Fundamental retinal circuitry for circadian rhythms

Abstract

A remarkable piece of tissue, the retina is a true outpost of the brain, peripheral only for its location on the back of the eye. Downstream of the photoreceptors, the specialized cells which transduce light energy into electric signals then conveyed to the brain by the optic nerve, approximately 60 types of neurons belonging to five classes are arranged in a sophisticated architecture and provide the substrate for extracting information pertinent to contrast, position in space, intensity, chromatic content, and movement. Light reaching photoreceptors and other photosensitive retinal neurons is also coded as temporal information pertinent to the alternation of night and day and to seasonal changes. This information is transmitted to a central clock located in the brain, which tunes biological rhythms to environmental light-dark cycles. Thus, a single sensory organ, the retina, informs the brain of light changes functional to vision, as well as to variations of light occurring in time, providing the core information for the existence of circadian rhythms. Correspondingly, this chapter summarizes fundamental features of retinal organization providing an overview of the main principles according to which the mammalian retina is built and operates as an organ of the visual system. The focus is, however, on retinal neuronal types and circuits forming the substrate for the establishment and function of circadian rhythms. Indications are given for appreciating the elaborate architecture of the whole retinal neurome and the likely existence of retinal channels deputed to code features of the visual scene of so far unsuspected complexity.