hearing - from sound to synapses Flashcards
what are the main causes of hearing loss *
loud traumatic sounds eg military, industrial, clubs
200 genetic conditions that can cause hearing problems
infections like meningitis or congenital ones eg rubella/syphilis
drugs that are used for severe heart infections and chemo
aging
what is the frequency range heard by humans *
20Hz to 20KHz
what is frequency *
cycles/second
percieved as pitch
define pitch *
the perception of frequency
most sensitive at 1-3kHz
what is timbre
it is what distinguishes 2 sounds at the same frequency and intensity - ie what distinguishes between 2 instruments playing the same note
how sensitive is the ear
the most sensitive sense
the internal ear can detect movements of size a fration of a nm
what is intensity *
the amount of energy given per second - how many Joules per second pass through 1 square m
percieved as volume
amplitude
what is the range of energy that we can hear
we can hear 1 watt of energy over size 3x UK - lowest intensity
highest intenstity - 12 orders of magntitude higher (this is pain)
ie range 10(power -12)W/sq m -> 1w/sq m
why do we use a decibel scale *
to make the numbers managable
what is teh decibel scale *
instead of measuring the intensity with respect to faintest percievable intesity of sound Io - compare logs - this is the bel scale
multiply everything by 10 = decibel scale
10Log(I) - 10log(Io) = 10log(I/Io) = dB
summarise the path of sound waves from the air to the hair cells *
ear detects sound waves in the air - stimulate the tympanic membrane
vibration go through the 3 small bones to the cochlear
here is the basilar membrane where there are 4 rows of hair cells - the sensory receptor of the internal ear
describe the hair cells in the cochlear *
they take their name from the hair bundle
the hair bundle is a cluster of modified microvilli called stereocilia
there are 3 rows of outer hair cells - 20000 in 3 rows, 1 axon innervates several cells
one row of inner hair cells - 3500 in 1 row - innervated by 10 sensory neurons per cell
describe how the ossicles are involved in hearing *
they transmit the vibration of the tympanic membrane to the cochlear
they match the impedance to reduce loss in energy as the vibration goes from the air to the cochlear
when sound moves from 1 medium to another - eg from air to fluid some sound is transmitted and some is reflected
the impedence measures the reluctance of a system in recieving energy from a source
if the impedence is the same between 2 mediums - all of the sound will be transmitted - the impedence depends on the mechanical properties locally
the position of incus and malleeus can be adjusted by the tensor tympanic muscle and stapedius muscle to control the tension of the tympanic membrane - providing an inbetween inpedance to air and liquid = increased transmission
what is the resonant frequency *
the frequency at which the impedence of the system is minimal - ie transmission of sound is max
what is the structure of the cochlear *
a snail shaped organ
filled with liquid
it is divided longitudinally by the basilar and vestibular membranes into 3 compartments
sound wavees casue these membranes to vibrate
the air cells are on the basilar membrane

what is conductive hearing loss *
when the ear is not capable of transmitting the vibration of sound waves onto the cochlear
what are the causes of conductive hearing loss *
earwax (cerumen)
infections eg otitis
tumours
in children fluid accumulation in the inner ear
perforated tympanic membrane
abnormal growth of bone (otosclerosis) can obstruct ear canal
barotrauma (temporary) - because of increased air or water pressure eg on a plane
congenital malformations
How is the cochlear involved in hearing *
the motion of the stapes generates a pressure difference between the 2 fluid filled chambers of the cochlear - this causes the vibration of the basilar membrane
the relative movement of the tectorial and basilar membrane stimulate the hair cells
what makes up the organ of Corti *
the basilar membrane
tectoral membrane - gelatinous - doesnt vibrate with sound
hair cells
supporting cells - surround the hair cells
spiral ganglion is embedded in modiolus and innervates hair cells
stria vascularis secretes endolymph - high in K low in Na
what is the function of the basilar membrane *
it is an elastic structure - it has heterrogenous mechanical properties at different positions along its length in response to different frequencies - ie vibrates at different positions depending on the frequencies
it breaks complex sounds down - distribute the energy of each component freq along length
sensory receptors (hair cells) are along the length of the membrane to detect frequencies
this is a tonotopic map - higher frequencies vibrate the membrane nearer the base, lower frequencies nearer the apex
describe how the hair cells act as sensory receptors *
the motion of the basilar membrane deflects the hair bundles of the hair cells
the bending of the stereocilia towards the largest stereocillium changes the internal voltage of the cell -> electric signal that travels towards the brain - this is mechano-transduction
how do stereocilia change the voltage in a cell *
they are connected by tip-links - filamentous linkages that act as springs
tip links share their location with ion channels
response currents in cells are because of the opening of these channels
the opening of channels relaxes the tip links -> measured stiffness of the hairbundles becomes -ve = bundle move actively when triggered by stimulus
describe the 4 aspects of the active process with the hair cells *
amplification - living basilar membrane vibrates more than a dead membrane (passive)
frequency tuning - dead mem produces broadd response - not tuned for specific frequency, living selectively amplifies a single frequencies
competitive non-linearity - at low intensities, tehe basialr membrane moves more with increasing intesity - this ability decreases as intensity increases
spontanaeous otoacoustic emission - work produced as hair cells in normal conditions of low levels of sound to counteract the drag in the cochlear
what are the differences between outer and inner hair cells *
more outer than inner
95% afferent projections are from inner - ie sensory
most efferent are to outer - they are involved in the active process and do the work