The Auditory System

Question 1

What is the difference between the perilymph and endolymph?

Answer 1

Perilymph:

• High in Na⁺, low in K⁺

Endolymph:

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

Question 2

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

Answer 2

• 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

Question 3

What is conductive hearing loss?

Answer 3

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

Question 4

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

Answer 4

• 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

Question 5

What are tip links between the stereocilia made of?

Answer 5

Glycoproteins

Question 6

Describe the course of sound waves through the ear.

Answer 6

• 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
  *

Question 7

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

Answer 7

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

Question 8

How is pitch encoded?

Answer 8

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

Question 9

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

Answer 9

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

Question 10

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

Answer 10

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

Question 11

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

Answer 11

• High pitched sound is more energetic and therefore more damaging

Question 12

Describe the pathway that enables perception of sound localisation.

Answer 12

• 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

Question 13

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

Answer 13

3500

Question 14

What do the outer hair cells do?

Answer 14

• 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

Question 15

Why is the cochlear nerve vulnerable to damage?

Answer 15

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

Question 16

What are the ossicles and what are their function?

Answer 16

• 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

Question 17

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

Answer 17

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