The rod and cone photopigments in the eye consist of two parts: opsin and retinal (an abbreviated name for retinaldehyde, which is vitamin A aldehyde). (In this book the noun retinal, standing for the molecule, is printed in small capital letters to distinguish it from the adjective retinal, meaning “pertaining to the retina.”) The visual receptor molecules span the membranes of receptor discs (see Figure 1a).

Figure 1  Photoreceptors Amplify the Effect of Light
A rod photoreceptor (a) is hyperpolarized when stimulated by light (b). (c) The hyperpolarization is caused by a cascade of neurochemical events that enormously multiplies the effect of each photon captured by a receptor cell.

When hit by light, retinal dissociates rapidly from the opsin molecule. This liberated opsin molecule combines rapidly with many molecules of a protein called transducin (see Figure 1c). The many molecules of transducin, in turn, trigger a cascade of chemical reactions that rapidly closes channels for sodium ions (Na+). Capture of a single light particle can lead to the closing of hundreds of sodium channels in the photoreceptor membrane, thereby blocking the entry of more than a million Na+ ions (Schnapf and Baylor, 1987). Closing the Na+ channels creates a hyperpolarizing potential (see Figure 1b). This vast amplification—a single photon affecting a million Na+ ions—is one reason why our vision works even in very low light levels. On the other hand, that cascade of chemical reactions takes time, which makes vision relatively slow compared, for example, to audition. This is why we cannot see objects that are moving very fast, and why viewing pictures that gradually change 16 times per second or faster gives us the illusion that we’re seeing motion, as in motion pictures and video.

References

Schnapf, J. L., and Baylor, D. A. (1987). How photoreceptor cells respond to light. Scientific American 256(4): 40–47.