The base of the basilar membrane is narrow and stiff, resulting in it responding best to high frequency sounds. The apex of the basilar membrane is wider and much less stiff in comparison to the base, causing it to respond best to low frequencies.This selectivity to certain frequencies can be illustrated by neural tuning curves. These demonstrate the frequencies a fiber responds to, by showing threshold levels (dB SPL) of auditory nerve fibers as a function of different frequencies. This demonstrates that auditory nerve fibers respond best, and hence have better thresholds at the fibers characteristic frequency and frequencies immediately surrounding it. The basilar membrane is said to be ‘sharply tuned’ due to the sharp ‘V’ shaped curve, with its ‘tip’ centered at the auditory fibers characteristic frequency. This shape shows how few frequencies a fiber responds to. If it were a broader ‘V’ shape, it would be responding to more frequencies (See Figure 4). IHC vs OHC hearing loss
A normal neural tuning curve is characterised by a broadly tuned low frequency ‘tail’, with a finely tuned middle frequency ‘tip’. However, where there is partial or complete damage to the OHCs, but with unharmed IHCs, the resulting tuning curve would show the elimination of sensitivity at the quiet sounds. I.e. where the neural tuning curve would normally be most sensitive (at the ‘tip’) (See Figure 5).Where both the OHCs and the IHCs are damaged, the resulting neural tuning curve would show the elimination of sensitivity at the ‘tip.
Thus, damage to the OHCs results in the reduction of sensitivity of the basilar membrane to weak sounds. Amplification to these sounds is therefore required, in order for the basilar membrane to respond efficiently. IHCs are less susceptible to damage in comparison to the OHCs. However, if they become damaged, this will result in an overall loss of sensitivity. Neural tuning curves
Frequency selectivity
The traveling wave along the basilar membrane peaks at different places along it, depending on whether the sound is low or high frequency. Due to the mass and stiffness of the basilar membrane, low frequency waves peak in the apex, while high frequency sounds peak in the basal end of the cochlea. Therefore, each position along the basilar membrane is finely tuned to a particular frequency. These specifically tuned frequencies are referred to as characteristic frequencies (CF).If a sound entering the ear is displaced from the characteristic frequency, then the strength of response from the basilar membrane will progressively lessen. The fine tuning of the basilar membrane is created by the input of two separate mechanisms. The first mechanism being a linear passive mechanism, which is dependent on the mechanical structure of the basilar membrane and its surrounding structures. The second mechanism is a non-linear active mechanism, which is primarily dependent on the functioning of the OHCs, and also the general physiological condition of the cochlea itself. The base and apex of the basilar membrane differ in stiffness and width, which cause the basilar membrane to respond to varying frequencies differently along its length.