The Auditory System Flashcards

1
Q

What is the difference between the perilymph and endolymph?

A

Perilymph:

  • High in Na+, low in K+

Endolymph:

  • High in K+, low in Na+
  • The endolymph is where sound is transduced
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2
Q

In the spiral organ, where are the hair cells found? Describe their anatomy.

A
  • The outer and inner hair cells are embedded in the basilar membrane within the cochlear duct
  • The stereocilia of the outer hair cells tug on the tectorial membrane above, which amplifies vibrations created by sound waves
  • The stereocilia are arranged in height order, with tip links connecting the adjacent stereocilia
  • The inner hair cells transduce the sound
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3
Q

What is conductive hearing loss?

A
  • Sound doesn’t reach the hair cells
  • Hair cells are still intact
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4
Q

What is the mechanism underlying sound transduction in the hair cell?

A
  • The apical surface of the hair cells contain stereocilia
  • Sound waves in the fluid causes stereocilia to tilt back and forth
  • Alternating tugging and release of tip links causes sound to be transduced
  • Tilt towards taller stereocilia causes tugging of the tip links, opening mechanically-gated Na+ channels
  • Cell is depolarised and releases glutamate from its synaptic terminals
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5
Q

What are tip links between the stereocilia made of?

A

Glycoproteins

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6
Q

Describe the course of sound waves through the ear.

A
  • Sound waves hit the pinna and travel through the ear canal, causing the tympanic membrane to vibrate
  • This causes vibration of the ossicles - the membrane covering the inner ear
    *
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7
Q

How is sound amplitude encoded? Why is this a vulnerability of the auditory system?

A
  • Louder sounds produce larger vibrations
  • Bigger receptor potentials
  • More glutamate released
  • More action potentials
  • Strong vibrations can damage the stereocilia and kill hair cells
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8
Q

How is pitch encoded?

A

The mechanics of the cochlea separate sounds of different pitch:

  • Low and high pitch sounds will produce maximal vibrations at different locations
  • Thus different hair cells are activated depending on the pitch
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9
Q

Describe the primary auditory pathway, which is used for discriminative hearing.

A
  1. Cochlear nerve > cochlear nuclei
  2. Inferior colliculus
  3. Medial geniculate nucleus
  4. Primary auditory cortex
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10
Q

The auditory pathway is said to be tonotopically organised. What does this mean?

A
  • Cells at anterior of A1 respond to low frequency sounds
  • Cells at posterior of A1 resond to high frequency sounds
  • Isofrequency bands
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11
Q

Presbyacusis refers to the loss of high frequency hearing as we age. Why does this happen?

A
  • High pitched sound is more energetic and therefore more damaging
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12
Q

Describe the pathway that enables perception of sound localisation.

A
  • Primary auditory pathway - except origin is superior olivary nucleus
  • Superior olivary nuclei receive input from cochlear nuclei
  • Lateral superior olivary nuclei compare loudness of sounds in the two ears
  • Medial superior olivary nuclei compare the moment when a sound reaches the two ears - only low frequency sounds compared unambiguously
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13
Q

Inner ear cells are irreplacable cells that transduce sound into action potentials. How many of them are there in each ear?

A

3500

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14
Q

What do the outer hair cells do?

A
  • They amplify the vibrations of the cochlea
  • Without outer hair cells, inner hair cells lose the ability to transduce sound as the vibrations are not loud enough
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15
Q

Why is the cochlear nerve vulnerable to damage?

A
  • The cochlear nerve runs through the narrow internal auditory meatus, surrounded by solid bone
  • Space-taking lesions e.g. acoustic neuroma damage the nerve
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16
Q

What are the ossicles and what are their function?

A
  • The ossicles are 3 tiny bones in the middle ear that convert sound waves into vibrations in the endolymph
  • These vibrations initially travel through the oval window and into the endolymph
17
Q

Explain the difference between the roles of the lateral and medial parts of the superior olivary nucleus.

A
  • Lateral - detects loudness
  • Medial - detects moment that sound reaches each ear
  • Allows the superior olivary nucleus to localise sound