Sound waves reach the cochlea via the outer and middle ear. As they enter, a sound wave vibrates a specific location along the basilar membrane. The basilar membrane runs the length of the cochlea and is lined with hair cells and neurons which transmit electrical pulses to the brain. Low-frequency stimulation occurs in the uppermost region called the apex, while high frequencies are stimulated at the base. This principal relationship between region-specific stimulation and processed frequency is called the ‘tonotopical principal of the cochlea.’ This is called place coding and is one of two important frequency coding mechanisms which help human beings to hear and differentiate pitch.