auditory system

Question 1

step 1

Answer 1

stimulation of hair cells at specific point of basilar membrane activates sensory neurones

Question 2

step 2

Answer 2

sensory neurones carry auditory information down cochlear nerve to cochlear nucleus on the same side

Question 3

step 3

Answer 3

information ascends from each cochlear nucleus to the superior olivary nuclei of the pons and inferior colliculi of the midbrain

Question 4

step 4

Answer 4

inferior colliculi directs a variety of unconscious motor responses to sound

Question 5

step 5

Answer 5

ascending auditory information goes to the medial geniculate body

Question 6

step 6

Answer 6

projections then deliver the information
to specific locations within the auditory cortex of the temporal lobe

Question 7

what is sound

Answer 7

audible variation in air pressure (compression and rarefaction)

Question 8

sound measurement

Answer 8

measured in decibels, every 10 decibels is 10 times louder than threshold

Question 9

2 key properties of sound

Answer 9

frequency (pitch or tone), intensity (amplitude or loudness)

Question 10

middle ear

Answer 10

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

Question 11

acoustic reflex

Answer 11

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

Question 12

the cochlear

Answer 12

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

Question 13

function of the cochlear - step 1

Answer 13

sound wave arrives at tympanic membrane

Question 14

function of the cochlear - step 2

Answer 14

movement of the tympanic membrane displaces the auditory ossicles

Question 15

function of the cochlear - step 3

Answer 15

movement of the stapes at the oval window produces pressure waves in the perilymph of the Scala vestibuli

Question 16

function of the cochlear - step 4

Answer 16

pressure waves distort the basilar membrane on their way to the round window of the Scala tympani

Question 17

function of the cochlear - step 5

Answer 17

vibration of the basilar membrane causes vibration of hair cells against the tectoral membrane

Question 18

function of the cochlear - step 6

Answer 18

information about the region and intensity of stimulation is relayed to the CNS over the cochlear nerve

Question 19

tonotopy

Answer 19

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

Question 20

organ of corti

Answer 20

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

Question 21

hair cells

Answer 21

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

Question 22

mechanotransduction - depolarisation

Answer 22

basilar membrane moves down wards, reticular laminar down and away from modiolus, sterocillia bend in, depolarisation, opposite is true for hyper-polarisation

Question 23

stereocillia

Answer 23

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

Question 24

adaptation

Answer 24

neurones have a limited dynamic range, adapting allows the response to a sustained stimuli but maintains sensitivity to further increases

Question 25

mechanisms for hair cell adaptation

Answer 25

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

Question 26

myosin motors

Answer 26

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

Question 27

cochlear amplifier - outer hair cells

Answer 27

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

Question 28

central feedback to outer hair cells

Answer 28

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),

Question 29

adapt to maintain sensitivity

Answer 29

molecular adaptation of MET channel (calcium), modify cochlear amplifier (olivochoclear system), acoustic reflex (muscles in middle ear)

Question 30

frequency

Answer 30

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

Question 31

auditory pathway

Answer 31

cochlear, spiral ganglion, ventral cochlear nucleus, superior olive, inferior colliculus, medial geniculate nucleus (thalamus), auditory cortex

Question 32

200 Hz

Answer 32

no location on cochlear for less than 200 Hz, frequencies >200 Hz uses tonotopy (mapping), frequencies <200 Hz uses phase locking

Question 33

amplitude

Answer 33

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

Question 34

sound source localisation

Answer 34

horizontal localisation (both ears) - binaural, vertical localisation - monaural, horizontal = compare sounds at two ears using interaural intensity difference and intramural time difference

Question 35

intramural intensity difference

Answer 35

head casts sound shadow, lower intensity received at ear in shadow, comparison at each ear = localisation, higher frequency sounds

Question 36

lateral superior olive

Answer 36

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

Question 37

interaural time difference

Answer 37

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

Question 38

medial superior olive

Answer 38

neurones act as coincidence detectors, synchronous excitation from both ears causes firing, based on delay lines, length of axon introduces compensatory delay