrum perforation or infection (myringitis) which reduces the vibrational impulses to the auditory bones
• Middle ear infections which can cause fluid buildup that restricts auditory bone movement
• Genetic disorders that interfere with the normal development of the auditory ossicles