Auditory ossicles
Auditory ossicles is a collective term for the three smallest bones in the human body connected by joints, which enable sound to be transmitted to the inner ear in the tympanic cavity.
Definition
The auditory ossicles are also known in medicine as ossicula auditiva, ossicula auditus or ossicula auditoria. It is a collective term for the bones stapes (stirrups), malleus (hammer) and incus (anvil).
Anatomy
With a total weight of approx. 53 mg, the auditory ossicles are the smallest bones in the human body. Surrounded by a mucous membrane in the middle ear , the stapes, malleus and incus are connected by two joints.
Starting from the eardrum, mechanical vibrations are passed on to the inner ear via the bones. The malleus is fused with the tympanic membrane on its stem. The tensor tympani muscle connects the malleus and the eardrum. Its tension prevents excessive movement of the bones, such as when sneezing.
The head of the hammer is connected to the incus by the hammer-anvil joint. The lenticular process, a narrow web of bone and part of the incus, is linked to the stapes by the incus-stapes joint.
The stirrup, which weighs around 3 mg and is the smallest of the three miniature bones, is named for its special shape. It consists of a foot or base plate, from which two legs lead to the neck, which ends with the stirrup head. The bottom plate is connected to the oval window and transmits the vibrations to the fluid-filled cochlea behind the window.
Function
The chain of bones of malleus, incus and stapes transmits the sound coming from the eardrum to the oval window. In the process, the acoustic vibrations are first converted into mechanical ones at the eardrum. The mechanical impedance from the eardrum to the inner ear roughly corresponds to the acoustic impedance of the auditory canal.
The Fenestra ovales connects the timpani with the labyrinth forecourt. The footplate of the stapes usually occludes the oval window. Due to the vibrations triggered by the sound, the footplate vibrates with it, so the sound waves are transmitted and amplified about 30 times.
The auditory ossicles, which lie between the low mechanical impedance at the eardrum and the high one at the inner ear, act as a kind of transformer. At the same time, it is the task of the auditory ossicles to protect the inner ear from excessively loud sound impressions.
Diseases
A typical disease of the auditory ossicles is otosclerosis. This leads to a hardening and remodeling of the auditory ossicles and as a result to a firm attachment of the base plate of the stapes to the oval window.
The hardening of the connection causes conductive hearing loss. The sound waves can no longer be fully transmitted mechanically by the eardrum. Areas within the cochlea can also be affected by the condition, which then also leads to hearing loss in the inner ear.
Although the condition often affects both ears, the first symptoms are usually unilateral hearing loss. The cause of the disease is not yet known, but statistically it affects women more often than men. Treatment is surgical. In the advanced stage, parts of the auditory ossicles have to be removed and replaced with artificial building blocks.
Another disease affecting the ossicles is cholesteatoma. This is an overgrowth of squamous epithelial cells , cells of the superficial layer of skin, that proliferate from the outer ear canal into the middle ear. This prevents fluid drainage from the middle ear.
The existing structures of malleus, incus and stapes can be severely disturbed by the skin growths. Chronic otitis media develops . In this case, only surgical intervention can help, accompanied by antibiotic therapy .
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