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

The parietal eye of the lizard responds to illumination by sending afferent impulses to the pineal gland during daylight, the photophase. The pineal gland has efferently conducting neurons which are especially sensitive to norepinephrine and whose feedback to the parietal eye enhances its photo responsiveness. During the scotophase, at night, the eye generates afferent impulses to the cessation of light and the pineal efferents are most sensitive to serotonin. Thus, the photo-and chemoresponses of this system of interacting neurons are nearly reversed during the two phases of the daily photoperiod of the lizard.