Hyperpolarization

Neurons (namely the Photoreceptors) have a resting potential of about -40 millivolts. This means their inside stands at a slightly lower voltage than their outside when it is not being excited (is exposed to darkness). In order for a photoreceptor to initiate a signal for the Phototransduction Pathway, it must further increase this voltage potential, i.e. hyperpolarize. Note that other neurons may instead instigate a signal via depolarizing, i.e. decreasing the voltage potential.

Photoreceptors also work with a Graded Potential response, meaning the degree of hyperpolarization is proportional to the light the photoreceptors detects (its Quantum Catch). This is opposed to an Action Potential, which is the all-or-nothing “Boolean” signal that activates when the potential passes a certain threshold (e.g. depolarizing to >-30mV).

The graded potential can increase (hyperpolarize) until it reaches about -65 millivolts, at which point the photoreceptor is Saturated. Saturation for extended periods of time is called Photobleaching.