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
what is electromotility *
the cell bodies of the outer hair cells shorten when their internal voltage is changed
due to the reorientation of the protein prestin
may be the orrigin of the cochlear amplificationand otoacoustic emissions
describe the connection of the hair cells to cochlear ganglion *
there are synapses
there are many channels - in case one breaks
the ganglion cells in a particualr area of the spinal ganglion respond best top the resonant frequency of teh basilar membrane in the same area - this is the tonotopic map
what is sensorineural hearing loss *
when the problem is rooted in the sensory apparatus of the inner ear or in the vestibulocochlear nerve
what is the cause of sensineural hearing loss *
load noises can cause temp/permenant damage
many genetics affect the organ of Corti
aminoglycoside AB - toxic to hair cells
congenital disease- rubella/toxoplasmosis
acoustic neuroma (tumour on the cochlear nerve)
meniere’s disease
hereditory disorders
viral infection
injury to central auditory pathway
aging (presbycusis)
malfunctioning of auditory pathways - demyelination and blast injuries can affect the balance between inhibition and excitation in the superior olivary complex
how can you treat hearing loss
bypass the hair cells and stimulate the nerve cells directly - ie detect cells, break them into the constituent frequencies and send signal to auditory nerves by antennas
elongated coil inserted into cochlear - pairs of electrodes corresponding to single frequencies
describe the organisation of the ventral cochlear nucleus *
nerve fibres arranged tonotopically
low frequencies ventrally, high frequencies dorsally
describe the role of the dorsal cochlear nucleus
it locates sound in a vertical plane
high frequency sounds produce interference with the body ie when the wavelength is the size of the head and external ear
the ears detect and react to these differently depending on their spectral cues ie when sounds come from different directions and have assymetrical shape
what is the role of the superior olivary complex *
compares the bilateral activity of the cochlear nuclei
localise sounds in the horizontal plane
medial superior olive - computes the interneural time difference - sounds at near ear detected first, a map of interneural delay can be formed due to delay lines - neuron fires when stimulated by both ears at the same time
lateral superior olive - detects differences in intensity between the 2 ears >2kHz due to head size - this is interneural level difference because of head shadow. excitation that arrives ipsilaterally must arrive at same time as inhibition contralaterally - inhib signal carried by large axons with large synapses (the large calyxes of Held) axons for excitation are smaller and conduct more slowly
what is the connection between the lateral superior olive and the cochlear *
superior olivary complex neurons send feedback to hair cells - to afferent fibres and to outer hair cells
this modulates the amplification done by the cochlear - protect the ears
describe the inferior collicus *
made of the central nucleus (tonotopically mapped), dorsal cortex, external cortex
carries information about sound location
all ascending pathways converge here - sensitive to complex features
involved in precedence effect - brain filters out sound not necessary to localise it ie sounds of loer intesnity withing 30sec window
involved in reflexes - cause head to turn
describe the superior collicus *
all ascending pathways converge here too
auditory and visual maps merge
neurons are tuned to respond to stimuli with specific sound directions
auditory map here is fundamental for reflexes - head turn in direction of sound
describe the auditory cortex *
neurons respond to complex sounds
- primary auditory cortex located in superior bank of the temporal lobe - central area of the auditory cortex - tonotopically mapped - loudness rate and freq modulation are mapped here - reflexes for complex tasks related to gaze control - can be trained to help dyslexia etc which have impaired auditory function
describe the anatomy of the ear
outer - auricle and external acoustic meatus
middle - Air-filled chamber in bone, lying between tympanic membrane laterally, and oval and round windows medially
inner - cochlear and organs of balance, hair cells
describe how the middle ear can be protective *
Reflex contraction of tensor tympani and stapedius muscles reduces amplitude of vibrations passing through ossicles
Protects against natural sounds but maybe not against man-made sounds
Auditory tube allows equilibration of air pressure on either side of tympanic membrane
summarise how the middle ear is involved in amplication *
achieved by the lever system of the ossicles
also the ratio of the tympanic membrane to the oval window
what is sound
pressure wave in air
function of the outer ear
collects and conducts sound waves to tympanic membrane
describe the transduction mechanism in the cochlear *
basilar membrane vibrates in response to sound
upward movement displaces cilia away from mediolus - K channels open - K enters from endolymph - hair cells depolarise
downward movement displace cilia to modiolus - K ion channels close - hair cell hyperpolarises
this requires the stria vascularis to maintain the endolymph at +80mV by stria vascularis
depolarisation opens Ca channels in body of hair cell
glutamate released from base depolarises axon of spinal ganglion cell -> AP
summarise the auditory pathways *
complex bilateral pathways through the brain
colateral pathways go to reticular formation and cerebellum
lateral inhibition in ascending pathways enhances resolution of similar frequencies
descending pathways provide feedback at all levels
primary auditory cortex is divided according to frequency response
cells respond to specific features of sound - on/off, duration, repitition, intensity, rising and falling frequencies, animal vocalisations
secondary cortex - neurons respond to more compelx sound patterns
describe the mechanism of amplification *
conduction through middle ear amplifies sound by 30db
lever system of articulated ossicles and ratio of area of tympanic membrane to oval window
