Electrical response of single photoreceptors

The responses to be dealt with are potential changes with latencies of several milliseconds recorded from within single photoreceptors using micropipette electrodes having tip diameters less than 0.1 micron. These responses were obtained from single cones in the carp and single photoreceptors in the octopus. The intracellular placement of the electrode was achieved by mounting the retina on a vibrating table which gives it a large acceleration against the slowly advancing electrode tip. The electrode penetration is signaled by the appearance of a resting potential of - 30 to - 40 mV. In single carp cones, responses to light are hyperpolarizing. The response spectra studied statistically are classified into three groups in terms of wavelengths of maximum sensitivity. These are in close agreement with those of the absorption spectra of single goldfish cones reported by Marks and Mac-Nichol. The response of single octopus photoreceptors, studied as a control, is a graded sustained depolarization, common in most invertebrate photoreceptors. The action spectrum calculated therefrom agrees well with the absorption spectrum of the receptor photopigment. Evidence is provided that each octopus photoreceptor is selectively sensitive to a certain plane of polarized light.