Neuroscience Analysis of Occupational Performance (OCTH 455)
and
Neuroscience (PTRS 850)

Auditory system

Three parts make up the auditory apparatus:
External ear - sound capture (pinna to tympanic membrane)
Middle ear - sound amplification (ossicles)
Inner ear - sound (cochlea) & motion (vestibular apparatus) sensory receptors

Cochlea

Spiral-shaped organ, with three chambers. The middle chamber (scala media) houses the organ of Corti, which contains the:

Properties of sound

Intensity The amplitude of changes in sound pressure. The pressure change moves through the cochlea as a traveling wave. The positions of tectorial & basilar membranes shift positions relative to one another as pressure changes. The cilia of auditory hair cells are stretched as scala media & basilar membrane shift position. The stretching of the stereocilia opens ion channels and activates the receptor.

Frequency The rate of pressure changes. Different auditory hair cell recpetors are sensitive to different frequencies of sound (tone), with higher frequencies sensed by receptors near the oval window & lower frequencies senses by receptors nearer to the apex of the cochlea. Tonotopic organization occurs not only in the auditory receptors, but also in the auditory pathways, auditory nuclei, and auditory cortex.

Central projections

The auditory system is unique with respect to other sensory systems. Because auditory information from the same ear ascends in pathways on both sides of the brain, unilateral loss of ascending information does not result in a total loss of hearing (although localization may be impaired).

Auditory pathways diagram:

Hair cell sensitivity & changing sensory thresholds

Recruitment - louder sounds activate more receptors and produce bigger responses
Mechanical - basilar membrane is stiff near the oval window & more flexible near the apex; high frequencies have more energy; low frequencies travel further

Efferent control

1) Descending control through CN V:
This reflex pathway controls tension of the tensor tympani muscle of middle ear. This muscle attaches to the malleus, and produces a tensing of the tympanic membrane when contracted. This reflex acts to limit the amount of tympanic movement associated with loud, low frequency noise.

2) Descending control through CN VII:
This reflex pathway contracts the stapedius muscle, which is attached to the stapes. This dampens the movement of the stapes, and helps to protect against sudden, loud high frequency noises. Why, then, is ear protection a good idea if you visit a firing range?

3) Descending control of outer hair cells:
Inner hair cells give rise to 90% of the sensory fibers. Outer hair cells are more numerous but only supply 10% of sensory fibers. Outer hair cells modulate the sensitivity of inner hair cells by expanding/contracting the cell body to alter tension on inner hair cells. One viewpoint holds that this process may contribute to auditory figure-ground distinctions:

Thought exercises:

Once you've read through these questions and formulated answers, you can compare your answers to my own attempts at answers; remember that there can be multiple correct answers to some of these questions ....

Deficits associated with the auditory system may lead to a vast range of functional impairments. Describe behavioral, clinical, and neurological signs which may occur in the following five disorders. What other sensory or motor systems may become involved as the disorder progresses? If the functional impairment was mild, how would you modify the person's activities or lifestyle? If the impairment was severe?

End of the Auditory System lecture outline

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Last Update: October 2000