Auditory processing pathways Flashcards
(23 cards)
what is meant by tonotopic organisation?
where is it found?
tonotopic organisation refers to the fact that different parts respond to different frequencies of sound -
first place of tonotopic organisation is the basiliar membrane, and this organisation is preserved up to the auditory cortex
explain how the basiliar membrane has tonotopic organisation? and what two factors determine BM vibration? why is that helpful?
the two factors determining BM vibration is its mass and stiffness
different frequencies will cause the BM to resonate at different length.
for example a high frequency noise, will cause the BM to resonate near the base of the cochlea, while a low frequency noise causes the BM to resonate near its apex (because BM becomes wider and less stiff)
this allows the decomposition of a travelling wave (A complex wave) into its components
what are the functions of IHCs? what type of transducers are they? what are their characteristics?
they’re used for auditory discrimination
they’re mechano-electrical transducers (their depolarisation causes NT release)
each IHC is innervated by many ANFs, their stereocilia is not embedded within the tectorial membrane
what are the functions of the OHCs? what type of transducers are they? what are their characteristics?
OHCs have the function of increasing sensitivity of hearing (thus able to hear low noises - by increasing BM vibration, therefore, increasing the amplitude of a wave for the same noise) they’re also used for fine tuning of noise
OHCs are electro-mechano transducers (where depolarisation causes a change in their shape, which causes BM vibration)
OHCs are embedded into the tectorial membrane, they’re innervated by type II ANFs
what is the stapedius reflex?
This reflex is used as a protective reflex, when external auditory noise are very loud
reflex is activated by VIIth nerve, causing muscle to contract, causes chain of ossicles to stiffen, this reduces their vibrations, therefore, less vibrations are transferred to oval window and the cochlear duct - this prevents damage to the inner ear
what is impedment matching? where does it occur? why is it necessary? and how is it achieved?
most sound waves will be reflected off by water in the inner ear, because water has high impedence compared to air - this would reduce sound amplitude b 30dB
to reduce this sound loss this matching mechanism occurs in the middle ear and it is achieved by:
- area ratio of tympanic membrane to stapes (20:1)
this is the most important factor, increases the pressure applied by the vibrations
- the lever action of the osciles
this is because they’re not all the same length
buckling of the tympanic membrane
how does signal transduction occur in the hair cells?
mention:
- current responsible for depolarisation
- type of receptors involved
on vibration of the basiliar membrane the hair cells move up and down
moving up: stereocilia towards kinocilia (excitation)
moving down: stereo cilia way from kinocilia (inhibition)
stereocilia are linked together via tip links, those contain stretch sensitive potassium channels, on movement those receptors open and cause and influx of potassium into the hair cells which causes their depolarisation
in OHC depolarisation - changes shape of cell
in IHC depolarisation - causes calcium dependant transmitter release
the current responsible for depolarisation is a potassium current, because endolymph has high potassium content
what is the main way in which we are able to hear a range of sound frequencies?
IHCs are supplied by as many as 20 ANFs (although each ANF makes contact with a single IHCs)
Those ANFs have a range of spontaneous firing rates:
- those with high spontaneous firing rate, are activated by sounds of low frequencies (low stimulus) and they’re probably responsible for hearing tonic background noise (As they have a low saturation point)
as sound intensity increases: medium and low spontaneous firing rate nerves are recruited, this allows an increase in the average spike rate - and thus allowing neuronal coding for intensity of sound
what is the link between receptor potential and phase locking?
receptor potential refers to IHCs:
- when there is a sound wave of low frequency, IHCs are able to depolarise and polarise in phase with the peaks and troughs of the sound waves (phase locking)
- thus the AP produced have a time pattern, this is because the stereocilia, depolarise in synchronousy with peak of sound wave
phase locking does not occur with high frequency noises, so AP firing occurs with no time pattern - and this is because the stereocilia mechanically cannot keep up with the sound wave pattern and instead remains depolarised
the average firing from ANFs is more indicative of phase locking then recording from a single ANF
what is oto-acoustic emission? why is it used?
any health ear, when hears a noise will transmit a third wave - and this is because of the active mechanical feedback of the OHCs (the third wave is a sign of a functioning cochlea)
it is used as a screening for cochlear function in new borns
what is interaural timing difference? which nuclei does it use?
the basis on which bianural hearing is used to localise sound of low frequency - it uses the fact that sound will reach one ear quicker than the other
- it uses the medial superior olivary nucleus
what is interaural level difference? which nuclei does it use?
the basis on which binaural hearing is used to localise high frequency noises - it uses the fact that a sound shadow will be cast due to the head blocking the transmission of sound to the contralateral ear -
this means the ipsilateral LSO will be stimulated more strongly than the contralateral LSO
what are cones of confusion?
cones of confusion are points along the midline of the ear - in which binaural signals for sound localisation are rendered useless (and this is because there is no level or timing differences) spectral cues (Which are collected by the cartilage of the outer ear) are used to localise sound
what is thought to be the purpose of the auditory cortex? and what evidence is there to suggest so?
it is thought to integrate information from the
MSO: concerning ITd
LSO: concerning ILD
and information from spectral cues: concerning broadband sounds
damage to A1 - means subject is no longer able to localise sound, but if damage to the anterior part, sound localisation is not affected -
theory of dorsal and ventral stream
ventral stream – ‘what a sound is
dorsals stream – where a sound is
what does a tuning frequency curve show?
how can the shape of this curve be changed by drugs?
a tuning curve shows the frequency sensitivity of auditory nerve fibres to different amplitudes
it is plotted as the log of amplitude - aganist the log of frequency
certain drugs will increase the threshold frequency - meaning losing hearing sensitivity (cannot hear quiet noises) because higher amplitude noises are required to elicit a response
furthermore the bandwidth responses becomes wider meaning the ability of localisation is reduced
what is the specialisation of the VCN?
used for localisation of sound (through intensity and timing detection?
what is the specialisation of the DCN?
contains complex cells - probably used in assessing pitch and quality of the sound
what is the function of the cochlear nucleus
the cochlear nucleus - is where the auditory nerve fibres terminate
each nucleus receives nerve fibres from the ipsilateral ear
can be thought of as the first site of auditory processing, because fibres of different frequencies come together - and it is the site of first inhibitory neurones
it projects bilaterally to the superior olivary nucleus
what is the superior olivary nucleus?
this is the first site of binaural hearing - because it recieves bilateral supply from the cochlear nuclei -
negative input form the contralateral nuclei and excitatory nuclei form the ipsilateral nuclei
what happens when there is a lesion to Werincke’s area?
there receptive aphasia:
- patients cannot understand spoken language
- they cannot repeat words
what is transcortical aphasia?
transcortical aphasia, is when there is damage to the posterior language area.
- patients cannot understand speech
- but they can repeat something, even if they do not understand it
- this is because they is a transcortical link, between broca’s area and Werincke’s area
what is the descending pathway influencing IHCs?
where does it originate? and how does it work?
the descending pathway is from olivo-cochlear pathway
it arises from the lateral superior olivary nucleus (from small neurones within the nucleus and around it.
those terminate under the IHCs in the dendrities of the ANFs and they’re in a position in controlling firing in the ANFs - thereby controlling transmission of ANFs to the brain
this system shifts the response of ANFs to higher sound levels, thereby increasing their dynamic range (shift the dynamic range to the right)
what is the descending pathway controlling OHCs?
where does it originate? and how does it work?
the medial system from the olivo-cochlear pathway- it arises from large neurones within the medial superior cochlear nucleus.
the fibres terminates within the cell bodies of the OHCs - they control the gain in OHCs - thereby controlling by how much a sound is amplified
