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Anatomy and Physiology of Middle Ear

ANATOMY OF THE MIDDLE EAR

The middle ear or tympanic cavity is an irregular, laterally compressed space within the temporal bone. It is filled with air, which is conveyed to it from the nasal part of the pharynx through the auditory tube. It contains a chain of movable bones, which connect its lateral to its medial wall, and serve to convey the vibrations communicated to the tympanic membrane across the cavity to the internal ear.

Middle ear

The middle ear is connected by the Eustachian tube to the nasopharynx and is continuous with air-filled cells in the adjacent mastoid portion of the temporal bone. The eustachian tube is approximately 1 mm wide and 35 mm long. Normally, the eustachian tube is closed, but it opens by action of the tensor veli palatini muscle when performing a Valsalva maneuver or when yawning or swallowing. The tube serves as a drainage channel for normal and abnormal secretions of the middle ear and equalizes pressure in the middle ear with that of the atmosphere.

Tympanic Membrane

The tympanic membrane (ie, eardrum), about 1 cm in diameter and very thin, is normally pearly gray and translucent. The tympanic membrane consists of three layers of tissue: an outer layer, continuous with the skin of the ear canal; a fibrous middle layer; and an inner mucosal layer, continuous with the lining of the middle ear cavity. Approximately 80% of the tympanic membrane is composed of all three layers and is called the parstensa. The other 20% of the tympanic membrane lacks the middle layer and is called the pars flaccida. The absence of this fibrous middle layer makes the pars flaccida more vulnerable to pathologic disorders than the pars tensa. Distinguishing landmarks of the tympanic membrane include the annulus, the fibrous border that attaches the eardrum to the temporal bone; the short process of the malleus; the long process of the malleus; the umbo of the malleus, which attaches to the tympanic membrane in the center; the pars flaccida; and the pars tensa. The tympanic membrane protects the middle ear and conducts sound vibrations from the external canal to the ossicles. The sound pressure is magnified 22 times as a result of transmission from a larger area to a smaller one.

Ossicles

The ossicles (also called auditory ossicles) are the three smallest bones in the human body. They are contained within the middle ear space and serve to transmit sounds from the air to the fluid-filled labyrinth (cochlea ). The term “ossicles” literally means “tiny bones” and commonly refers to the auditory ossicles, though the term may refer to any small bone throughout the body.

The malleus (hammer) articulates with the incus and is attached to the tympanic membrane , from which vibrational sound pressure motion is passed.
The incus (anvil) is connected to both the other bones.
The stapes (stirrup) articulates with the incus and is attached to the membrane of the fenestra ovalis , the elliptical or oval window or opening between the middle ear and the vestibule of the inner ear . The footplate of the stapes sits in the oval window, secured by a fibrous annulus, or ring-shaped structure. The footplate transmits sound to the inner ear. The round window, covered by a thin membrane, provides an exit for sound vibrations.

PHYSIOLOGY OF THE MIDDLE EAR

  1. Transmit sound: As a result of sounds conveyed by the ear canal, the tympanic membrane is thrown into tiny vibrations. The middle ear will transmit the disturbance caused by the movement of the tympanic membrane to the inner ear.
  2. Amplify sound-wave: The middle ear amplifies the pressure of the sound waves which is transmits, so that the inner ear can be stimulated in spite of its higher impedance. The mechanisms to amplify the sound waves is the lever action of the ossicles and the curved membrane mechanism.
  3. Protects the ear: The middle ear play an important role in protecting the ear from the damaging effects of very loud sounds, and in protecting our mind from the disturbing effects of our own voice, with the help of the tensor tympani and stapedius muscles.