Visual Cycle

The cycle that a Retinal molecule – the Chromophore in opsins – travels between being excited by a Photon, transmuting, then being refreshed in an RPE cell. The simplified steps of the Visual Cycle are:

  • The 11-cis-Retinal (bent tail) is nestled in an Opsin protein.
  • It is struck by and absorbs a photon, transmuting into all-trans-retinal (straight tail)
  • The spent retinal is ejected from the Opsin, travels through the Photoreceptor cell, and is transferred to an RPE cell.
  • Meanwhile, the Phototransduction Pathway carries the electrical visual signal to the brain.
  • In the RPE cell, a series of enzymes transmute the molecule back to 11-cis-Retinal.
  • The “charged” Retinal moved back to the Photoreceptor outer segments
  • The Retinal binds with an empty Opsin and waits to be excited by another Photon.
Path of Retinal in the Visual Cycle