Circadian clock in Limulus brain increases response and decreases noise of retinal photoreceptors.

The sensitivity and structure of ommatidia in the Limulus lateral eye exhibit circadian rhythms. The cyclic changes are mediated by the activity of efferent optic nerve fibres that originate in the brain and terminate in the retina. We report here that the effects of the efferent input to the retina are detectable in single photoreceptor cells. At night the efferent input decreases photoreceptor noise (discrete waves of the membrane potential in darkness) and increases the photoreceptor response to light (amplitude of the receptor potential).