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 (SPFs)--the well-known quantum bumps--were normally less than 20 mV in amplitude. The large potential fluctuations (LPFs) were up to 80 mV in amplitude. LPFs appear to be regenerative events triggered by SPFs that enable single photon absorptions in retinular cells to fire off nerve impulses in the eccentric cell. In the dark, SPFs and LPFs occur spontaneously. At low light intensities, LPFs are the major components of the receptor potential. At high intensities, LPFs are suppressed and SPFs become the major components. SPFs and LPFs together enable single photoreceptor cells to encode approximately a 9-log unit range of light intensity. Excising the eye from the animal or cutting off its blood supply generally abolishes LPFs and thereby reduces the range of light intensity coded in the optic nerve discharge.

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