Model of Phototransduction in Retinal Rods

Phototransduction is the process by which the absorption of a photon by a molecule of rhodopsin in a photoreceptor is transformed into an electrical signal. This electrical signal is caused by the modulation of a current, usually called photocurrent, flowing across the light-sensitive channels in the plasma membrane of outer segments. Na+ carries the large fraction of this current, but Ca++ also contributes by about one fourth of the photocurrent. Phototransduction occurs through changes in the concentration of a substance, usually referred to as the internal messenger, recently identified as guanosine 3′,5′-cyclic monophosphate (cGMP) (Caretta and Cavaggioni 1983; Fesenko et al. 1985; Yau and Nakatani 1985; Haynes et al. 1986; Zimmermann and Baylor 1986). It is now known that Ca++ is not a positive transmitter (Matthews et al. 1985; Lamb et al. 1986), as originally proposed (Hagins 1972), but plays a crucial role in light adaptation (Torre et al. 1986;...