Parietal eye of the lizard: neuronal photoresponses and feedback from the pineal gland.

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Abstract
The parietal eye of the lizard [Crotaphytus collaris] responded to illumination by sending afferent impulses to the pineal gland during daylight, the photophase. The pineal gland had efferent neurons which were especially sensitive to norepinephrine and whose feedback to the parietal eye enhanced its photo responsiveness. During the scotophase, at night, the eye generated afferent impulses to the cessation of light, and the pineal efferents are most sensitive to serotonin. The photo- and chemoresponses of this system of interacting neurons are nearly reversed during the 2 phases of the daily photoperiod of the lizard.