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.