Medicine Notes Neuroscience 1 Notes
Contains notes for the neuroscience module covered in Michaelmas Term...
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How does the cochlea respond to different frequencies of sound?
Intro
Frequency of a sound refers to the number of cycles of sound waves in one second
Humans the audible range is between 20Hz to 20000 Hz
Ability to detect these different frequencies allows humans to distinguish between different pitches of sound with a higher pitched sound having a higher frequency
Natural speech, sounds contain several different frequencies which vary in amplitude so ability to resolve individual frequency components are needed to understand speech
Basilar membrane
Key structure of the auditory system that plays a key role in distinguishing between different frequencies is the basilar membrane
Basilar membrane is found in the cochlea and separates the scala media from the scala tympani
Properties of this membrane were first discovered by von Bekesy who observed the motion of membrane by looking at the movement of apex of the cochlea of human cadavers in response to high intensity sounds using light microscopy. He observed the following things
Motion of the vibrating basilar membrane was in the form of a travelling wave with oscillations at the same frequency as the sound
The wave always starts at the base and propagates to the apex
Peak displacement of the membrane is related to the frequency of the sound
Observations are supported by the structure of membrane- which is made up of elastic fibres
Thick, floppy apex
Thin stiff base
From this von Bekesy concluded that the basilar membrane creates a topographical map across a spectrum of frequencies
Peak displacement for high frequencies occuing at the base
Peak displacements for low frequencies occur at the apex
So in complex sounds- each frequency component establishes a peak displacement at a point on the basilar membrane that is almost completely independent to the other components
Hair cells
The hair cells of the auditory system are embedded in the basilar membrane and are of two types
Inner
Outer
Hair cells are also topographically organised as each hair cell is most sensitive to a particular frequency which is known as the characteristic frequency
Frequency sensitivity of hair cell is displayed as a tuning curve- constructed by frequency vs threshold experiments for certain points of the cochlea
Intensity of stimulation is adjusted for each frequency until a response reaches a predefined magnitude
Tuning curve is V shaped- tip of curve- characteristic frequency
DRAW GRAPH IN EXAM
Clinical
In the presence of aminoglycoside antibiotics such as gentamicin โ inhibits the 30S ribosome in the mitochondria- ototoxic damage
Specific frequency is reduced but still exists which shows there are two components to the tuning of a hair cell
Acellular โ basilar membrane acting as a mechanical analyser distributing specific stimulus energies arrayed along its length
Cellular
Inner hair cell
Inner hair cells mostly contribute to hearing as 90% of afferent neurons that form the spiral ganglion arise from the inner hair cells
The cellular component of a frequency tuning in these cells occurs by two mechanisms
Mechanical properties of the sterocilia that make up the hair bundle
Depending on the flexibility and the mass each hair cell has a resonant frequency
Hair cells that respond to low frequency sound waves have longest bundles whilst those that respond to high frequency of sond waves have shortest bundles
Electrical properties of the individual hair cells
When the cell is depolarised by a square current pulse, the membrane voltage showed sinusoidal oscillation and responded most strongly to sound that had same frequency at which the cellโs membrane potential resonates
This occurs as there is a cycle of changes in the membrane...
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Contains notes for the neuroscience module covered in Michaelmas Term...
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