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The Auditory System

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Competencies:

• Draw the receptor apparatus for hearing.
• Diagram the central connections of the auditory nerve.
• Explain the limited loss of hearing that results from lesions of the central pathways.

To master the material presented in this lecture:

Read ...

Purves text, Chapter 13
Haines pp 266.

Look at the Review Questions below ...

Listen to the lecture and focus on the following points ...

• Sound characteristics:
  • Frequency, i.e. wave length, and intensity, i.e. loudness.
  • Best hearing is in the range of 1000 to 3000 Hz, i.e. requires the lowest intensity to be heard. Young adults may hear sounds from 20 to 20,000 Hz.
  • How can one effectively block sound overload which may damage the receptors?
    
• Transmission of sound in air into the fluid filled inner ear and to the receptor/transducer.
  • The ear drum, middle ear ossicles and oval window provide a gain of about 1.5.
    
• The cochlea is a curved sensory apparatus in which minuscule displacement of the microvilli atop hair cells results in a change of potential across the hair cell membrane.
  • The hair cells are attached to a basilar membrane which vibrates in response to the pressure waves of sound.
  • The length and mass of the basilar membrane result in a tonotopic organization with the high frequencies transduced at the base of the cochlea and the low frequencies transduced at the apex.
  • What are the causes of hearing loss at the cochlea? What are the effects of losing hair cells for the transduction of high frequencies?
    
• The tonotopic map of hearing is maintained throughout the central pathways.
  
• Localization of sound is dependent upon comparing time of arrival at each ear and relative intensity at each ear. Various sites in the CNS make such comparisons, but the first place where the comparison is made is in the Superior Olivary Nuclei.
  
• After the first relay in the CNS at the dorsal and ventral cochlear nuclei, there may be several relay sites on the way to the cerebral cortex. Each relay (synapse) provides the brain with an opportunity to sharpen the messages and to establish reflexes. Above the dorsal and ventral cochlear nuclei, it may be assumed that auditory pathways and nuclei carry bilateral information, i.e. from both ears, rather than only contralateral as in many other sensory system pathways.
  
• The following outline of relays should not be taken to imply that every message stream must synapse at every possible level.
  • Receptor transduction in cochlea to the primary neurons (cell bodies in the spiral ganglion) »
  • Dorsal and ventral Cochlear Nuclei  »
  • Brachium of inferior colliculus  »
  • Medial geniculate nucleus of thalamus  »
  • Transverse Gyri of Heschl on superior aspect of the temporal lobe