hearing Flashcards
what is sound
varying distribution of air molecules in space over time, about a fixed point in space
molecules affect neighbouring molecules, giving rise to propogation of sound wave
how can sound be represented, what are its features
by a sin (as in cos and sin) wave
the period is the time taken for the wave to cross the centre line 3 times (go above and below it once each), or as seen in a sin wave between 0 and 360 degrees
amplitude is distance between top of peak and bottom of trough
frequency = 1/period, measured in Hz
pitch of sound is determined by frequency, loudness is determined by amplitude
what is human hearing range
20-20,000 Hz but most sensitive range is 1,000-4,000Hz
how is sound pressure calculated
sound pressure level (dB)= 20 x log (Pt/Pr)
Pt is test pressure (the sound pressure)
Pr is the reference pressure (20microPa), 1Pa = 1N/M^2
Pr is the threshold (just audible) sound pressure
if sound is 10 times Pr then it is 20dB, if it is 100 times Pr then it is 40dB
describe anatomy and function of the outer ear
outer ear is known as auricle or pinna
consists of:
the helix (large back part of the ear)
the lobule (ear lobe)
the concha (hard protrusion right in front of the hole)
external auditory meatus
function of the auricle: gathers sound and thus transforms sound pressure at the tympanic membrane, amplifies sound and filtration of sound which aids in sound localisation
describe anatomy and function of the middle ear
function: transfers sound energy from the external environment to the cochlea
comprises of the tympanic membrane (the ear drum), the ossicles, the middle ear muscles and the eustachian tube
the ossicles reside just inwards and above of the tympanic membrane
the eustichian tube is inwards of the tympanic membrane and goes downwards
middle ear muscles in order of outwards to inwards and downwards: malleus, incus, stapes and tensor tympani
the function of the ossicles is lever action and pressure amplification
reflex contraction of middle ear muscles can reduce ossicle movement which has protective function
what is impedence matching
the difference in area between tympanic membrane and oval window compensates for impedance mismatch between air and cochlear fluid, provides up to 30dB sound increase at 2.5Khz, since tympanic membrane has higher area the pressure will be higher at oval window
what does damage of different parts of ear result in
middle ear damage is termed conductive deafness
inner ear damage is termed nerve deafness
what is function of the inner ear
function of the cochlea (inner ear): splits sound into simple components, amplifies signal, transduces mechanical vibrations into action potentials
what is the basilar membrane
the basilar membrane is tuned (has a inwards helical shape (like snail shell) which gets thinner as you go in) this allows cochlea to seperate frequencies, since at the apex the membrane is wide thin and flexible, at the base (outside) it is narrow, thick and stiff, there is a place on the BM at which each frequency is processed (place principle)
the thinner the membrane, the lower the frequency sound it detects, so the base detects 20,000 Hz and apex detects 20Hz
travelling wave displaces chochlear fluids which cause basilar membrane to move up and down
where are sensory hair cells located in the ear
in the organ of corti, in the inner ear
describe the inner ear anatomy
primary afferent neurones of cranial nerve 8 synapse onto inner hair cells (IHCs)
in the middle of inner ear is the basilar membrane, it seperates on one side the scala tympani and on the other side the scala vestibuli and scala media
scala media has contact with basilar membrane, scala vestibuli does not, it is seperated from scala media by reissner’s membrane
in scala media is the tectorial membrane which contacts the inner and outer hair cells
what effects inner hair cells receptor potentials
basilar membrane movements drive receptor potential
describe the pattern of innervation in the inner ear
type 1 auditory nerve fibres are connected to IHCs, they have the same tuning as the part of the basilar membrane to which they are connected
tonotopic map is carried into the central auditory system
selective innervation pattern of afferent neurones allows for chochlear frequency map
how many auditory nerve fibres are there in humans, how are they distributed
roughly 30,000
95% are type 1; myelinated, synapse with IHCs, there are up to 20 afferents per IHC however each afferent only synapses one IHC
5% are type 2; unmyelinated, synapse onto OHCs, one fibre synapses between 5-100 OHCs
