Hearing Flashcards
what is sound?
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)
hearing
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
the outer/external ear
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
the middle ear
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
the inner ear
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
sound transduction
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
the cochlea
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)
closer look at the cochlea
cochlear duct
organ of corti
tectorial membrane
hair cell - the hairs of hair cells are embedded in the tectorial membrane
basilar membrane
hair cells
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
frequency mapping on the cochlea “tonotopy”
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
coding of sound frequency
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
coding of sound intensity
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
auditory pathway in the CNS
Ears -> Medulla -> Midbrain -> Thalamus -> Auditory Cortex
Medulla - where the nerves cross the body midline
Midbrain - projections to cerebellum
primary auditory cortex
sound localization
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
types of hearing loss
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
