The organ of Corti is made up of sensory cells called hair cells. These hair cells are in contact with the fibres of cochlear nerve (a part of 8th cranial nerve). More vibrations cause (sound waves at higher frequency) more hair cells fire in that region.
The auditory nerve reaches the nuclei in the brainstem and projects to the inferior colliculus in the midbrain and the medial geniculate nucleus in the thalamus.
Thus, the blank can be filled with inferior colliculus; medial geniculate nucleus.