Phototransduction

Light is defined by its wave properties – frequency ( ) and wavelength ( ) where the velocity c . The intensity of a beam of light is the rate at which energy is delivered per unit area. It is measured in W m 2. Light also possesses particle properties. The quanta are called photons and the energy (in J) of a single photon is given

by E h , where h is Planck’s constant (6.626 10 34 J s).

Sensitivity of photoreceptors

Significant advances in the understanding of molecular mechanisms in biology have, from time to time, emerged from opportunities that allow us to observe unitary events. For example, appreciation of ion channels has benefited from the observation of the opening and closing of individual channels. In a similar way, our understanding of the first steps of the visual transduction mechanism has followed from the investigation of the interaction of light with single photoreceptor molecules. To detect this, it is necessary to illuminate photoreceptors with very low light intensities so as to establish the minimal conditions for excitation. The first attempts were

made in the 19th century, well before it had been realized that light possesses both wave and particle properties. Only then could estimates of the minimal intensity in terms of quanta be made.

With little knowledge of the basic physiology of the human eye, Hecht, Shlaer, and Pirenne in 1942 set about measuring its quantum sensitivity.2 Using darkadapted human subjects responding verbally to precisely calibrated single flashes (1 ms duration at 510 nm, close to the optimal wavelength for vision in dim light), they determined that the eye’s detection limit corresponds to energies, incident at the cornea, in the range 2–6 10 17 J, equivalent to 54–148 photons. To calculate the energy actually absorbed by the retinal receptors, they applied corrections to take account of losses due to reflection (4% at the cornea) and absorption by the ocular medium (50%). Also, at least 80% of the light falling on the retina passes through unabsorbed. After all this, they estimated that number of photons absorbed by the visual pigment was in the range 5–14. Since this is very small in comparison with the number of retinal photoreceptors in the field illuminated, it was concluded that they are of such sensitivity that the coincidence of single photons impinging simultaneously on five cells is sufficient to strike consciousness in a human being.2

The fact that for the absolute visual threshold, the number of quanta is small makes one realize the limitation set on vision by the quantum

structure of light. Obviously the amount of energy required to stimulate any eye must be large enough to supply at least one quantum to the photosensitive material. No eye need be so sensitive as this. But it is a tribute to the excellence of natural selection that our own eye comes so remarkably close to the lowest level.2

Much later, estimation of the quantum sensitivity of individual photoreceptors was performed electrophysiologically using suction electrodes applied to single rod cells obtained from the toad.3 This approach avoids the subjectivity of human psychophysical experimentation and many of the assumptions made concerning the proportion of the light signal that actually reaches