The lacertilian parietal eye is a highly organized photoreceptive organ found on the dorsal midline of the head. Developmentally, it is related to the pineal gland but structurally, it resembles the retina of the eye. In contrast to the retina, the parietal eye has a reduced number of components. The photoreceptors synapse directly onto the ganglion cells and there is no convincing evidence of interneurons between these cell types. Retrograde transport studies have revealed two populations of ganglion cells, one in the ganglion cell layer and the other a population of #xAB;displaced#xBB; ganglion cells located in the photoreceptor layer. The pars dorsolateralis of the left medial habenular nucleus is specialized to receive the projection of the parietal eye ganglion cells. A few centrifugal neurons project to the parietal eye and affect its photoresponsiveness. These centrifugal neurons are stimulated differentially by norepinephrine and serotonin.
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