Are neurons sensitive to quantum effects?

[Iamu]

How sensitive are neurons and chemical receptors like olfactory receptors to the chemicals that stimulate them? Can a single molecule stimulate a nerve? How many photons does it take to stimulate a rod or a cone in the eye?

I ask because I'm curious as to whether or not human thought and behavior is generally sensitive to random quantum effects. If a single photon could stimulate a rod, or a single molecule of a neurotransmitter could cause a neuron to fire, then our behavior might be truly random, because it would depend on truly random quantum events, but if it takes thousands of photons or thousands of molecules of a neurotransmitter, I figure our behavior would probably be governed by more-or-less deterministic classical physics.

 

[Cincinnatus]

There are really two questions here.

1. Is the behavior of single neurons ever affected by quantum mechanics?

2. Is the behavior of a larger nervous system affected by quantum mechanics?

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To the first question the answer is certainly yes. Membrane potential is constantly fluctuating due to random openings and closing of ion channels and the like. Mini synaptic events also occur stochastically probably due to quantum fluctuations.

As for the second question. This seems to be untrue. It is likely that just as with any other macroscopic system the basic component parts may feel quantum effects but the assembly as a whole behaves classically. Psychophysicists and behavioral neuroscientists have described and quantified many perceptual behaviors but never encountered one where they could convincingly make the case that something quantum must be going on.

Though, not everyone agrees with me on the above point. See Penrose and Hameroff's work for an opposing view on this.

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Either way, randomness is not the same as free will. I'm not sure anyone has a particularly satisfying answer as to how "free will" gets into a neural system classical or not.

 

[Ygggdrasil]

IIRC, a single photon is enough to alter the membrane potential of a rod cells, so they definitely do have single photon sensitivity. I'm not sure whether olfactory neurons have single molecule sensitivity.

However, also keep in mind that biological systems are very noisy and often the cell employs a variety of mechanisms to cope with that noise. Even though single photon/molecule sensitivity of sensory neurons would lead to stochastic fluctuation in the signals going to the brain, the brain may have evolved control systems to deal with those fluctuations.