Auditory Ossicles · What Are They, Location, Function, and More

The auditory ossicles—malleus, incus, and stapes—are three small bones in the middle ear that transmit air vibrations from the outer ear into the inner ear, so they can be processed as sound. They are named after their resemblance to a hammer, anvil, and stirrup, respectively.

The auditory ossicles are located in the middle ear, which is the portion of the ear between the external part of the ear, formed by the auricle, or pinna, and the ear canal; and the inner ear, which contains the cochlea, and semicircular canals. The cochlea is a special structure that converts sound waves into electrical impulses for the brain, and the semicircular canals help with balance. 

The middle ear is an air-filled cavity inside the temporal bone of the skull that is shaped like a small chamber with four walls, a floor, and a roof. The tympanic membrane, or eardrum, makes up the lateral wall of this cavity, anVAL WINd separates the middle ear from the external ear. The medial or labyrinthine wall separates the middle ear from the inner ear. The labyrinthine wall has two openings: an oval window located superiorly and a round window located inferiorly. The two other walls of the middle ear are the posterior wall, towards the back of the head, and the anterior wall, towards the front. The posterior wall has an opening called the mastoid antrum, which connects the middle ear with the mastoid cavity within the temporal bone. The anterior wall has an opening for the eustachian tube, which connects the middle ear to the nasopharynx.

The auditory ossicles are arranged from the tympanic membrane to the oval window, forming an ossicular chain that transfers sound waves from the eardrum to the inner ear. The malleus is the largest and most lateral of the auditory ossicles. The handle of the malleus rests on the internal surface of the eardrum, where it causes a central depression called the umbo of the tympanic membrane. The incus is found in the middle of the three ossicles, attached to the innermost bone, called the stapes. The body of the incus articulates with the head of the malleus by the incudomalleolar joint; another projection of the incus, called the lenticular process, articulates with the head of the stapes by the incudostapedial joint. The stapes is not only the smallest of the auditory ossicles but also the smallest bone in the body. Its foot attaches to the oval window, which connects to the fluid-filled cochlea of the inner ear.

The function of the auditory ossicles is to transfer and amplify air vibrations into the inner ear to be processed as sound. Sound waves travel through the air to the external auditory canal, causing the eardrum to vibrate. These vibrations are transmitted from the head of the malleus to the incus and stapes, and finally to the oval window, which transfers the vibrations to the inner ear. Since the oval window is around 20 times smaller than the eardrum, sound waves are amplified as they vibrate their way across the auditory ossicular chain. In the inner ear, the motion of fluid within the fluid-filled cochlea converts sound waves to electrical impulses, to be sent to the brain via the vestibulocochlear nerve. 

Air vibrations made by loud noises can easily damage the inner ear, which is why there are two muscles that dampen the movement of the auditory ossicles: the tensor tympani and the stapedius. The tensor tympani muscle pulls on the malleus and tenses the tympanic membrane to reduce the amplitude of its oscillations; whereas the stapedius inserts on the stapes and pulls it posteriorly reducing the amplitude of sound waves transmitted to the oval window. 

The auditory muscles can be damaged due to a variety of causes, both directly and indirectly. For example, severe head injuries, blast injuries, and sudden changes in barometric pressure (i.e., barotrauma) can damage the auditory ossicles, thereby leading to an ossicular fracture or ossicular chain dislocation. These conditions are often managed with ossicular chain reconstructive surgery. 

A tympanic perforation may reduce the vibrational impulses transmitted from the tympanic membrane to the auditory ossicles; middle ear infections, like otitis media, can cause fluid build-up that results in restricted auditory bone movement, thereby reducing the amplitude of sound waves transmitted to the inner ear. Both conditions may result in conductive hearing loss and are often treated with topical medications as well as antibiotic therapy as necessary. 

Otosclerosis is a condition that involves increased bone growth in the middle ear, resulting in the fusion of the auditory ossicles and progressive hearing loss. While its cause is currently unknown, it may involve a genetic predisposition. Treatment is often required in late stages and generally involves hearing aids and ossicular surgery. Genetic mutations interfering with the fetal development of auditory ossicles may also result in the partial or total absence of one or more ear bones. 

Finally, hyperacusis refers to an increased sensitivity to sound, where normal noises appear painfully loud. Usually, the stapedius muscle, which is innervated by the facial nerve, helps dampen the excessive vibrations caused by loud noises. With damage to the facial nerve, the stapedius muscle is unable to contract and properly dampen the oscillations of the ossicles, thereby resulting in the perception of unusually loud sounds on the affected side.

The auditory ossicles—malleus, incus, and stapes—are three small bones in the middle ear that transmit air vibrations from the outer ear to the inner ear to be processed as sound. Sound waves enter through the outer ear and cause eardrum vibrations. These vibrations are amplified through the auditory ossicles of the middle ear, finally reaching the inner ear. In the inner ear, the fluid-filled cochlea converts sound waves to electrical impulses which are sent to the brain.