Properties of visual cells in the lateral eye of Limulus in situ: intracellular recordings.

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Abstract
Two types of potential fluctuations, large and small, recorded intracellularly from photoreceptors in the dark-adapted Limulus eye in situ underlie the dual properties of the impulse discharge of the optic nerve fibers. The small potential fluctuations (SPF).sbd.the well-known quantum bumps.sbd.were normally < 20 mV in amplitude. The large potential fluctuations (LPF) were up to 80 mV in amplitude. LPF appear to be regenerative events triggered by SPF that enable single photon absorptions in retinular cells to fire off nerve impulses in the eccentric cell. In the dark, SPF and LPF occur spontaneously. At low light intensities, LPF are the major components of the receptor potential. At high intensities, LPF are suppressed and SPF become the major components. SPF and LPF together enable single photoreceptor cells to encode an .apprx. 9-log unit range of light intensity. Excising the eye from the animal or cutting off its blood supply generally abolishes LPF and thereby reduces the range of light intensity coded in the optic nerve discharge.