Neural tuning curves Sensorineural hearing loss

figure 4: neural tuning curve normal hearing.

the traveling wave along basilar membrane peaks @ different places along it, depending on whether sound low or high frequency. due mass , stiffness of basilar membrane, low frequency waves peak in apex, while high frequency sounds peak in basal end of cochlea. therefore, each position along basilar membrane finely tuned particular frequency. these tuned frequencies referred characteristic frequencies (cf).

if sound entering ear displaced characteristic frequency, strength of response basilar membrane progressively lessen. fine tuning of basilar membrane created input of 2 separate mechanisms. first mechanism being linear passive mechanism, dependent on mechanical structure of basilar membrane , surrounding structures. second mechanism non-linear active mechanism, dependent on functioning of ohcs, , general physiological condition of cochlea itself. base , apex of basilar membrane differ in stiffness , width, cause basilar membrane respond varying frequencies differently along length. base of basilar membrane narrow , stiff, resulting in responding best high frequency sounds. apex of basilar membrane wider , less stiff in comparison base, causing respond best low frequencies.

this selectivity frequencies can illustrated neural tuning curves. these demonstrate frequencies fiber responds to, showing threshold levels (db spl) of auditory nerve fibers function of different frequencies. demonstrates auditory nerve fibers respond best, , hence have better thresholds @ fiber s characteristic frequency , frequencies surrounding it. basilar membrane said ‘sharply tuned’ due sharp ‘v’ shaped curve, ‘tip’ centered @ auditory fibers characteristic frequency. shape shows how few frequencies fiber responds to. if broader ‘v’ shape, responding more frequencies (see figure 4).

ihc vs ohc hearing loss

a normal neural tuning curve characterised broadly tuned low frequency ‘tail’, finely tuned middle frequency ‘tip’. however, there partial or complete damage ohcs, unharmed ihcs, resulting tuning curve show elimination of sensitivity @ quiet sounds. i.e. neural tuning curve sensitive (at ‘tip’) (see figure 5).

where both ohcs , ihcs damaged, resulting neural tuning curve show elimination of sensitivity @ ‘tip . however, due ihc damage, whole tuning curve becomes raised, giving loss of sensitivity across frequencies (see figure 6). necessary first row of ohcs damaged elimination of finely tuned ‘tip’ occur. supports idea incidence of ohc damage , loss of sensitivity quiet sounds, occurs more ihc loss.

when ihcs or part of basilar membrane damaged or destroyed, no longer function transducers, result ‘dead region’. dead regions can defined in terms of characteristic frequencies of ihc, related specific place along basilar membrane dead region occurs. assuming there has been no shift in characteristic frequencies relating regions of basilar membrane, due damage of ohcs. occurs ihc damage. dead regions can defined anatomical place of non-functioning ihc (such “apical dead region”), or characteristic frequencies of ihc adjacent dead region.