Hearing Flashcards
What is the volume of a sound defined as? What is it measured as?
Decibels (dB)
Loudness defined relative to a sound which is just audible (0dB)
Conversation = ~60dB
Painful sound = ~120dB
Why is it important that the round window is closed by a membrane?
Fluid within the inner ear compressed by the stapes can bulge out slightly
How does the ear amplify sounds of lower frequency?
Basilar membrane resonates, mechanically amplifying sound with progressively lower frequencies along the length of the cochlea
The site along the basilar membrane determines the frequency detected
How are hair cells arranged in the inner ear? How does this signify their function?
1 row of inner hair cells
- sense sound by moving Type 1 spiral ganglion afferents (95%)
3 rows of outer hair cells
- amplify sound by moving Type 2 spiral ganglion afferents (5%)
- modulated by the olivocochlear system
How are hair cells tuned?
Mechanically tuned by location along the cochlea
Electrically tuned by expression of particular ion channels
How does a sound become transmitted into a nervous impulse?
Bending of stereocilia by pressure of sound causes tension
Mechano-transducer channels activated, generating a membrane potential (depolarisation)
Voltage-gated Ca2+ channels open
Release of neurotransmitters onto spiral ganglion
note: the [K+]o = 140mM (normally it is only ~5mM)
What is the olivocochlear system?
As the outer hair cells elongate and contract, they generate a force detected by the olivocochlear bundle
note: the louder the sound, the more APs are produced and neighbouring fibres recruited
What are the components of the auditory pathway?
APs —> spiral ganglion cells —> cochlear nucleus —> superior olivary complex —> inferior colliculus —> medial geniculate nucleus —> auditory cortex and brainstem (sound localisation)
How does the perceived speed and volume of a sound differ between ears?
Sound transmitted to one ear has to cross the midline to be detected in both ears, causing ~0.5msec delay as the AP is transmitted
Volume difference between ears is due to the sound being refracted around the head (the sound is perceived as quieter in the ear opposite to the sound origin)
Give some examples of causes of hearing impairment.
Loud noise
Congenital defects affecting:
- hair cells
- tectorial membrane proteins
- non-sensory cells
- mitochondrial proteins
Infections e.g. rubella, otitis media
Ototoxic compounds e.g. aminoglycoside antibiotics
Trauma (damage to temporal bone)
Age
How is hearing assessed?
Visual inspection (otoscope)
Audiograms = sensitivity v.s. frequency (overall hearing sensitivity)
Otoacoustic emissions = sound generated by outer hair cells (normally not audible) is detected and measured (amplifier function)
Auditory brainstem response = assess general hearing function in babies
What are some of the causes of hearing loss due to conductive damage? Which sites are affected?
Blockage
Ruptured eardrum
Fluid accumulation (otitis media)
Otosclerosis (progressive ossicle immobilisation)
What are some of the causes of hearing loss due to sensory damage? Which sites are affected?
Hair cell destruction e.g. physical, loud noises
Hair cell death e.g. ototoxic compounds
What are some of the causes of hearing loss due to neural damage? Which sites are affected?
Spiral ganglion damage e.g. acoustic neuroma
Age-related hearing loss
Tinnitus (“phantom sound”)
Auditory neuropathy (associated with neonatal jaundice)
Monoaural deafness (deaf on one side; destroys ability to localise a sound)
Outline some of the treatments for hearing loss.
Hearing aids
Cochlea implants (direct electrical stimulation of spiral ganglion)
Hair cell regeneration (not yet applied in clinical practice)
Cochlear nucleus implants (direct stimulation of neurones in first nucleus of auditory pathway)
