BMS248 Lecture 5 - The Auditory System Flashcards
What are the 4 features of sound that need encoding?
- Frequency (Pitch) - Hz
- Intensity (Loudness) - dB
- Onset
- Duration
What is the relationship between frequency and wavelength/energy?
Inverse
What are sound frequency, intensity, onset, and duration encoded by and where? (2)
. Hair cells
2. Nerve fibres
- found in the cochlea (Organ of Corti)
What is sound frequency (pitch), and how can we hear a range of frequencies?
The number of cycles (waves) per second.
Achieved by mechanics of cochlea, and physiology of hair cells.
What is sound intensity (loudness), and how can we hear a range of sound intensity?
The amplitude of the wave from peak-to-peak.
Achieved by the firing rate of many nerve fibres.
Why is a rapid onset important?
For localising different sounds and creating a map of the auditory world around us
What are the 3 chambers that form the cochlear spiral from top to bottom?
- Scala Vestibuli (SV): 0mV
- Scala Media (SM): +80mV
- Scala Tympani (ST): 0 mV
What two things are found in the Scala Media? What innervates these?
- Organ of Corti (-60mV): innervated by the Auditory nerve
- Basilar membrane: sits underneath organ of corti
The SV and ST contain what? What are the compositions of K+, Ca2+, and Na+ ?
Perilymph:
- Low K+
- Normal Ca2+
- High Na+
The SM contains what? What are the compositions of K+, Ca2+, and Na+ ?
Endolymph:
- High K+
- Low Ca2+
- Low Na+
What creates the high K+ concentration in the SM - The Endocochlear Potential? What is the value of this?
Stria Vascularis creates Endocochlear potential of +80mV
What is the Hair Cell resting potential in the Organ of Corti? Therefore what is the driving force into the Hair Cells from the SM?
-60mV
Driving force of 140mV - vital for function
What is the tonotopic organisation of the cochlea? (Inner hair cells at apex vs base of cochlea)
Hair cells at the APEX respond to LOW frequency (pitch)
Hair cells at the BASE respond to HIGH frequency (pitch)
How is cochlear tonotopicity established? (How does specific frequency activate a specific IHC)
By the basilar membrane travelling wave - from base to apex
Sound of one frequency causes maximal movement of the BM at one location (the characteristic frequency location) - CF
E.g. a LOW frequency sound will travel further along the BM and cause maximal movement near the apex - CF location closer to apex
What are the characteristics of the apex vs base of the BM?
Apex: wide + floppy
Base: narrow + stiff
What is the “place-frequency” code?
Sound frequency is encoded in the location of the active IHC, not the firing pattern - this positional information is preserved along the entire auditory pathway.
What is the role of the inner hair cells of the cochlea?
Primary sensory receptors - they encode all of the auditory information and pass it onto the nerve fibres
What channels are found on the tips of the shorter stereocilia on IHCs? What are these connected to?
Mechanoelectrical Transducer Channels (MET) generate an inward K+ current - these are connected to tip links which pull the channels open
At rest what happens at IHCs?
- Slight tension on tip links - some MET channels open
- Resting inward MET current - K+ enters down an electrical gradient
- Large gradient for K+ exit and small gradient for K+ entry
- Depolarised resting potential (-55mV)
- Resting activity in nerve fibres
What happens during Excitatory stimulation of IHCs?
- Large deflection of the hair bundle towards the taller stereocilia
- Increased tension of tip links
- Opening of MET channels - large MET current (K+)
- Depolarisation of hair cell (-30mV) - activates Ca2+ channels
- Increased activity of afferent nerve fibres
- K+ channels repolarise the cell
What happens during Inhibitory stimulation of the IHCs?
- Large deflection of the hair bundle towards the shorter stereocilia
- Tip links slacken
- MET channels close - turns off MET current
- Hyperpolarises cell below resting potential to -65mV
- None or little neuronal activity
- K+ channels open for longer to repolarise the cell - ready for the next cycle
Why does K+ enter down an electrical gradient but leave down a chemical gradient?
Due to the separation between endo- and perilymph.
K+ both depolarises and repolarises the cell:
- Depolarisation by entry
- Repolarisation by exit
What is the role of the outer hair cells of the cochlea?
Function as the cochlear amplifier
OHCs have Prestin in their cell membrane - what does this molecule allow?
Allows the cell to shorten or elongate in response to changes in membrane potential - electromotility
What is OHC resting potential?
-40mV
What happens to OHC length when they DEPOLARISE? (Excitatory)
Shorten (-20mV)
What happens to OHC length when they HYPERPOLARISE? (Inhibitory)
Lengthen (-50mV)
How do OHCs function as the cochlear amplifier?
Combined movement of 3 rows of OHCs increase the movement of the basilar membrane - this increases the stimulation of the IHC bundles at the CF region
