Middle Ear Blood Vessels

The tympanic membane is thought of as the boundary between the outer ear and the middle ear, but it is technically part of the middle ear. The middle ear includes the tympanic membrane, the tympanic cavity, and three small bones called auditory ossicles.

The tympanic cavity is an air-filled space in the temporal bone that separates the external and internal ears. The tympanic membrane is a semitransparent membrane covered by a thin layer of skin on its outer surface and by mucous membrane on the inside. It has an oval margin and is cone-shaped, with the apex of the cone directed inward. One of the auditory ossicles (malleus) maintains its cone shape.

The three auditory ossicles, called the malleus, the incus, and the stapes, are attached to the wall of the tympanic cavity by tiny ligaments and are covered by mucous membrane. These bones bridge the tympanic membrane and the inner ear, transmitting vibrations between these parts. Specifically, the malleus is attached to the tympanic membrane, and when the tympanic membrane vibrates, the malleus vibrates in unison with it. The malleus vibrates the incus, and the incus passes the movement on to the stapes. Ligaments hold the stapes to an opening in the wall of the tympanic cavity called the oval window. Vibration of the stapes, which acts like a piston at the oval window, moves a fluid within the inner ear. These vibrations of the fluid stimulate the hearing receptors (fig. 12.10).

In addition to transmitting vibrations, the auditory ossicles form a lever system that helps increase (amplify) the force of the vibrations as they pass from the tympanic membrane to the oval window. Also, because the ossicles transmit vibrations from the large surface of the tympanic membrane to a much smaller area at the oval window, the vibrational force concentrates as it travels from the external to the inner ear. As a result of these two factors, the pressure (per square millimeter) that the stapes applies at the oval window is about

Figure 12.10

Major parts of the ear.

Semicircular canals

- Cochlea

Vestibulocochlear nerve

Semicircular canals

- Cochlea

Conducto Auditivo Externo Netter

Oval window (under stapes)

j^'-i-Round window

.-Tympanic cavity

Oval window (under stapes)

j^'-i-Round window

.-Tympanic cavity

Temporal bone

Malleus

Incus

Temporal bone

Malleus

Incus

Tympanic membrane

Tympanic cavity

Major Parts The Middle Ear

Auditory tube

Figure

Two small muscles attached to the (a) malleus and (b) stapes, the tensor tympani and the stapedius, are effectors in the tympanic reflex. Figure 12.10 does not show these muscles.

Tendon of stapedius m.

Stapes

Tympanic membrane

Tympanic cavity

Auditory tube

Figure

Two small muscles attached to the (a) malleus and (b) stapes, the tensor tympani and the stapedius, are effectors in the tympanic reflex. Figure 12.10 does not show these muscles.

twenty-two times greater than that which sound waves exert on the tympanic membrane.

The middle ear also contains two small skeletal muscles that are attached to the auditory ossicles and are controlled involuntarily. One of them, the tensor tympani, is inserted on the medial surface of the malleus and is anchored to the cartilaginous wall of the auditory tube. When it contracts, it pulls the malleus inward. The other muscle, the stapedius, is attached to the posterior side of the stapes and the inner wall of the tympanic cavity. It pulls the stapes outward (fig. 12.11). These muscles are the effectors in the tympanic reflex, which is elicited in about one-tenth second following a long, external sound. When the reflex occurs, the muscles contract, and the malleus and stapes move. As a result, the bridge of ossicles in the middle ear becomes more rigid, reducing its effectiveness in transmitting vibrations to the inner ear.

The tympanic reflex reduces pressure from loud sounds that might otherwise damage the hearing receptors. The tympanic reflex is also elicited by ordinary vocal sounds, as when a person speaks or sings, and this action muffles the lower frequencies of such sounds, improving the hearing of higher frequencies, which are common in human vocal sounds. In addition, the tensor tympani muscle also steadily pulls on the tympanic membrane. This is important because a loose tympanic membrane would not be able to transmit vibrations effectively to the auditory ossicles.

The muscles of the middle ear take 100 to 200 milliseconds to contract. For this reason, the tympanic reflex cannot protect the hearing receptors from the effects of loud sounds that occur very rapidly, such as those from an explosion or a gunshot. On the other hand, this protective mechanism can reduce the effects of intense sounds that arise slowly, such as the roar of thunder.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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