MIDDLE EAR: ANATOMY (PARTS), FUNCTIONS, DISEASES - ANATOMY AND PHYSIOLOGY - 2025

The middle ear is one of the three regions that make up the hearing organ of many animals. It has a special function in amplifying the vibrations that accompany the sounds we hear.

In humans, the organ of hearing is made up of three parts or regions that are known as the outer ear, the middle ear, and the inner ear; each with special features and functions.

Anatomical scheme of the middle ear (Source: BruceBlaus via Wikimedia Commons)

The external ear corresponds to the pinna or ear, an internal canal called the external auditory meatus and the eardrum, which is a membrane that covers the final portion of the meatus. This region is responsible for receiving sound waves and converting them into mechanical vibrations.

The middle ear, also called the “tympanic cavity,” is made up of a chain of small bones (ossicles) and forms an air-filled cavity.

Finally, the inner ear is made up of a cavity known as the "bony labyrinth", within which a "membranous labyrinth" is suspended. This portion of the organ of hearing receives vibrations from the middle ear and transfers them to a liquid inside.

Not only does hearing depend on the inner ear, but it also controls balance; This last portion is in charge of the transmission of sensory impulses to the brain.

Anatomy (parts)

The middle ear is an air-filled cavity that is lined by a mucous membrane and contains three small bones known as the hammer, anvil, and stapes. This region of the ear is connected to the pharynx through the auditory tube, Eustachian tube or pharyngotympanic tube.

Tympanic cavity or middle ear

The space comprised by the middle ear, also known as the tympanic cavity, is lined by an epithelium that continues with the inner lining of the tympanic membrane until it reaches the auditory tube, where the bone in the cavity changes to cartilage.

In the bony portion of this cavity there are no associated glands, otherwise the cartilaginous portion, where there are multiple mucous glands that open into the middle ear cavity.

It can be said that one of the edges of the tympanic cavity is made up of the eardrum or the tympanic membrane, which is the only membrane in the human body that is exposed to air on both sides.

This membrane is composed of 3 layers, the middle layer being a layer rich in collagen fibers, thus providing mechanical stability and firmness to the membrane. The eardrum is essential for the acoustic functions of the middle ear.

Mucosa associated with the middle ear

Many authors consider that the epithelium associated with the tympanic cavity consists of cells of cuboidal or flattened morphology, lacking cilia and mucus-secreting elements.

However, much of the literature indicates that some hair cells can be found in the inner lining of the middle ear, although not in its entirety, but in some defined regions, together with mucus-secreting structures.

These hair cells in the tympanic cavity are involved in many processes related to middle ear health and disease.

Eustachian tube

The Eustachian tube is normally a closed tube, but it has the ability to equalize the pressures between the middle ear and the outer space. In addition, it is internally covered by a ciliated epithelium, whose movement is directed from the cavity of the middle ear towards the pharynx.

Several small muscles from the upper pharynx are attached to the Eustachian tube, which dilate when swallowing, which explains why the action of swallowing or opening the mouth helps us equalize the pressure in the ear in different situations.

Chain of ossicles of the middle ear

Hammer, anvil and stapes are the names of the three ossicles that form a kind of interconnected chain in the cavity of the middle ear. The movement of these ossicles is one of the events that allows the “translation” of sound waves into vibrations in the hearing process.

Graphic representation of the middle ear ossicles chain (Source: Modified from Marc Giacone via Wikimedia Commons)

The chain formed by these three ossicles is housed in part of the space formed by the tympanic cavity (middle ear) and they are located throughout the distance between the tympanic membrane and the membrane of the oval window, which is part of the cochlea of ​​the ear. internal.

The tympanic membrane is attached to the ossicle known as the hammer, to which the incus joins, ending the chain with the stapes, which firmly connects it with the membrane of the oval window in the cochlea.

The chain formed by these three ossicles is covered by a simple squamous epithelium and to these ossicles are associated two small skeletal muscles called the tensor tympani and the stapedium.

How is vibrational "translation" produced?

The tensor tympani and stapedium muscle participate in both the movement of the tympanic membrane and the hammer, incus, and stapes. The tensor tympani is attached to the "handle" of the hammer, while the stapedium is attached to the stapes.

When a sound wave is perceived by the eardrum, these vibrations pass from the tympanic membrane to the ossicles.

When the vibration reaches the stapes, which is the last bone in the chain, it is transmitted to the membrane of the oval window, in the inner ear, and comes into contact with the liquid medium that is in the cochlear division of this.

The volume displacements produced by the movement of the stirrup in the oval window membrane are compensated by displacements of the same magnitude in said membrane.

Both muscles associated with the ossicles of the middle ear act as "buffers", preventing damage induced by loud noise.

Features

As discussed above, the main function of the middle ear is to convert or "translate" sound waves associated with sounds into physically perceptible mechanical waves or vibrations that can produce movement in the fluid within the inner ear..

Its efficiency in this process is related to the disproportion in the diameters between the tympanic membrane (the eardrum, which is much larger) and the oval window membrane (which is much smaller), which favors the “concentration” of the sound.

Some researchers have shown that, with sounds of low or moderate frequencies, the central portion of the tympanic membrane moves like a rigid body and in the same way the hammer, which is attached to it, moves.

However, said membrane does not move in its entirety, since it has been shown that the edges the net movement is zero.

Faced with high-frequency sound stimuli, the movement of the tympanic membrane is different, since different sections of it vibrate in different phases, which means that the coupling between the membrane and the hammer is not perfect, and that certain acoustic energies that make the eardrum vibrate are not always transmitted to the ossicles.

Diseases

There are some congenital anomalies of the middle ear that occur for one in 3,000-20,000 newborns and that are related to deviations in the anatomical development of the middle ear, as well as its normal function.

These abnormalities are classified as minor (those that only involve the middle ear) and major (those also associated with the tympanic membrane and the outer ear). Also, according to severity, they are classified as mild, moderate, and severe.

Some of these abnormalities are associated with some syndromes such as Treacher Collins, Goldenhar, Klippel-Feil, which have to do with genetic mutations that result in anatomical malformations of the vertebrae, the face, etc.

Minor anomalies

Some of the “minor” defects in the middle ear are related to changes in the configuration or size of the tympanic cavity, as well as changes in the anatomical distances between the main structures of the middle ear: the tympanic membrane, the ossicles or the oval membrane.

Major anomalies

These almost always have to do with the ossicles of the middle ear. Among the most common of all are the poor or defective development of the stapes, its thickening or thinning or its fusion with other bone portions.

Other diseases

Many infectious diseases of the middle ear occur due to damage or interference in the function of the ciliated epithelium in the Eustachian tube, since the sense of ciliary movement functions in the elimination of mucus and pathogens from the middle ear cavity.

The tympanic membrane, as an essential part of the outer and middle ear, can also be the site of formation of a common hearing disease known as cholesteatoma.

Cholesteatoma is an abnormal growth of the skin within the air cavity of the middle ear, behind the plane of the tympanic membrane. Since the skin grows in a different place than its normal place, it "invades" space comprised by the internal mucosa, and this can have serious implications in the stability of the ossicles of the middle ear.

Chronic otitis media is another common pathological condition of the middle ear and has to do with chronic inflammation of the middle ear, which affects the integrity of the hammer, anvil and stapes. It is also related to the tympanic membrane and is thought to occur because it does not heal spontaneously.

References

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