II. Sound waves are usually generated by vibrating objects (e.g., vocal cords).

III. Similar to light waves, sound waves are characterized by
  1. their amplitude

  2. wavelength (frequency),
 

 I.  External ear: consists of
  1. the pinna: a sound-collecting cone

  2. auditory canal (external auditory meatus):
 

II. Middle ear: it begins at the ear drum & consists of 3 tiny bones (ossicles)
  1. first step of transduction
 
  2. the bones include
    A. malleus (hammer): attached to ear drum

    B. incus (anvil): middle of the 3 bones

    C. stapes (stirrup): connect to the oval window

  3. The Eustachian tube connects the middle ear & to our throat

  4. anything that obstructs bone movement (e.g., fluid) can impair hearing
 

III. Inner ear: consists largely of the cochlea (land snail),
 1. it is composed of 3 tunnels (scalae)
   A. scala vestibuli
   B. scala tympani
   C. scala media:

 2. scala media is the tunnel important for audition
   A. Reissner's membrane forms the upper boundary
   B. stria vascularis: forms the lateral boundary
   C. basilar membrane forms the lower boundary
     b. organ or corti (hair cells) lies on the  basilar membrane
     c. tectorial membrane covers the surface of the organ or corti

 3. bending of inner hair cells causes ion exchange (2nd step of transduction)
   A. axons of the cochlear nerve synapse on inner hair cells & can cause APs

 4. round window deformation relieves the pressure applied by the stapes on
     the oval window (creates space for endolymph to move)
 

I. afferent neurons for hearing (two types: type I & type II)
  1. bipolar neurons with cell bodies in spiral ganglia (in the cochlea)

  2. distal end of type I axons synapse on inner hair cells

  3. distal end of type II axons synapse on outer hair cells
 

II. efferent axons
  1. cell bodies in the superior olive
  2. efferent axons (i.e., the distal end) synapse on (Kingsley p. 399)
    A. the soma of outer hair cells
 

 2. Frequency coding: works best for frequencies under 200 Hz
 

II. detection of loudness
 1. intense vibrations of the oval window cause a greater deformation of hair
     cells & more neurotransmitter is released
  A. this causes more rapid firing of the auditory nerve fibers
  B. the number of axons that are active at a given time
 

III. How do we Perceive Sources (direction) of Sound?
 

I. cochlear nerve axons (part of cranial nerve 8) synapse on hair cells

II. The axons of the auditory nerve carry the signal to the medulla where they
     synapse on the cochlear nuclear complex.

III. most axons form cochlear nuclei carry the signal to the superior olive

IV. axons form the superior olive ascend & send collaterals to the inferior
      colliculus & nucleus of the lateral lemniscus (midbrain)

V. inferior colliculus axons ascend to the medial geniculate nucleus (thalamus)

VI. medial geniculate nucleus axons ascend to the primary auditory cortex.
 

VII. auditory cortex is on the temporal lobe mostly in the lateral fissure
 1. the auditory cortex is arranged into columns

 2. Auditory information is relayed to other areas of the brain after the cortex
 

I. conductive hearing loss: caused by a restriction of the ossicles
  1. due to
    A. middle ear infections

    B. serous otitis media: eustachian tube not draining appropriately

    C. otosclerosis: a growth of bone near the oval window (impedes ossicles)
 

II. presbyacusis: loss of hearing associated with aging
 

III. ototoxicity:
 

IV. noise exposure: very intense sounds can kill hair cells
 

V. acoustic neuroma: tumor growing on the 8th cranial nerve
 

VI. damage to brain areas important for hearing
 

VII. tinnitus: perception of sounds that originate in the ear (e.g., ringing or
         whistling)
 

VIII. Auditory agnosia: is the impaired capacity to recognize the nature of nonverbal sounds

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