What is the slight negative electrical charge maintained by photoreceptors in the dark?

The slight negative electrical charge maintained by photoreceptors in the dark is typically around -70 mV. This resting membrane potential is due to the permeability of the photoreceptor cell membrane to potassium ions, as well as the activity of ion pumps, particularly the sodium-potassium pump, which helps maintain the ion gradients essential for phototransduction.

In the dark, photoreceptors are depolarized relative to their surrounding environment because the channels for sodium ions are open. This depolarization causes the release of neurotransmitters at the synapse with bipolar cells, effectively communicating the state of the photoreceptors to the downstream retinal cells. When light hits the photoreceptors, it causes a cascading biochemical response that closes these sodium channels, leading to hyperpolarization and a decrease in neurotransmitter release. The -70 mV resting potential is therefore crucial for the phototransduction process, which is the conversion of light into electrical signals in the retina.

This resting membrane potential is slightly less negative than -90 mV, indicating that while there is a steady state of negative charge, it fluctuates based on the activation of specific ion channels during the phototransduction pathway. A value of -30 mV would suggest a significant depolarization that