 In human rod cells, these events occur about once every 100 seconds on average, which, taking into account the number of rhodopsin molecules in a rod cell, implies that the half-life of a rhodopsin molecule is about 420 years.The indistinguishability of dark events from photon responses supports this explanation, because rhodopsin is at the input of the transduction chain.On the other hand, processes such as the spontaneous release of neurotransmitters cannot be completely ruled out.