What electrical charge do photoreceptors maintain, approximately, in darkness?

In darkness, photoreceptors maintain an electrical charge of approximately -40 mV. This hyperpolarized state is established due to the high levels of cyclic GMP which keeps the cGMP-gated sodium channels open, allowing an influx of sodium ions and resulting in a depolarized membrane potential.

It's important to understand that this resting potential is more positive than that of other neurons, where resting potentials typically sit closer to -70 mV. The -40 mV charge in photoreceptors indicates that they are ready to respond to light stimuli. When light photons hit the photopigments in the outer segments of photoreceptors, a biochemical cascade starts that leads to the closure of these cGMP-gated channels, ultimately causing hyperpolarization and a reduction in neurotransmitter release to bipolar cells.

In summary, the approximate resting potential of -40 mV in darkness reflects the unique ionic currents and biochemical environments within photoreceptors, setting them up for their functional response to light.