Auditory System Flashcards
What does intensity of sound refer to ?
Amplitude of the wave
It is the difference in pressure between compressed and rarefied patches of air in a sound wave
It determines loudness
What is sound ?
Audible variations in the air pressure
What does the frequency of sound refer to ?
Number of waves per second
Expressed as Hz
It determines pitch
What does the middle ear do ?
It converts the wave in air to a wave in fluid
What are the ossicles of the Middle ear ?
They are within air filled cavities
They provide amplification of the sound - they amplify the signal from the tympanic membrane to cause the fluid in the cochlear to move
- malleus, incus and stapes
What is the attenuation reflex ?
It dampens the sound when it is very loud
The stapedius and tensor tympani muscles contract to reduce the movements of the ossicles so the fluid ,movement doesn’t damage the hair cells in the cochlear
What is conductive hearing loss ?
Scar tissue due to an infection or otosclerosis( proliferation of bone) causes decreased movement of ossicles
What is a major part of the outer ear ?
Pinna
Made up of elastic cartilage which funnels the sound wave down the auditory canal so it hits the tympanic membrane
What is the inner ear like ?
It is a snail shaped fluid filled cavity sitting in the skull
3 channels- scala vestibuli, media and tympani
Contains the basilar membrane which is floppy and moves up and down due to fluid flow
How does the basilar membrane move ?
Pressure changes cause the oval window to move
Moves the perilymph in the scala vestibuli
This moves the reissners membrane which moves the endolymph
This causes the basilar membrane to move creating a travelling wave
How are different frequency sounds heard ?
Different frequency sounds cause movement of the basilar membrane at a specific part
This determines which hair cells move
What is the organ of corti ?
It is on top of the basilar membrane and contains hair cells and auditory receptor neurons
The hair cells extend and implant into the rigid tectorial membrane
What does the round windows do ?
Allows the cochlear to bulge out a little to reduce the pressure
What is the helicotrema ?
It connects the scala vestibuli and tympani and allows fluid to flow from one side to the other causing the membrane to move
Where are high frequency sounds heard on the basilar membrane ?
They are able to vibrate the stiff base but they don’t travel far
Where are low frequency sounds heard on the basilar membrane ?
There energy doesn’t dissipate so they can travel further but they don’t have enough energy to vibrate the stiff base
What is the purpose of the tonotopoc organisation of the basilar membrane?
It’s called frequency coding
Enables the brain to know which part of the basilar membrane has moved and therefore which hair cells have moved enabling it to determine the frequency of the sound
Works to about 200Hz
What happens is the main purpose of the auditory receptors (hair cells) ?
They are the point where mechanical energy is changed into membrane potential
What are the types of hair cells ?
Inner and outer hair cells
3x more outer hair cells but it is the inner hair cells which pass on most of the information about movement of the membrane
What structures are linked to the basilar membrane to form a structural unit ?
Basilar membrane
Reticular laminar
Rods of corti
Hair cells
What happens when the basilar membrane moves downwards ?
Reticular laminar also moves downwards and away from the modiolus
This causes the stereocillia to bend in
What happens when the basilar membrane moves upward ?
Reticular laminar moves upwards and in towards the modiolus
This causes stereocillia to bend out
What makes the hair cells rigid ?
The actin in the filaments
What are the concentration of ions in the endolymph ?
Potassium ions - 150mM
Sodium ions - 1mM
What is the concentration of ions in the perilymph ?
Potassium ions - 7mM
Sodium ions - 140mM
This bathes the hair cells
How much more positive is the endolymph,oh compared to the perilymph ?
80mV
Endocochlear potential
This bathes the base of the hair cells
What happens when potassium channels on hair cells open ?
Potassium ions flow into the hair cells due to the high concentration of potassium ions in the endolymph
What are the potassium channels like in the stereocillia ?
They are mechanically gated channels
Elastic filaments link the lids of channels on different hair cells together - lids are only slightly open when the basilar membrane is still so there is only bey slight movement of potassium ions
What is the kinacillium ?
It is the largest stereocillia
What happens when the stereocillia move towards the kinacillium ?
Tips move away from each other and this causes the elastic filaments linking the lids to stretch and pull the lids open so more potassium ions can flow into the cells
What happens when the stereocillia move away from the kinacillium ?
The elastic filaments connecting the lids contract and the lids close completely
How many spiral ganglion cells does a single inner hair cells synapse onto ?
About 20
These cells control neurotransmitter release
What is the purpose of the outer hair cells ?
They amplify the movement of the basilar membrane
Contain motor proteins which can lengthen by contracting and this will further exaggerate the movement of the basilar membrane
Causing greater bending of the stereocillia so more chance of depolarisation and higher rate of AP firing
What happens in sensorineural hearing loss ?
It is hearing loss which occurs when cells dies meaning that signal transduction can no longer occur
Can be caused by congenital or environmental damage
What is the treatment for sensorineural hearing loss ?
Cochlear implant
Directly stimulates specific points on he basilar membrane to activate specific neurons in cranial nerve 8
How are frequencies below 200Hz encoded ?
By phase locking
Where is tonotopy seen for sound frequencies greater than 200Hz ?
Basilar membrane-spiral ganglion- cochlear nucleus
What is phase locking ?
It is the timing of neuronal firing
It complement the tonotopic map
It occurs with sound waves up to 4kHz because above this the sound wave cycles too fast for the action potentials of a single neuron to match the timings
When does neuronal firing= frequency ?
At low frequencies when the cell fires everytime the sound wave is in a particular phase
When does group firing rate= frequency ?
Where one cell does not Fire everytime but as part of a group so it is a combined effort - coverage
What is the ascending auditory pathway ?
Spiral ganglion Ventral cochlear nucleus Superior olive Inferior colliculus Medial geniculate nucleus of thalamus Auditory cortex
Where does decussation occur in the ascending auditory pathway ?
2nd order neuron from the ventral cochlear nucleus decussates to the contralateral superior olive
What is the function of the inferior colliculus ?
Detection of sudden/strange/loud sounds - cause your head to turn
What are the 2 mechanisms for encoding sound localisation ?
Horizontal localisation - both ears
Vertical localisation - mono aural
What are the 2 mechanisms for horizOntal sound localisation ?
Interaural time difference
Inter aural intensity differenc e
What is interaural time difference ?
Time difference of sounds between both ears
- if the sound is closer to the left ear then It takes longer to hear it in the right ear
Sounds at a 45 degree angle causes a 0.3msec difference
Sounds at 90 degree able cause a 0.6msec difference
Where is the coincidence detector ?
Medial superior olive - contains lots of different neurons to help tell where sound is coming from
The neuron will only fire if it receive inputs from both ears
What is interaural intensity difference ?
Compares intensity of sound in each ear
Head causes sound to be muffled
Binaural inputs - excitatory inputs from AVCN and inhibitory inputs from MNTB to lateral superior olive
