auditory system Flashcards
step 1
stimulation of hair cells at specific point of basilar membrane activates sensory neurones
step 2
sensory neurones carry auditory information down cochlear nerve to cochlear nucleus on the same side
step 3
information ascends from each cochlear nucleus to the superior olivary nuclei of the pons and inferior colliculi of the midbrain
step 4
inferior colliculi directs a variety of unconscious motor responses to sound
step 5
ascending auditory information goes to the medial geniculate body
step 6
projections then deliver the information
to specific locations within the auditory cortex of the temporal lobe
what is sound
audible variation in air pressure (compression and rarefaction)
sound measurement
measured in decibels, every 10 decibels is 10 times louder than threshold
2 key properties of sound
frequency (pitch or tone), intensity (amplitude or loudness)
middle ear
malleus (hammer), incus (anvil), stapes (stirrup), these transmit pressure waves in air into waves in liquid, the reduction in surface area from the ear drum to the stapes on the oval window combined with the malleus being longer than the incus forming. lever results in a 22 tomes grater force at the oval window
acoustic reflex
evoked at 20-100 dB, tensor tympani stiffens the ear drum, stapedius pulls stapes away from the oval window, lowers sound transmission by 20 dB
the cochlear
Scala vestibuli is a perilymph filled chamber connected from the oval window to the tip of the cochlear, Scala tympani is a perilymph filled chamber connected from the cochlear tip to the round window
function of the cochlear - step 1
sound wave arrives at tympanic membrane
function of the cochlear - step 2
movement of the tympanic membrane displaces the auditory ossicles
function of the cochlear - step 3
movement of the stapes at the oval window produces pressure waves in the perilymph of the Scala vestibuli
function of the cochlear - step 4
pressure waves distort the basilar membrane on their way to the round window of the Scala tympani
function of the cochlear - step 5
vibration of the basilar membrane causes vibration of hair cells against the tectoral membrane
function of the cochlear - step 6
information about the region and intensity of stimulation is relayed to the CNS over the cochlear nerve
tonotopy
different frequencies resonate at different places on the cochlear (basilar membrane), apex is wide and floppy whilst the base is narrow and stiff so resonates with high frequencies
organ of corti
transducer mechanical stimuli into electrical stimuli, the cochlear duct portion of the organ of corti is filled with endolymph which is high extracellular potassium and low extracellular sodium, inner hair cells transduce sound
hair cells
perform auditory transduction, are analogue, stereocillia that are exquisitely sensitive to movement, synapse (glutaminergic) onto spiral ganglion afferents from the 8th nerve, each inner hair cells synapses onto multiple spiral ganglion afferent
mechanotransduction - depolarisation
basilar membrane moves down wards, reticular laminar down and away from modiolus, sterocillia bend in, depolarisation, opposite is true for hyper-polarisation
stereocillia
stereocillia are connected via tip links, top links are connected to MET channels, at rest there is a small potassium leak, when sterocillia are deflected the MET channels open and close, high potassium endolymph means that MET channels have an Erev of ~ 0mV’: therefore potassium influx at rest causes depolarisation
adaptation
neurones have a limited dynamic range, adapting allows the response to a sustained stimuli but maintains sensitivity to further increases
mechanisms for hair cell adaptation
fast component thought to be due to calcium binding directly to the MET channel reducing sensitivity (Met channel also permeable to calcium), slower component thought to be due to movement of the MET channel to reduce tension on the tip links
myosin motors
place rising tension on tip links, MET channel bound to myosin via adapter proteins, myosin pulls MET channel along actin filaments placing tension on tip link, calcium enters and binds to calmodulin causing myosin to unbind from actin, results in slippage of MET channel reducing tension
cochlear amplifier - outer hair cells
the stereocillia of outer hair cells are connected to tectoral membrane, motor protein prestin contracts outer hair cells moving basilar membrane up or down, amplifies movement of the membrane and therefore inner hair cells, 100 times amplification of movement, movement of outer hair cells generates a noise; used to test newborn cochlear health
central feedback to outer hair cells
olivocochlear system sends axons to outer hair cells, form nicotinic/GABAergic synapses directly onto hair cells, protects against loud sounds (decouple cochlear amplifier), detection and discrimination of sounds in noise; suppress broad band noise (cocktail party effect),
adapt to maintain sensitivity
molecular adaptation of MET channel (calcium), modify cochlear amplifier (olivochoclear system), acoustic reflex (muscles in middle ear)
frequency
auditory nerves are sensitive to sounds of different frequency (tone), high sensitivity; respond to very quiet sounds, different frequencies carried by different nerves (tonotopy), encodes by place (which neurones are firing
auditory pathway
cochlear, spiral ganglion, ventral cochlear nucleus, superior olive, inferior colliculus, medial geniculate nucleus (thalamus), auditory cortex
200 Hz
no location on cochlear for less than 200 Hz, frequencies >200 Hz uses tonotopy (mapping), frequencies <200 Hz uses phase locking
amplitude
each inner hair cell synapses ~10 spiral ganglion fibres, louder sounds = more spiral ganglion cells recruited and eventually fibres with different best frequencies, encoded by rate (number of action potentials for given frequencies
sound source localisation
horizontal localisation (both ears) - binaural, vertical localisation - monaural, horizontal = compare sounds at two ears using interaural intensity difference and intramural time difference
intramural intensity difference
head casts sound shadow, lower intensity received at ear in shadow, comparison at each ear = localisation, higher frequency sounds
lateral superior olive
receives binaural inputs - excitatory from anterioventricular cochlear nucleus, inhibitory from medial nucleus of trapezoid body, if sound comes from ipsilateral side = excitatory input to LSO but weaker inhibitory input from MNTB , gives maximal signal, for midline sounds excitatory and inhibitory inputs are equal so LSO stimulus is zero
interaural time difference
temporal difference between sounds arriving at each ear; phase at each ear will be different, compare timing at each ear, only works for lower frequency where phase locking is present
medial superior olive
neurones act as coincidence detectors, synchronous excitation from both ears causes firing, based on delay lines, length of axon introduces compensatory delay
