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How is Frequency Represented in the Auditory Nerve (Georg Von Bekesy…
How is Frequency Represented in the Auditory Nerve
Georg Von Bekesy Discovers how the Basilar Membrane Vibrates
He found that the Basilar Membranes vibration was like a TRAVELLING WAVE - like the motion that occurs when a person holds the end of a rope and "snaps it", sending a wave travelling down the rope
Bekesy found that most of the membrane vibrates, but that some parts vibrated more than others
One portion of the Basilar Membrane appears to vibrate more than the others
This led to the finding that the place that vibrates the most depends on the frequency of the tone
As the frequency of the sound increases, the place on the membrane that vibrates the most moves from the APEX at the end of the cochlea, toward the BASE at the oval window
Because the place of maximum vibration depends on frequency, this means that basilar membrane vibration effectively functions as a filter that sorts tones by frequency
The Cochlea Functions as a Filter
The different places of maximum vibration along the length of the basilar membrane separate sound stimuli by frequency
High Frequencies cause more vibration near the base end of the cochlea, and low frequencies cause more vibration at the apex of the cochlea
Thus, vibration of the basilar membrane "sorts" or "filters" by frequency so hair cells are activated at different places along the cochlea for different frequencies
A map of frequencies was created using guinea pigs and is called a TONOTOPIC MAP
Another way of demonstrating the connection between frequency and place is to record from single auditory nerve3 fibres located at different parts along the cochlea
Measurement of the response of auditory nerve fibres to frequency is depicted by a fibres NEURAL FREQUENCY TUNING CURVE
A neurons frequency tuning curve is determined by presenting pure tones of different frequencies and measuring the sound level necessary to cause the neuron to increase its firing above the 'baseline' or "spontaneous" rate in the absence of sounds
This level is the threshold for that frequency
The frequency to which a neuron is most sensitive is called the characteristic frequency of that particular auditory nerve fibre
The cochlea's filtering action is reflected by the following three characteristics
Each frequency is associated with nerve fibres located at a specific place along the basilar membrane, with fibres originating near the base of the cochlea having high characteristic frequencies and those originating near the apex having low characteristic frequencies
The curves become wider at higher frequencies
The neurons respond best to one frequency
The filtering is most selective at low frequencies than at high frequencies
Outer hair cells: The Cochlear Amplifier
Later researchers realized that one reason for Bekesy's broad vibration patterns was that his measurements were carried out on "dead" cochleas that were isolated from human and animal cadavers
When tuModern researchers used more advanced technology that enabled them to measure vibration in live cochleas, they showed that the pattern of vibration for specific frequencies was much narrower than what Bekesy had observed
What was responsible for the narrower vibration???
It was proposed that a mechanism called the COCHLEAR AMPLIFIER, which explained why neural turning curves were narrower than what would be expected based on Bekesy's measurements of basilar membrane vibration
He also proposed that the cochlear amplifier was an active mechanical process that took place in the OUTER CELLS
It is now known that the major purpose of the outer hair cells is to influence the way the basilar membrane vibrates, and they accomplish this by changing length
While ion flow in inner hair cells causes an electrical response in auditory nerve fibres, ion flow in outer hair cells causes mechanical changes inside the cell that causes the cell to expand and contract
The outer hair cells become elongated when the cilia bend in one direction and contract when they bend in the other direction
This mechanical response of elongation and contraction pushes and pulls on the basilar membrane, which increases the motion of the basilar membrane and sharpens its response to specific frequencies
Basically, the cochlear amplifier greatly sharpens the tuning of each place along the cochlea