Efferent innervation to Limulus eyes

Abstract

The visual system of the horseshoe crab Limulus polyphemus has been important model for studies of basic aspects of vision. Experiments investigating phototransduction and contrast enhancement by lateral inhibition using Limulus eyes are famous. Recent studies with this animal are providing new information on still another feature of the visual system common to many species: the modulation of retinal function by efferent innervation from the brain. Retinal efferent systems have been identified in a variety of animals from invertebrates to mammals (see Ref. 1 for selected references) and of these the efferent system projecting to Limulus eyes is now the most thoroughly understood. Studies with Limulus have revealed that efferent innervation controls circadian changes in the sensitivity of the eye, and influences the eye's integration of visual information, adaptation to ambient light levels, and renewal of photosensitive membrane. Postsynaptic targets of efferent projections have been identified as has an efferent fiber neurotransmitter. The retinal efferent system in Limulus has opened the way for detailed studies of mechanisms underlying the central control of visual function.