Hearing Impairments

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There are two major categories of deafness: (1) conduction deafness, in which the transmission of sound waves through the middle ear to the oval window is impaired, and (2) sensorineural, or perceptive, deafness, in which the transmission of nerve impulses anywhere from the cochlea to the auditory cortex is impaired. Conduction deafness can be caused by middle-ear damage from otitis media or otosclerosis (discussed in the previous clinical applications box, p. 257). Sensorineural deafness may result from a wide variety of pathological processes and from exposure to extremely loud sounds. Unfortunately, the hair cells in the inner ears of mammals cannot regenerate once they are destroyed. Experiments have shown, however, that the hair cells of reptiles and birds can regenerate by cell division when they are damaged. Scientists are currently trying to determine if mammalian sensory hair cells might be made to respond in a similar fashion.

Conduction deafness impairs hearing at all sound frequencies. Sensorineural deafness, by contrast, often impairs the ability to hear some pitches more than others. This may be due to

Inferior colliculus

Midbrain

Medulla oblongata

Inferior colliculus

Midbrain

Medulla oblongata

Physiology Thalamus

-Thalamus

Auditory cortex (temporal lobe)

Medial geniculate body of thalamus

Cochlear nucleus

Vestibulocochlear nerve

From spiral organ (of Corti)

-Thalamus

Auditory cortex (temporal lobe)

Medial geniculate body of thalamus

Cochlear nucleus

Vestibulocochlear nerve

From spiral organ (of Corti)

■ Figure 10.23 Neural pathways for hearing. These pathways extend from the spiral organ in the cochlea to the auditory cortex.

Cerebral cortex

Correspondence between the cochlea and the acoustic area of the cortex: Blue—low tones Red—medium tones Yellow—high tones

Auditory Cortex Pitch Tone Frequency
Cochlea

■ Figure 10.24 Correlation between pitch location in the cochlea and auditory cortex. Sounds of different frequencies (pitches) cause vibration of different parts of the basilar membrane, exciting different sensory neurons in the cochlea. These in turn send their input to different regions of the auditory cortex.

pathological processes or to changes that occur during aging. Age-related hearing impairment—called presbycusis—begins after age 20 when the ability to hear high frequencies (18,000 to 20,000 Hz) diminishes. Men are affected to a greater degree than women, and although the progression is variable, the deficits may gradually extend into the 4,000-to-8,000-Hz range. These impairments can be detected by audiometry, a technique in which the threshold intensity of different pitches is determined. The ability to hear speech is particularly affected by hearing loss in the higher frequencies.

People with conduction deafness can be helped by hearing aids—devices that amplify sounds and conduct the sound waves through bone to the inner ear. People with sensorineural deafness sometimes choose to have cochlear implants, which electrically stimulate the fibers of the vestibulocochlear nerve in response to sounds. Experiments with animals suggest that such devices produce a reorganization of the auditory cortex, demonstrating a plasticity similar to that described previously in the somatosensory cortex (the postcentral gyrus) in people with amputated limbs.

Test Yourself Before You Continue

1. Use a flowchart to describe how sound waves in air within the external auditory meatus are transduced into movements of the basilar membrane.

2. Explain how movements of the basilar membrane affect hair cells, and how hair cells can stimulate associated sensory neurons.

3. Explain how sounds of different intensities affect the function of the cochlea. How are different pitches of sounds distinguished by the cochlea?

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Responses

  • ELIISA
    How is the organ of corti affected by the different pitches of sound?
    8 years ago

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