Light-induced fluctuations in membrane current of single toad rod outer segments

VERTEBRATE photoreceptors respond to illumination with a reduction in the steady current of Na ions which in darkness flows inwards across the outer segment membrane; this results in hyperpolarisation of the cells¹. The light responses of the receptors can be studied by measuring extracellular voltage gradients² or by intracellular recording¹, but these methods can only provide information averaged over many photoreceptors owing respectively to high extracellular conductivity and to the presence of electrical coupling between cells^3-6. This averaging and the presence of dark voltage noise in photoreceptors^6,7 have prevented observation of the electrical effects of individual photoisomerisations. To try to record these elementary events and to localise the source of the dark noise, we have developed a method for recording the membrane current of a single rod outer segment. The technique is based on that used by Neher and Sakmann ⁸ on muscle fibres.