 Hello and welcome to Physiology Open. Any pressure wave or sound has two characteristics. Its frequency which accounts for the pitch of the sound and its amplitude which accounts for loudness. So we need to quote for these characteristics of the pressure wave in form of action potential for the perception of the sound. Now we know that sound waves travel from outer to middle ear and then to inner ear. When waves cause the movement of steps in middle ear, it leads to movement of the oval window. Now, as you might be aware that oval window actually communicates with the scalar vestibuli of cochlea. So here we have a diagram where we have opened up the coil cochlea showing its different regions. So oval window movements lead to generation of some waves in the peri-lymph in scalar vestibuli. So this generation of waves in a scalar vestibuli is known as traveling waves. This causes depression in the basilar membrane which again vibrates and leads to stimulation of the receptors. So you can very well guess that wherever this traveling wave reaches maximum height, the vibration of the basilar membrane in that region of cochlea will be maximum and hence stimulation of receptors in that region will be maximum. So these traveling waves reach a maximum height at a particular place in cochlea depending on the frequency of the pressure wave and then they wane off. Now the high frequency sound reach the peak near the base of the cochlea while low frequency sounds reach the peak near the apex of cochlea. See the part of the cochlea near the oval window is known as base while the last part is known as apex. So wherever the maximum height of the wave is reached, it sets up maximum vibrations there. So high frequency will set up maximum vibrations of basilar membrane near the base while lower frequencies at the apex of cochlea. Moreover, the basilar membrane of cochlea has a structure which promotes a response to this type of stimulus. At the base, the basilar membrane is taut and thin while at the apex, it is loose and thicker. So this portion is maximum sensitive to high frequency waves and sensitivity changes to lower and lower frequency as we move towards the apex. So information from a particular region of cochlea represents a certain frequency. This representation of frequency in a space that is along the region of the basilar membrane is known as tonotopy. Now this tonotopy continues throughout the auditory pathway until the auditory cortex. So there are separate lines for different pitch or frequency of the sound coming from different place in cochlea. So pitch is encoded by receptor stimulation at a particular place in cochlea, this is known as place theory of hearing. But you see the ear has to encode frequencies from 20 hertz to 20,000 hertz. So imagine the entire range being represented in this small area of basilar membrane. Basically, what I am suggesting is that place theory is not enough to encode all the frequencies. Instead, pitch coding also occurs by two more mechanisms which are known as frequency or temporal theory and another one is volley theory. According to frequency theory, a pressure wave will generate action potentials in the afferent with each cycle of the sound wave. So suppose this is the pressure wave with each cycle, a set of action potentials are generated. So this pattern depicts a frequency of sound and this pattern is recognized by auditory cortex as a particular frequency. But you see a single neuron cannot fire faster than 1000 action potentials per second. So closer to this frequency, it will start getting action potentials like this with no pattern, isn't it? Obviously, that means frequencies faster than 1000 hertz have to be encoded by some other means. So that is by a volley principle. In a volley principle, many neurons combine and quote frequency of the sound. So suppose this is the pressure wave. First neuron will respond something like this. The second neuron to the next part of the wave in that pattern. And then third neuron to next part. All these information from different neuron reaches to auditory cortex, which sees this pattern of firing of many neurons together and recognizes it as a particular frequency. So the limitations of frequency theory caused by firing of single neuron is overcome by many neurons coming together and firing in a particular set. So pitch of sound is encoded by place theory, frequency or temporal theory and by volley principle. OK, what about the loudness of sound? See, for a particular pitch, loudness of sound is coded as number of action potentials in afferent nerve. So pitch is from where the action potential is coming from. That is place theory or in what set pattern it is coming from. That is a frequency and volley theory. And loudness is how many action potentials are coming from a particular place. Well, that's how pitch and loudness of sound are coded. Thanks for watching the video. If you liked it, do like and share the video. And don't forget to subscribe to the channel Physiology Open. Thank you.