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Jul17-13, 01:07 AM
I've been studying the cochlea, and how the hair cells (IHCs) receive sound, and some amplify it (OHCs) ; and am trying to get an idea of the density profile of hair cells acting as a spatial filter.
The smallest discrimination in frequency that is positional (not temporal) coded, would be the frequency difference of the position of one hair cell ; so I want to know the profile (density) change of cells along the whole cochlea.
After looking at photomicrographs/sem online, I was only able to measure the spacing between cells in one (arbitrary/unknown) place in the cochlea; and it was about 10microns -- center to center -- axially along a single row of OHC's.
That good enough for a crude estimate test (assuming it was constant everywhere); for a human can discriminate a pitch change of about 5 "cents" near middle "C"; which works out to ~.77Hz ;
I used human cadaver data from ( Greenwood // or // Zwisloski ) to make a curve fit of resonance distance along the cochlea (passive resonance) vs. frequency -- and I get a result that .77Hz is roughly 15microns of distance near middle "C". -- So, about 1.5 hair cells... ( which is what I would expect 1 to 2 cells... )
Now, I just want to improve on that:
Does anyone know of a source that shows cell spacings from one end of the cochlea (basal/oval window) to the other (helicotrema) ?
I know it doesn't vary a lot, eg: it's less than 2:1 ratio ; but I can't seem to find any photographs of more than just one end of the cochlea at a time...
P.S. Here's my estimate of passive resonance frequency as a function of distance along the cochlea.
f( x[cm] ) [Hz] ~= 10**( -0.069*x**3 + 0.3293*x**2 - 1.115*x + 4.522 )
Jul17-13, 02:30 PM
Well, I still haven't found any pictures.... esp. human ones...
but I stumbled across something I didn't know that is suggestive:
Researcher "Barbara A. Bohne" notes in her "morphological analysis of hair cells in the Chinchila cochlea", that across several mammals, the outer hair cell has a systematic length change across the length of the cochlea; eg: longer at the apical end (low frequencies) and shorter at the basal end.
The length of the cells (not the width/spacing), I would estimate, changes by a factor of ~2.25:1 to ~3:1 ; so, if you see that number show up -- it's probably about length and not width spacing.
Larger cells = suggests more energy capability / larger mass / longer power stroke for amplification at low frequency ; eg: the cell size/length change is suggestive of tuning.
She also notes that hair density is supposedly lower in longer cochleas....
But, she notes that the width does not change significantly in her study -- so that it must be cell "spacing" which causes the density change.... hmmm...
Cell sizes in her table, on average, show only a trend of around a 4% change in width from basal to apical measurements in her cochleas.... (less than 1 std deviation in size chg, though, so not significant).
So, I am definitely looking for spacing changes between hair cells, as the cells themselves aren't expected to become wider or narrower.
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