Physiology of Hearing Flashcards
What range of Hz can humans hear within?
- between 20Hz and 20,000Hz
Over 140dB is the
threshold of pain
What receptors convert sound pressure waves to receptor potential and ultimately neuronal signalling events
Mechanoreceptors are sensory receptors whose membrane contains cation channels that convert a mechanical signal to a graded receptor potential for the release of neurotransmitters or the generation of an action potential
The Ear:
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What are the auditory ossicles?
- three small bones located in the middle
ear - malleus = hammer
- incus = anvil
- stapes = stirrup
the malleus is attached to the tympanic membrane
the stapes is attached to the oval window
Purpose of the auditory ossicles?
to convert sound waves received through the outer ear to fluid waves in the inner ear
Middle Ear:
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Inner Ear:
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The Cochlear Canal:
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The Upper Cochlear Canal:
scala vestibuli contains perilymph
The Middle Cochlear Canal:
scala media (cochlear duct) contains endolymph and the organ of corti
The Lower Cochlear Canal:
scala tympani contains perilymph which ends in the narrow window
Oscillation of the Stapes causes
pressure waves to the perilymph in the scala vestibuli and scala tympani
which causes the vibration of the basilar membrane
What is the organ of corti?
Where is it located?
Contains?
- the primary sensory receptor of the
auditory system? - sits on the basilar membrane
- contains the inner and outer hair cells
which are sensory mechanoreceptors - the hair cells are supported by epithelial
cells
vibrations of the basilar membrane are received by the hair cells
Organ of Corti:
Peri, endo, peri
Hair Cells:
- contain stereocilia
- sits on the basilar membrane in scala
media - the hair cells are sensory
mechanoreceptors - supported by epithelial cells
- inner = inner side of organ of corti
- outer = outside of organ corti = embedded
in the tectorial membrane
stereocilia are projections of the hair cell membrane; structure is achieved through actin filaments
Sound pressure is
localised deviation of pressure caused by a sound wave from the atmospheric pressure
Hearing loss can be caused by a high sound pressure levels or low sound pressure levels?
High sound pressure levels
The bony labyrinth is filled with
perilymph (high Na+, low K+)
The membranous labyrinth is filled with
endolymph (low Na+, high K+)
Stereocilia:
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Journey of vibration through the ear
- vibration of the tympanic membrane
- vibration of the ossicles
- waves travel through perilymph
- basilar membrane vibrates
- results in outer hair cells bending
- as they bend membrane depolarisation
begins - the inner hair cells move in response to
endolymph movement - oscillation of outer hair cells and vibration
of membranes influence the sensitivity of
inner hair cells - both inner and outer hair cells depolarise
- opening of K+ channels on stereocilia so
influx of K+ from endolymph into
perilymph - depolarisation of perilymph
- which opens Ca2+ voltage gated channels
- influx of Ca2+ and efflux of K+ causes
repolarisation - neurotransmitter released
What does medial displacement of stereocilia cause?
- hyperpolarisation
- K+ influx does not open Ca2+ channel
- no neurotransmitter released
How can the CNS modulate the auditory system at the receptor level?
- outer hair cells are directly innervated by
afferent and efferent nerves - inner hair cells are directly innervated by
afferent nerves - CNS can modulate the auditory system at
the receptor level
Tonotopic Organisation:
Hair cells respond to different sound frequencies depending on their location along the basilar membrane of the cochlear canal
The Central Auditory Pathway:
- spiral ganglion bipolar sensory neurons
innervate the stereocillia - travels to the cochlear part of CNVIII
- CNVIII projects to the ventral and dorsal
cochlear nuclei - The nerve from the dorsal cochlear
nucleus will travel to the superior olivary
complex and then to the inferior colliculus
from where the signal will be directed to
the relevant cortical area
the path travelled from the superior olivary complex to the inferior colliculus is referred to as the lateral lemniscus
The Central Auditory Pathway:
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Summary of Auditory Nuclei:
- superior olivary complex
- lateral leminscus
- inferior colliculus
- medial geniculate nucleus
- auditory cortex
- superior olivary complex = receives
bilateral signal from the cochlear nuclei,
can distinguish sounds based on the time
of arrival and the perceived intensity in
each year - lateral leminscus = projects to the inferior
colliculus - inferior colliculus = tonotopically organised
- medial geniculate nucleus (thalamus) =
tonotopically organised - auditory cortex = receives all input and is
tonotopically organised
What areas of the brain are involved in auditory signal processing?
- the auditory cortex is located in the
transverse temporal gyrus of Heschl (area
41) - 42 is the auditory association area
- Wernicke’s area is a secondary auditory
area and is important for the
interpretation of the spoken word - Broca’s area in the frontal cortex is
involved in speech - 41 and 42 send descending projections to
the auditory system, involving efferent
nerves
Auditory Signal Processing:
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Conductive Hearing Loss:
disease in the outer ear canal or in the middle ear
Sensorineural Hearing Loss:
disease in the cochlea or in the neural pathway from the cochlea to the brain
Occupational Hearing Loss:
a noisy work environment can use the deterioration of the organ of corti
Ototoxic Hearing Loss:
noisy work environment can cause deterioration of the organ of corti streptomycin, quinine)
Infectious Hearing Loss:
drug induced (mumps or rubella)
Tinnitus
hearing a sound that has no external source
Conductive Hearing Loss Examples and Causes:
- earwax
- outer ear infection
- otitis media (middle ear inflammation)
- perforated eardrum
- tumour in the ear canal or middle ear
- otosclerosis
Ototsclerosis: Causes:
- genetic or external factors like viral
infection - temporal bone sclerosis affects the stapes
that becomes fixed to the oval window
leading to impaired vibration - treatment: involves external hearing aids
to amplify sounds or stapedectomy
Stapedectomy: Risk:
can damage a branch of the facial nerve called corda tympani which passes through the inner ear
lose sense of taste
Sensorineural Hearing Loss:
- GJB2 encodes for protein connexin 26
involved in innner ear K+ homestasis so
struggle to depolarise sterocillia
membrane - SLC26A4 encodes for anion transporter
Cochlear Implants:
- electronic device that provides a sense of
sound to someone with severe hearing
loss - surgically implanted
- contains microphone, speech processor,
transmitter and receiver, electrode array - electrodes transform sounds to nerve
stimulations that bypass the damage part
of the inner ear - does not completely restore hearing
Cochlear Implants:
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