Special Senses 2

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The Ear

Hearing is the detection of sound waves and sound is the result of vibration, or waves, travelling through a medium. Auditory stimuli travel farther and more quickly than chemical ones, but not as fast as light. Auditory receptors provide better directional information than chemoreceptors.

The cochlea is a bony structure containing part of the cochlear duct. The vestibular canal lies above this duct, while the tympanic canal lies below it. All three chambers are filled with fluid. Pressure waves travel down the tympanic canal to the round window, which transmits pressure back to the middle ear.

The organ or court transduces sound in the cochlea:
  • The basilar membrane is the bottom of the cochlear duct, where hairs are attached
  • Hair cells are found with their associated neurons
  • The tectorial membrane is an overhanging, gelatinous membrane where hair cells hit.
The stereocilia of hair cells bend in response to vibrations of the basilar membrane. Nerve impulses then travel to the auditory cortex of the temporal lobe.

How do we hear?


  1. Air vibrations are channeled through the auditory canal of the external ear.
  2. Vibrations reach the tympanic membrane (eardrum).
  3. Movement of the tympanic membrane causes displacement of the auditory ossicles.
  4. Bones in the middle ear move (malleus, incus, stapes) and the oval window vibrates, establishing pressure waves in the perilymph of the scala vestibuli (surrounding the cochlear duct).
  5. The pressure waves distort the basilar membrane on their way to the round window of the scala tympani.
  6. Vibration of the basilar membrane causes vibration of hair cells against the tectorial membrane.
  7. Information about the region and the intensity of stimulation is relayed to the CNS over the cochlear branch of cranial nerve VIII.

Neural Pathway

  1. Hair cells are stimulated at a specific location along the basilar membrane, activating sensory neurons.
  2. Sensory neurons carry the sound information in the cochlear branch of the vestibulocochlear nerve (VIII) to the cochlear nucleus on that side.
  3. Information ascends from each cochlear nucleus to the inferior colliculi of the midbrain.
  4. The inferior colliculi direct a variety of unconscious motor responses to sounds.
  5. Ascending acoustic information goes to the medial geniculate nucleus.
  6. Projection fibres then deliver the information of specific locations within the auditory cortex of the temporal lobe.

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