Hearing

Question 1

what is sound?

Answer 1

pressure waves generated by vibrating air molecules, alternately compressed and released

sound wave amplitude is measured indecibels (dB)

sound wave frequency is measured in cycles/sec, or Hertz (Hz)

Question 2

hearing

Answer 2

inner ear - oval window, Cochlea

middle ear - ossicles (three small bones), tympanic membrane (ear drum)

outer ear - auricle (pinna) - moveable in some animals

auditory canal (external auditory meatus

Question 3

the outer/external ear

Answer 3

pinna or outer ear - directs the sound into the ear

ear canal - funnels and conducts the sound to the middle ear

tympanic membrane - separates the external ear from the middle ear

Question 4

the middle ear

Answer 4

small bones - transfer the sound from the external environment to the inner ear

eustachian tube - it connects the middle ear with the pharynx and helps equilibrate the middle ear pressure

explains the unpleasant pain/pressure that we feel in our ears during flights

Question 5

the inner ear

Answer 5

oval and round window - separates the cochlea (filled with liquids) from the middle ear (filled with air)

cochlea - has structures that converts the physical motion of the ear’s structures into a neuronal response

vestibular apparatus - responsible for our sense of balance

Question 6

sound transduction

Answer 6

1) sound waves strike the tympanic membrane and become vibrations
2) the sound wave energy is transferred to the three bones of the middle ear, which vibrate
3) the stapes is attatched to the membrane of the oval window. vibrations of the oval window create fluid waves within the cochlea

Question 7

the cochlea

Answer 7

fluid waves initiated at oval window (#3)

these waves push on the flexible membrane of the cochlea duct (#4)

pressure from the wave releases at the round window (#5)

mechanoreceptors within the cochlear duct transduce movement into action potentials on the auditory nerve (#6)

Question 8

closer look at the cochlea

Answer 8

cochlear duct

organ of corti

tectorial membrane

hair cell - the hairs of hair cells are embedded in the tectorial membrane

basilar membrane

Question 9

hair cells

Answer 9

1) as the basilar membrane vibrates, the hairs of the hair cells bend back and forth
2) bending toward the biggest hair opens mechanically-linked ion channels, depolarizing the hair cell
3) depolarization increases the release of neurotransmitter, increasing the action potential frequency on the afferent nerve
4) bending in the opposite direction hyperpolarizes the hair cell, decreasing the release of neurotransmitters

Question 10

frequency mapping on the cochlea “tonotopy”

Answer 10

fluid waves travel along the basilar membrane

the location of peak displacement varies with frequency

narrow, thick base tuned for high frequencies

wide, thin apex tuned for low frequencies

middle portion tuned for medium frequencies

Question 11

coding of sound frequency

Answer 11

recall that the brain can hear frequencies up to 20,000 Hz

this occurs because of tonotopy (frequency mapping)

activation of nerve fibers from the base of the cochlea is interpreted as high frequency sound, even though the nerve fiber can only sustain action potentials up to a rate of ~1,000 Hz. this is a form of “labeled line” encoding

tonotopy is maintained throughout auditory system

Question 12

coding of sound intensity

Answer 12

recall that sound intensity can vary over a 120 dB range.

how does the brain cover this range?

1) increased rate of firing on a single nerve fiber (up to ~1,000 Hz)
2) multiple sets of neurons with different thresholds
3) recruitment of additional neurons as loudness increases

Question 13

auditory pathway in the CNS

Answer 13

Ears -> Medulla -> Midbrain -> Thalamus -> Auditory Cortex

Medulla - where the nerves cross the body midline

Midbrain - projections to cerebellum

Question 14

primary auditory cortex

Answer 14

Question 15

sound localization

Answer 15

unlike other sensory systems, transduction of sound provides no information about where a sound comes from

our brain exploits the fact that sound takes longer to reach one ear than the other -> inter-aural timing differences

timing differences can be very small (less than 1 ms). sensed by coincidence detectors in the brainstem

brainstem centres are also sensitive to the acoustic shadow produced by the head -> inter-aural intensity differences

*sound will reach closer ear sooner, and will be louder

Question 16

types of hearing loss

Answer 16

1) conductive: sound is unable to be transmitted through outer or middle ear. a mechanical defect
- extremely loud sounds rupture eardrum or damaged ossicles
- infection fills middle ear with fluid
- ear wax
2) sensorineural: damage to structures of inner ear that affects hair cells, or to auditory nerve (nerve deafness). a transductive and/or peripheral defect
- extremely loud sounds damage organ of corti
- ototoxic drugs that damage hair cells (e.g. streptomycin)
- presbycusis (old + hearing), i.e. degenerations in the cochlea
3) central: damage to auditory pathways upstream from cochlea. a defect in the Central Nervous System
- e.g. tumours or strokes in the central auditory pathways