1.5 The Auditory System Flashcards
What causes sound
disturbance of air molecules that makes up the sound wave consists of zones of compression (pressure increases) and zones of rarefaction (pressure decreases)
what determines the sound waves amplitude
the difference between pressure of molecules in zones of compression and rarefactions
Whats the external auditory canal
first step in hearing is sound waves entering this (auditory meatus)
what helps amplify and direct sound
the shapes of the outer ear (pinna or auricle) and the meatus help to amplify
- continuous vibrations of pressure waves spread down this
what happens as the sound reaches the tympanic membrane (ear drum)
it vibrates the same frequency as the sound wave
bows inwards during compression (higher freq)
returns to rest at rarefaction
distance moved depends on the pressure and so measure amplitude
what is the tympanic membrane (ear drum)
it is at the end of the meatus
- it is stretched across the canal separating outer and middle ears
what is the middle ear cavity
an air filled cavity in the temporal bone of the skull
what is the auditory (Eustachian) tube
connects the middle air to pharynx
exposes middle ear cavity to atmospheric air pressure
what is the second step in hearing
transmission of sound energy from tympanic membrane through middle ear to inner ear
what is the inner ear called
the cochlea
what is the cochlea
fluid filled spiral shaped passage in the temporal bone
it is a coiled structure of progressively decreasing diameter
what is in the temporal bone
the cochlea and semi-circular canals used in the vestibular system
what do ossicles do
as liquid is more difficult to move than air the sound pressure must be amplified which is what it does
what is the structure of ossicles
chain of 3 bones - malleus - incus - stapes that act a piston and couple the tympanic membrane vibratios to the oval window (effectively impedance matching)
what is the oval window
much smaller than the tympanic membrane so force per unit area increases by 15 to 20 time
what is scalae
3 liquid-filled compartments
- scala vestibui
- scala typmani
- scala media
what is scala vestibui
farthest from the base and has the oval window at the basal end
what is scala typmani
at the cochlear base and has the round window as its base
what is the scala media
it is within the cochlear duct which separates the 2 chambers
what is the one side of the cochlear formed of
by the basilar membrane upon which sits the organ of Corti (contains ear’s receptor cells)
what vibrates the basilar membrane
pressure differences across the cochlear duct
what is the frequency like at the distal end or apex of basilar membrane
membrane is fairly broad and flaccid so more sensitive to low frequency oscillations
what is the frequency like at the proximal end (near the tympanum) of basilar membrane
membrane is stiffer and more sensitive to high sound frequencies
what is the scale of frequencies of basilar membrane against displacement
logarithmic
what is a tonotopic map
neighbouring values in stimulus space are encoded by neighbouring sensory units
facilitates signal processing such as lateral
inhibition
what is the organ of Corti
receptor organ of the inner ear
what does the organ of Corti contain
16,000 hair cells innervated by approx 30,000 afferent nerve fibres which carry info to the brain
how are the hair cells in the organ of Corti organised
tonotopically organised
at any position along the basilar membrane, the hair cells are most sensitive to a particular frequency and these freq are logarithmically mapped in ascending order from the cochlea’s apex to its base
what do hair cell mechanoreceptors have
stereocilia (and one kinocilium) protruding from one end
- transform pressure waes into receptor potentials
- in contact with overhanging tectorial membrane
what happens as basilar membrane is displaced
hair cells more relative to the tectorial membrane causing shear of the stereocilia
what does shear of the stereocilia cause
opens ion channels that are gated with spring like structures
depending on direction, ion channels open for Ca of K
other channels contribute to the RP response
where is cochlea info sent
to the cochlear nucleus in the medulla
conveyed by central processes of cochlear ganglion cells
info relayed to different types of neurones in the cortex
(tonotopic arrangements maintained)
dorsal acoustic stria
pathway oicking up info from the dorsal cochlear nucleus
what connects to the inferior colliculus in mid brain
projections run through the pons to connect
what happens after inferior colliculus
info passed to medial geniculate nucleus and then to the primary auditory cortex
what does the intermediate acoustic stria do
connects the cochlear nucleus with the nucleus of the lateral lemniscus in the pons where the info is sent to the inferior colliculus
where does the trapezoid body send info to
the superior olivary nuclei
what happens at superior olivary nuclei
first binaural interactions occur which are important in sound localisation
where is the second binaural relay station
found in the nucleus of the lateral lemniscus in the pons
Afferent nerve fibres from cochlear ganglion cells
bundled in the cochlear or auditory component of the 8th cranial nerve and terminates exclusively in the cochlear nuclei in a tonotopic organisation
where do fibres that start at apical end of cochlea end
ventrally in the ventral and dorsal cochlear nuclei
where do fibres that start at basal end
terminate dorsally
stellate cells
o Encode sound frequency
o Each cell responds to a characteristic frequency
o Depolarising current injections induce regularly spaced spikes
bushy cells
o Single spike upon current injection
o Encode sound onset and horizontal sound localisation
fusiform cells
Vertical sound localisation
octopus cells
May be involved in recognition of sound patterns
whats the spatio-temporal correlation of signals used for
used to extract localisation information in the horizontal plan
what happens when Acoustic signal from right ear (ipsilateral input) is sent to a 1D spatial array of interneurons
o These also receive an input from the contralateral ear
o An interneuron generates an output signal if both inputs coincide.
Which interneuron fires encodes where the source of the sound is.
o Relies on the fact that electrical signals travel at finite speeds
o To obtain simultaneous arrival of a signal elicited by sound first picked up in the left ear, with a signal induced by the same sound source in the right ear, the left ear signals is made to travel along a long “delay line” which compensates for the later arrival of the right ear signal.
- Very short membrane time constants, otherwise also nearly coincident inputs would result in their activation.
what can cause sensorineural hearing loss
Intense stimulation of the hair cells may cause permanent damage as the stereocilia lose their ability to return to neutral position
o This results in sensorineural hearing loss
what do cochlear implants do
bypass the hair cells by frequency/positiondependent direct extracellular stimulation of afferent fibres
o Requires sound processing and tonotopic arrangement of electrodes
