What are the changes in hair cell density and length along the cochlea?

[andrewr]

Hi,

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...

Thanks!

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 )

:)

 

[andrewr]

:zzz:

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.

Secondly:
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.