Sound waves enter the external auditory meatus. 7.
Waves of changing pressures cause the tympanic membrane to reproduce the vibrations coming from the sound wave 8.
Auditory ossicles amplify and transmit vibrations to the end 9.
of the stapes.
Movement of the stapes at the oval window transmits 10.
vibrations to the perilymph in the scala vestibuli.
Vibrations pass through the vestibular membrane and enter 11.
the endolymph of the cochlear duct.
Different frequencies of vibration in endolymph move specific regions of the basilar membrane, thus stimulating specific sets of receptor cells.
A receptor cell becomes depolarized; its membrane becomes more permeable to calcium ions.
In the presence of calcium ions, vesicles at the base of the receptor cell release neurotransmitter.
Neurotransmitter stimulates the ends of nearby sensory neurons.
Sensory impulses are triggered on fibers of the cochlear branch of the vestibulocochlear nerve. The auditory cortex of the temporal lobe interprets the sensory impulses.
Several factors can impair hearing, including interference with transmission of vibrations to the inner ear (conductive deafness) or damage to the cochlea or the auditory nerve and its pathways (sensorineural deafness). Disease, injury, and heredity all can impair hearing. There are more than 100 forms of inherited deafness, many of which are part of other syndromes. About 8% of people have hearing loss.
About 95% of cases of hearing loss are conductive. One cause is accumulated dry wax or a foreign object in the ear, which plugs the auditory meatus. Changes in the tympanic membrane or auditory ossicles can also block hearing. The tympanic membrane may harden as a result of disease, becoming less responsive to sound waves, or an injury may tear or perforate it.
A common disorder of the auditory ossicles is otosclerosis, in which new bone is deposited abnormally around the base of the stapes. This interferes with the ossicles' movement, which is necessary to transmit vibrations to the inner ear. Surgery often can restore some hearing to a person with otosclerosis by chipping away the bone that holds the stapes in position, or replacing the stapes with a wire or plastic substitute.
Two tests used to diagnose conductive deafness are the Weber test and the Rinne test. In the Weber test, the handle of a vibrating tuning fork is pressed against the forehead. A person with normal hearing perceives the sound coming from directly in front, whereas a person with sound conduction blockage in one middle ear hears the sound coming from the impaired side.
In the Rinne test, a vibrating tuning fork is held against the bone behind the ear. After the sound is no longer heard by conduction through the bones of the skull, the fork is moved to just in front of the external auditory meatus. In middle ear conductive deafness, the vibrating fork can no longer be heard, but a normal ear will continue to hear its tone.
Very loud sounds can cause sensorineural deafness. If exposure is brief, hearing loss may be temporary, but when exposure is repeated and prolonged, such as occurs in foundries, near jackhammers, or on a firing range, impairment may be permanent. Pete Townshend, of the rock group The Who, suffers from hearing loss from many years of performing loud concerts. Former President Clinton was diagnosed with hearing loss, which often becomes noticeable in one's fifties. Other causes of sensorineural deafness include tumors in the central nervous system, brain damage as a result of vascular accidents, and the use of certain drugs.
Because hearing loss and other ear problems can begin gradually, it is important to be aware of their signs, which may include the following:
• difficulty hearing people talking softly
• inability to understand speech when there is background noise
New parents should notice whether their infant responds to sounds in a way that indicates normal hearing. Before 1993, 50% of hearing impaired infants were not diagnosed until age two. Since then, the federal government has advised hearing exams as part of a wellbaby visit to a doctor. If the baby's responses indicate a possible problem, the next step is to see an audiologist, who identifies and measures hearing loss.
Often a hearing aid can help people with conductive hearing loss. A hearing aid has a tiny microphone that picks up sound waves and converts them to electrical signals, which are then amplified so that the person can hear them. An ear mold holds the device in place, either behind the outer ear, in the outer ear, or in the ear canal.
A cochlear implant enables people with sensorineural hearing loss to detect some sounds, although it usually remains difficult to discern distinct words. The device converts sound waves to electrical signals, which stimulate neurons in the cochlea. ■
The saccule and utricle, which are expanded portions of the membranous labyrinth, are located within the bony chamber of the vestibule. (Compare with figure 12.12.)
movement, and the hairs embedded in it also bend. This bending of the hairs stimulates the hair cells to signal their associated nerve fibers, and, as a result, impulses travel to the brain (fig. 12.22).
Parts of the cerebellum are particularly important in interpreting impulses from the semicircular canals. Analysis of such information allows the brain to predict the consequences of rapid body movements, and by modifying signals to appropriate skeletal muscles, the cerebellum can maintain balance.
Other sensory structures aid in maintaining equilibrium. Various proprioceptors, particularly those associated with the joints of the neck, inform the brain about the position of body parts. The eyes detect changes in posture that result from body movements. Such visual information is so important that even if the organs of equilibrium are damaged, keeping the eyes open and moving slowly is sufficient to maintain normal balance.
99 Distinguish between the senses of static and dynamic equilibrium.
^9 Which structures provide the sense of static equilibrium? Of dynamic equilibrium?
^9 How does sensory information from other receptors help maintain equilibrium?
Motion sickness is a disturbance of the inner ear's sensation of balance. Nine out of ten people have experienced this nausea and vomiting, usually when riding in a car or on a boat. Astronauts began reporting a form of motion sickness called space adaptation syndrome in 1968, when spacecraft were made roomy enough for astronauts to move about while in flight.
Although the cause of motion sickness is not known, one theory is that it results when visual information contradicts the inner ear's sensation that one is motionless. Consider a woman riding in a car. Her inner ears tell her that she is not moving, but the passing scenery tells her eyes that she is moving. The problem is compounded if she tries to read. The brain reacts to these seemingly contradictory sensations by signaling a "vomiting center" in the medulla oblongata.
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