The auditory ossicles are a chain of three minuscule bones in the middle ear. These bones, the smallest in the human body, form a connection from the eardrum to the inner ear. They convert sound waves into vibrations that are transferred to the fluid-filled inner ear, allowing the brain to process sounds.
Meet the Malleus, Incus, and Stapes
The three auditory ossicles are named for their distinctive shapes: the malleus (hammer), incus (anvil), and stapes (stirrup). The malleus is the largest of the three and connects to the tympanic membrane, or eardrum. The head of the malleus joins with the incus, the second bone in the chain.
The incus acts as a bridge, connecting the malleus to the stapes. The joint between the incus and the stapes is a synovial joint, which allows for movement. This connection leads to the final bone in the chain, the stapes.
The stapes is composed of a head, two arches, and a base. The base, or footplate, of the stapes fits into the oval window of the inner ear. This arrangement of the three ossicles, held in place by ligaments, forms a chain that spans the middle ear.
The Mechanics of Hearing via the Ossicles
The process of hearing begins when sound waves cause the eardrum to vibrate. These vibrations are transmitted through the ossicular chain from the malleus to the incus and finally to the stapes. The movement of the stapes at the oval window creates waves in the inner ear’s fluid, which stimulates receptor cells. These cells then convert the vibrations into electrical signals sent to the brain for interpretation.
A primary function of the auditory ossicles is to address the impedance difference between the air in the middle ear and the fluid in the inner ear. The ossicles amplify sound pressure to prevent energy from being lost during this transfer. The vibratory forces are increased by about ten times from the eardrum to the base of the stapes.
This amplification is achieved through two main mechanisms. The first is the lever action of the ossicles. The second, and more significant, is the size difference between the large eardrum and the much smaller stapes footplate, which concentrates the vibrational energy.
Muscles of the Middle Ear and Ossicular Protection
Two small skeletal muscles are associated with the auditory ossicles and play a protective role in hearing. The tensor tympani muscle is connected to the malleus. When it contracts, it pulls the malleus medially, increasing the eardrum’s tension and dampening vibrations through the ossicular chain.
The stapedius muscle, the smallest skeletal muscle in the body, is attached to the stapes. Its contraction pulls the stapes posteriorly, restricting its movement and reducing the range of its oscillations. Both muscles contract reflexively in response to loud noises.
This acoustic reflex, also known as the middle ear reflex, protects the inner ear from damage caused by high-intensity sounds. By stiffening the ossicular chain, these muscles reduce the sound energy that reaches the inner ear. This mechanism can also reduce the intensity of self-generated sounds, such as chewing.
Common Auditory Ossicle Conditions
Several medical conditions can affect the auditory ossicles, leading to conductive hearing loss—a problem with the mechanical transmission of sound. One condition is otosclerosis, a disorder of abnormal bone growth. This growth often affects the stapes, impeding its movement and ability to transmit vibrations.
Another issue is ossicular chain discontinuity, a break or separation in the connection between the ossicles. This can be caused by head trauma or chronic ear infections, which disrupts the pathway for sound vibrations and results in hearing impairment.
Congenital malformations of the ossicles can also cause hearing loss. In some cases, the bones may be improperly formed or fused together, preventing them from functioning correctly.