>> It's true that in very low light conditions, vision takes on a 
>> "speckley" character, presumably due to some combination of small 
>> numbers of photons or small numbers of individual nerve impulses 
>> generating a fairly noisey signal. I'm not sure whether one single 
>> photon is enough to generate a nerve action potential though; maybe it 
>> takes 10 or so?
> 
> Not a bad guess.
> 
> http://math.ucr.edu/home/baez/physics/Quantum/see_a_photon.html

Well, I have a book - a truly awful book written by a guy who thinks 
that molecular biology *proves* that Darwin is wrong. The book spends 
most of its time with broken analogies and flawed logic, but inbetween 
is some really quite interesting stuff about various chemical processes 
of the cell - including the cascade for either retinol or rhodopsin, I 
forget which.

Essentially, when a single molecule of this protein absorbs a single 
photon, it undergoes a structural change which causes it to react with 
another protein, which causes one of the ion pumps on the cell wall to 
shut off, thereby macroscopically altering the ion concentration inside 
the cell, generating a minute but measurable net charge for the entire 
cell, which leads to a nerve impulse being fired... or something like 
that. It's *complicated*!

But anyway, I don't recall the exact pathway off the top of my head, but 
it would seem that you'd need a few photons for one of them to actually 
hit a rod (rather than a cone or some other random cell), in the right 
place to collide with a rhodopsin molecule (instead of some other 
protein floating around in the cytol), and it seems reasonable that 
you'd need to switch off a few ion pumps at once to make an action 
potential big enough to "fire" the neuron... so it seems reasonable that 
some smallish number of photons is required. Biological systems can 
become ludicrously efficient where there's some advantage to it, so it's 
not going to be thousands of photons either...

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

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