Figure 4. (A): Schematic of the organ of Corti, auditory nerve, cochlear nucleus and frequencythreshold tuning curves. High, mid and low frequency tones produce maximum vibration in the base, middle and apex of the cochlea and activate IHCs at the corresponding region of the cochlea. Auditory nerve (AN) fibre contacts a single IHCs in a one-to-one manner. Each AN fibre transmits the activity from a relatively restricted region of the organ of Corti. Axons from each AN fibre bifurcates as it enters the brainstem and axon terminals make excitatory (E) synaptic contacts on a sheet of neurons in the AVCN, PVCN (not shown) and DCN (not shown). High frequency and low frequency neurons in the cochlear nucleus are organized along a dorsal-ventral gradient. Neurons in adjacent isofrequency sheets make inhibitory (I) synaptic contacts. (B): Auditory nerve fibres have frequencythreshold tuning curves with a narrowly tuned low-threshold tip and a high-threshold, broadly tuned low-frequency tail. (C) Some neurons in the cochlear nucleus and higher levels of the auditory brain have excitatory response areas that flanked above and below CF by inhibitory response areas (shaded regions). (D) Some neurons in the DCN and higher levels of the auditory brain have inhibitory response areas that can completely envelop the excitatory response area (from Salvi et al., 1996 with permission).