what are the physical characteristics of sound?
- amplitude: difference in air pressure from the baseline to the peak of a wave. measured in decibel (dB).
- wavelength: distance from one peak wave to the next
- frequency: number of complete waves or cycles that pass by a given point in space every second.
what are the psychological dimension of sound?
- loudness/intensity: determined by amplitude. greater amplitude = louder
- pitch: how high or low a tone sounds. determined by frequency. high frequency = high pitch, low frequency = low pitch
- timbre: quality of sound and is related to characteristics of sound wave.
describe structure of auditory system.
- outer ear: pinna and auditory canal.
- middle ear: tympanic membrane, ossicles
- inner ear: cochlear
how does sound travel through auditory system?
sound vibrates through the air and reaches the eardrum. the malleus, the incus and the stapes vibrate and vibration spreads to cochlea.
vibration of air is converted to movement/vibration of fluids in the cochlea and this is captured by hair cells. this physical vibration is transduced to neural energy. signals to cochlear nucleus via the auditory vestibular nerve.
what is the outer ears role in how sounds travels?
- acoustic energy in the for of sound waves, passes pinna, ear canal. sound waves hit eardrum causing it to vibrate like a drum.
how does sound travel through middle?
three bones (malleus, incus, stapes) are moved by vibration, changing acoustic energy to mechanical energy. these middle ear bones mechanically amplify sound and compensate mismatched impedance
how does sound travel through inner ear and central auditory nervous system?
- when the stapes moves in and out of the oval window of the cochlea, it creates a fluid motion, hydrodynamic energy.
- causes membranes in organ of corti to shear against the hair cells. this creates an electrochemical signal which is sent via the auditory nerve to the brain.
describe transduction through middle ear?
fluids in the cochlear and it takes a lot of force to move so requires a lot of energy.
middle ear is responsible for impedance matching. excess energy is required to move cochlear fluids.
flat bottom part of stapes moves in and out like a piston at the oval window transmitting the sound vibrations.
what is impedance matching?
function of ossicles. 99.9% of sound is reflected due to high impedance of fluid in the cochlea. middle ear bones overcome the loss of sound by increasing sound pressure (+34dB).
what are the three mechanisms for impedance matching?
- area ratio of ear drum to the stapes footplate (+26dB)
- lever action of the ossicles (+2 dB)
- buckling of ear drum (+6dB)
what are the components of inner ear?
- cochlea: part of the auditory system
- labyrinth: part of vestibular system
- both structures contain hair cells and contribute to electrical signal that goes though cranial nerve 8. the stimulus for each is different. vestibular system detects movement and gravity.
anatomy of cochlea
- there are three fluid filled cavities (scala media, scala vestibuli, scala tympani)
- reissners membrane separates s.v from s.m
- basilar membrane separates s.m from s.t.
- transduction through organ of corti hair cells. from basilar membrane to tectorial membrane.
- scala media is smaller chamber between two other scalas. it contains organ of corti where hair cells reside. the organ sits on basilar membrane. above hair cells are on tectorial membrane.
- fluid from scal m doesn’t mix with other two.
- s.t and s.v connect at helicotrema.
- fluid in two chambers identical and called perilymph.
how does signal flow through cochlea?
when stapes pushes on oval window, the fluid goes through round chambers, through scala v and scala t. it then travels through opening at far end of cochlear.
describe organ of corti and associated structures
two types of hair cell separated by rods of corti.
- inner (3500)
- outer (15000-20000)
they synapse on bipolar neurons with cell body in spiral ganglion.
describe hair cells
- stereocilia are embedded in tectorial membrane. they get compressed and move in one direction or another. the kinocilium is the end of the stereocilia that provides with with direction.
- hair cells send signals to cochlear nucleus but also receives input from brainstem.
hair cell transduction
- the sensory neurons send signals to cochlear nucleus and synapse onto individual hair cells. hair receives information from brainstem and this info allows hair cells to adapt to sounds.
- efferent signal comes in and helps the hair cells adjust to the response.
- information is sent to the brainstem via afferent endings and info is received by efferent nerve endings.
- tip links connect hair cells.
- K channels are at the roof of stereocilia
- each cilium is linked by a protein structure and form a lid and prevents entry of K into channel. when soundwave arrives, the stereocilia will move and be tilted and this means the lid will move and the channel will be open and K enters.
- when stereocilia moves away, tip links will close channel.
- entry of potassium ions causes depolarisation of hair cell, influx of ca2+ and release of glutamate and is released by afferent nerve endings.
- hair cells dont cause depolarisation but membrane can be more/less depolarised or hyperpolarised and that will govern how much glutamate is released.
- when stereocilium move towards kinocilium, the membrane is depolarised to great extent and number of APs recorded in sensory neuron is much higher. when they are tilted in opposite direction the number of APs recorded in sensory neuron are fewer.
- tectorial membrane vibrates. the hair cells’ cilia bend and depedning on how they bend, the hair cells release neurostransitetr. transmitetrs released in the hair cell and captured in nerve fibres. the neural energy is sent to the brain.
why is the hair cell current inwards?
K concentration is high in extracellular fluids and low intracellularly, driving force into cell. channels present at apex of stereocilia and when they are open in these conditions the resting membrane of hair cells is positive.
what is the importance of recycling K
- K channels, transporters and the gap junctions networks composed of 5 different connexins are critical for reconstituting K+ in the endolymph.
- mutations in many of the genes present at scala media can result in deafness.
what is the effect of sound on basilar membrane?
- the b.m is flexible and vibrates in sync with fluid motion.
- the apex is wide and floppy, the base is narrow and stiff.
- sound travels from base to apex.
- high frequency sound is required to cause displacement at the stiff end.
- vibration of the basilar membrane -> mechanical stimulus -> transduction in cochlear hair cells
what does the auditory nerve (VIII) do?
Cranial nerve VIII brings sound and information about one’s position and movement in space into the brain.
describe tonotopy in the auditory system?
- tonotopy is the spatial representation of sound frequency around basilar membrane.
- the hair cell is displaced and releases glutamate onto sensory neurons which send their axons to different portions of the cochlear nucleus.
what do the two hairs cells (outer and inner) do?
- inner: main source of afferent signal in auditory (VIII) nerve. multiple outer HC innervated by single afferent.
- outer hair cells: primarily get efferent inputs. control stiffness amplify membrane vibration.
what do the outer hair cells do?
- called cochlear amplifier
- outnumber inner HC
- brainstem sends efferents to outer hair cell layer causing motor protein to change their length and causes them to expand and contract in response to signals received by nervous system.
- motor protein composed of “prestin” present in membrane and can change the length of the cell - shorten when depolarised, lengthen upon hyperpolarisation
- when it becomes larger it pushes the tectorial membrane away so the tilting of stereocilia is diminished.
- when contracted, the basilar and tectorial membrane are closer together which accounts for intensity of signal.
- outer hair cell can alter perception of sound. called gain of amplifier. some molecules like furosemide act as inhibitors of motor protein. it is clinically used when there are disturbances of ionic composition. . it can be ingested causing auditory difficulties as it inactivates prestin.
how does auditory nerve (VIII) transmit signal?
- neural information from inner hair cells is carried by cochlear division of the VIII cranial nerve.
- spiral ganglion is where cell bodies of bipolar neurons sit. these neurons send one process to hair cells and the other process will form part of the cranial nerve VIII which all goes to the cochlear nucleus.
