Coding of sound level

Auditory afferents have a dynamic range of around 30 dB, beyond which additional increase in SPL has no extra effect. The full range of SPL (0–100 dB) is signaled by afferents with various sensitivities. Cells with similar CF may vary in threshold SPL by 70 dB. Afferent sensitivity associates with its spontaneous rate (SR) of firing. High SR cells have the maximum sensitivity. Low SR cells are least sensitive and encode the frequencies of loud sounds.

The sensitivities of afferents can be altered by efferents that have their cell bodies in the superior olivary complex (SOC) and form inhibitory synapses on OHCs. This neurons modify the profit of the cochlear amplifier, decreasing the sensitivity of type I afferents as sound pressure levels increase.