The Auditory System

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Cranial nerves and nuclei II | Main Anatomy Index | The diencephalon

Last updated 30 March 2006

The Auditory System

Click here to go the "The Ear" under 1st year Gross Anatomy, Head and Neck (ANAT1006)

 

Peripheral Apparatus

 

The Conduction of Sound Energy

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Muscles of the Ossicles

Tensor Tympani Muscle

 

Stapedius Muscle

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The Auditory Part of the Inner Ear

 

The Cochlea

Click here for a diagram of a cross-section through the cochlea.

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The Cochlear Duct

  1. The vestibular membrane (or Reissner's membrane) borders the scala vestibuli and serves mainly as a barrier between the endolymph and perilymph;
  2. The stria vascularis adheres to the outer wall of the bony cochlea and is a specialised area that is rich in capillaries, producing most of the endolymph;
  3. The basilar membrane separates the scala media from the scala tympani.
  1. By higher frequencies at the base of the cochlea;
  2. And by lower frequencies at the apex of the cochlea.
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The Organ of Corti

Click here for a diagram of a cross-section through the Organ of Corti.

  1. A single row of inner hair cells near the osseous spiral lamina;
  2. And a band of outer hair cells 3-5 cells wide.

 

Role of Inner and Outer Hair Cells

Vibrations of basilar membrane --> oscillations of membrane potential --> vibration of the cells themselves

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Central Connections

Click here for a diagram of the auditory pathways.

  1. The dorsal cochlear nucleus;
  2. And the ventral cochlear nucleus.

 

Lateral Lemnisci

 

Inferior Colliculus and Medial Geniculate Nucleus

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Superior Olivary Nucleus

 

Strategies of Sound Localisation

  1. Comparison of the time-of-arrival of the sound;
  2. And the intensity of a sound at the two ears.
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Clinical Significance of the Auditory System

Stapedius Reflex

 

Pathway Involved in Stapedius Reflex

One ventral cochlear nucleus --> both superior olivary nuclei --> both facial motor nuclei.

 

Changes in Frequency Sensitivity with Age

Group Range
Children 20 Hz - 20 kHz
Middle Age 50 Hz - 8 kHz

 

Conduction Deafness

  1. Blockage of the external acoustic meatus;
  2. Impairment of the movement of the ossicles (due to bony overgrowth of the stapes);
  3. Infection of the middle ear;
  4. Or physical trauma.

 

Sensorineural Deafness

  1. Cochlea (noise, old age, drugs);
  2. Cochlear nerve (tumour);
  3. Or cochlear nuclei (vascular lesion).

 

Tests to Distinguish Between Conduction and Sensorineural Deafness

Ganong, William F. (1997) Review of Medical Physiology 18th Ed. Appleton & Lange, Stanford, Connecticut, USA. Table 9-1. p.172

  Weber Rinne Schwabach
Method Base of vibrating tuning fork placed on the vertex of skull Base of vibrating tuning fork placed on mastoid process until subject no longer hears it, then held in air next to ear Bone conduction of patient compared with that of a normal subject
Normal Hears equally on both sides Hears vibrations in air after bone conduction is over  
Conduction deafness Sound louder in affected ear because of masking effect of environmental noise is absent on diseased side Vibrations in air not heard after bone conduction is over Bone conduction better than normal (conduction defect excludes masking noise)
Nerve deafness Sound louder in normal ear Vibration hear in air after bone conduction is over, as long as nerve deafness is partial Bone conduction worse than normal
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Michael Tam (c) 1998