Control of Visual Receptor Cyclic GMP and Visual Excitation

Abstract

Psychophysical measurements have shown that human rods can detect single photons (Hecht et al, 1942). More recent electrophysiological recordings have demonstrated single photon responses (quantum bumps) in amphibian rods (Yau et al, 1977). These hyperpolarizing quantum bumps have a peak amplitude of about 1 millivolt or 1 picoampere and occur against a background of intrinsic rod electrical noise (Baylor et al, 1979). They are caused by a transient reduction in rod outer segment Na⁺ conductance near the site of photon absorption. Quantum bumps follow photon absorption with a constant delay, amplitude and duration illustrated in Figure 1. The onset of conductance change has a characteristic shape mathematically equivalent to a process where a sequence of four consecutive first order chemicals intervenes between photon absorption and conductance change. Two of these four reactions are materially slower than the others (Baylor et al, 1980).