Middle Ear

Synonyms

Latin: auris media

Introduction

The middle ear is an air-filled space lined with mucosa and located in the petrous bone of the skull. This is where the ossicles are located, through which sound or the vibrational energy of sound is transmitted from the external auditory canal via the eardrum and finally to the inner ear. The middle ear is anatomically made up of the ossicular chain consisting of the hammer (lat.

Malleus), anvil (lat. Incus) and stapes (lat. Stapes).

They are articulated with each other. The malleus is adjacent to the eardrum (Membrana tympani), which is the boundary between the outer ear and the middle ear. The malleus is followed by the anvil, which in turn is connected to the stapes in the middle ear.

The latter ends with its stirrup footplate at the oval window (fenestra vestibuli). The auditory ossicles in the middle ear are the smallest boils in the human body and, in addition to sound transmission, also have the function of amplifying sound 1.3 times. This is achieved by the leverage effect of the ossicles.

Overall, the movement of the ankle chain is a pendulum movement and its mobility is influenced by two muscles: Musculus tensor tympani (“tympanic tympanic muscle”) and Musculus stapedius (attaches to the stirrup). Both muscles reduce the sound transmission in the case of loud sound stimuli and thus fulfil a certain protective function. Contraction of the tensor tympani muscle causes the eardrum in the middle ear to tense; contraction of the stapedius muscle stiffens the sound conduction chain and reduces sound transmission to the inner ear.

This filter function is said to be particularly important for high-pitched tones (“high-pass filter”). The tympanic cavity in the middle ear is bounded by several walls. The lateral wall (paries membranaceus) represents the boundary to the outer ear.

It is mainly formed by the eardrum. The inner wall (Paries labyrinthicus) is the border to the inner ear. Here, a prominence is particularly noticeable; the so-called promontorium.

It is the basal cochlear coil of the inner ear. The lower wall (Paries jugularis) forms the floor of the tympanic cavity. Via the rear wall in the middle ear (Paries mastoideus) one reaches further air-filled cells (Cellulae mastoideae) of the petrous bone via a passage.

This is where an inflammation of the middle ear can spread, since there is a direct connection. The roof of the tympanic cavity limits the upper wall (Paries tegmentalis). Another important opening or connection of the middle ear contains the front wall (Paries caroticus) – the ear trumpet opening.

The ear trumpet (Tuba auditiva) in the middle ear creates an open connection between the middle ear and the throat. It consists of one third bony material and two thirds cartilaginous material. The corpulent part follows the bony part located in the petrous bone and widens towards the throat like a trumpet.

The tube guarantees constant ventilation of the middle ear and opens with each swallowing. This results in a pressure equalization between the air pressure in the middle ear and the environment. For this reason, it is often advisable to suck sweets or swallow frequently during flights to avoid “pressure on the ears”.

As an additional protective measure, the Eustachian tube has a special surface with cilia, which are intended to keep germs away from the middle ear by striking in the direction of the throat. If this system fails, it can lead to ascending middle ear inflammation caused by bacteria. Neighborly relations are of clinical importance, especially in diseases of the middle ear, since from here a severe purulent inflammation can spread to adjacent rooms.

This can result in meningitis, brain abscesses, inflammation of the mastoid process of the petrous bone (mastoiditis), visual disturbances, and paralysis of the facial muscles. Another anatomically important structure runs directly through the middle ear, protected only by a mucosal fold. It is a small nerve (Chorda tympani), which is responsible for the sensation of taste.

This nerve can be affected in middle ear infections. Affected persons report a disturbance of taste and reduced salivation. The most important task of the middle ear, apart from “simple” sound transmission, is the so-called sound wave resistance adjustment (impedance).

The incoming sound reaches the eardrum via the external auditory canal.If the fluid-filled inner ear were directly connected, about 99% of the sound waves would be reflected, since the acoustic impedance between air and inner ear fluid is too high. This problem is circumvented with the help of the middle ear. The sound energy is effectively transferred to the oval window via the hammer, anvil and stapes.

Two mechanisms for impedance matching are important. First, as mentioned above, the ossicles cause an increase in pressure at the oval window due to different lever arms. However, the second effect takes over the much larger part of the impedance matching process.

The principle here is the area effect between eardrum and oval window. Since the eardrum is approximately 17 times larger than the oval window, an equal force must be distributed over a smaller area. This results in an enormous sound pressure increase by a factor of 30. Overall, the middle ear and its impedance matching reduces the sound reflection to 35%, which results in an increase in hearing of 10-20 decibels (dB) depending on the frequency.