Term
| What are the major features of the medial and lateral ‘walls’ of the middle ear? |
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Definition
| The lateral wall is made up primarily of the tympanic membrane and the structures attached to it such as the chorda tympani nerve. The medial wall is composed primarily of the promontory, which is covered by fibers of the tympanic plexus. It also has two small openings, the oval window where the footplate of the stapes is inserted, and the round window sealed with a thin membrane. The promontory is the bulge formed by the first turn of the cochlea of the inner ear. |
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Term
| In which bone is the middle ear located? |
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Definition
| The petrous portion of the temporal bone. |
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Term
| What are the mastoid air cells and why they are important? |
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Definition
| The mastoid air cells are airspaces lined with mucosa that occupy the mastoid process of the temporal bone. They are connected with each other and are continuous with the middle ear cavity at the aditus to the mastoid antrum. The antrum is an enlarged space into which the air cells open. The cells are important since a middle ear infection (otitis media) may spread to these spaces where the infection can proliferate and access is limited. They can thus act as a reservoir of infection. |
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Term
| What is the roof of the tympanic cavity and what structure is close to it? |
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Definition
| The roof of the tympanic cavity is known as the tegmen tympani. It is relatively thin and is adjacent to the dural lining of the cranial cavity. Infection can erode through the tegmen and then can spread to and through the meningial lining causing meningitis. |
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Term
| What are the openings into the middle ear cavity? |
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Definition
| The openings to the cavity are the auditory or pharyngotympanic tube anteriorly and the aditus to the mastoid antrum posteriorly. |
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Term
| What are the ossicles and how do they function? |
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Definition
| The ossicles are the malleus, incus, and stapes. They articulate with one another, beginning with the malleus whose handle is attached to the tympanic membrane. The head of the malleus articulates with the incus, which in turn articulates with the stapes that is inserted into the oval window of the medial wall and transmits movement to the cochlea of the inner ear. |
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Term
| What muscles dampen movement of the ossicles? Why is that important? |
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Definition
| The tensor tympani is attached by a tendon to the handle of the malleus. It is innervated by the mandibular division of the trigeminal nerve (V3). The tendon of the stapedius extends out of the apex of the pyramid which enclosed the stapedius muscle and attaches to the stapes. It is innervated by a small branch of the facial nerve (VII). When these muscles contract they reduce the movement of the ossicles and thus reduce the amplitude of transmission and dampen the sound transmitted to the inner ear. This is a reflex function that helps protect the auditory apparatus from excessive noise and subsequent damage. |
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Term
| What is the function of the tympanic membrane, what structures cross it? How does the membrane change with a middle ear infection? |
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Definition
| The tympanic membrane vibrates with the sound pressure waves that hit it. It transmits that movement into movement of the ossicles, which convey the sound energy to the inner ear where it is translated. The chorda tympani nerve from the facial nerve (VII) crosses the upper inner surface of the tympanic membrane. Damage to this portion of the middle ear can damage the nerve, which interferes with the taste function of the anterior 2/3 of the tongue. During middle ear infections (otitis media) the tympanic membrane will become reddened and the presence of fluid in the tympanic cavity behind the membrane will cause it to change shape. This causes a change in the light reflective character of the membrane and results in the no longer being able to see the ‘cone of light’ present on a normal membrane. This is accompanied by pain or discomfort due to irritation of the sensory nerves of the cavity. |
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Term
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Definition
| The auricle is the external ear structure that functions to collect sound and funnel it into the external meatus where it contacts the tympanic membrane. It is composed of skin covering an elastic cartilage core. |
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Term
| Where is sound converted to neural impulses in the ear? |
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Definition
| Sound is converted into neural impulses within the inner ear. Movement of the footplate of the stapes causes movement of fluid in the turns of the cochlea of the middle ear. This fluid movement is sensed by neural cells within the cochlear duct that then transmit signals to the brain via the vestibulocochlear nerve (CN VIII). |
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Term
| What are the main functions of the inner ear? |
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Definition
| The main functions of the inner ear are hearing, detection of the position of the head in space and balance (semicircular canals), and movement, i.e., acceleration and deceleration. |
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Term
| What is otitis media? What are the two types of hearing loss? |
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Definition
| Otitis media is an inflammation and accumulation of fluid within the tympanic cavity caused in many cases by the closure or swelling of the musosa of the auditory tube. Two type of hearing loss are conductive hearing loss, that is, loss due to impaired movement of the ossicles or damage to the tympanic membrane and thus interference with transmission of the sound energy to the inner ear. The other is sensorineural loss that in due to problems within the cochlea, the nerve cells there, or within the brain. |
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