text: To understand the photoreceptor's behavior to light intensities, it is necessary to understand the roles of different currents.
  There is an ongoing outward potassium current through nongated K-selective channels.  This outward current tends to hyperpolarize the photoreceptor at around −70 mV (the equilibrium potential for K).
  There is also an inward sodium current carried by cGMP-gated sodium channels. This "dark current" depolarizes the cell to around −40 mV. This is significantly more depolarized than most other neurons.
  A high density of Na-K pumps enables the photoreceptor to maintain a steady intracellular concentration of Na and K.
  When light intensity increases, the potential of the membrane decreases (hyperpolarization). Because as the intensity increases, the release of the stimulating neurotransmitter glutamate of the photoreceptors is reduced. When light intensity decreases, that is, in the dark environment, glutamate release by photoreceptors increases. This increases the membrane potential and produces membrane depolarization.