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تسجيل

Ionic Basis of Photoreceptor Potentials

المؤلف:  Kim E. Barrett, Susan M. Barman, Heddwen L. Brooks, Jason X.-J. Yuan

المصدر:  Ganongs Review of Medical Physiology

الجزء والصفحة:  25th E, P182

2026-10-17

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Na+ channels in the outer segments of the rods and cones are open in the dark, so current flows from the inner to the outer segment (Figure 1). Current also flows to the synaptic ending of the photoreceptor. The Na+, K+ ATPase in the inner segment maintains ionic equilibrium. Release of synaptic transmitter (glutamate) is steady in the dark. When light strikes the outer segment, the reactions that are initiated close some of the Na+ channels, and the result is a hyperpolarizing receptor potential. The hyperpolarization reduces the release of glutamate, and this generates a signal in the bipolar cells that ultimately leads to action potentials in ganglion cells. The action potentials are transmitted to the brain.

Fig1. Effect of light on current flow in visual receptors. In the dark, Na+ channels in the outer segment are held open by cGMP. Light leads to increased conversion of cGMP to 5′-GMP, and some of the channels close. This produces hyperpolarization of the synaptic terminal of the photoreceptor.