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

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Abstract
The sensitivity and structure of ommatidia in the Limulus lateral eye exhibit circadian rhythms1–3. The cyclic changes are mediated by the activity of efferent optic nerve fibres1 that originate in the brain and terminate in the retina4,5. 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 photo-receptor response to light (amplitude of the receptor potential).