Journal of Neuroscience, Vol 7, 2135-2144, Copyright © 1987 by Society for Neuroscience
Control of structural rhythms in the lateral eye of Limulus: interactions of natural lighting and circadian efferent activity
SC Chamberlain and RB Barlow Jr
The structure of the Limulus polyphemus retina changes with the time of
day. The structural changes exhibit a circadian rhythm when the animal is
kept in constant darkness. During subjective night, the aperture of each
visual unit (ommatidium) widens and flattens, allowing more photons to
impinge on the underlying photoreceptors. In addition, the photosensitive
region (rhabdom) of the photoreceptors widens and shortens. During
subjective day, these changes reverse: both the aperture and rhabdom
elongate and narrow, reducing the number of photons caught by the
photoreceptors. The endogenous structural rhythm is mediated by efferent
optic nerve activity generated from a circadian clock located in the brain.
Eliminating the efferent input by cutting the optic nerve blocks the rhythm
and shocking the cut nerve restores it. Natural lighting magnifies the
structural rhythm. Daylight induces photomechanical movements, enhancing
the endogenous daytime changes that decrease photon catch. The onset of
darkness enhances endogenous nighttime changes that increase photon catch.
Cutting the optic nerve blocks all structural changes, both endogenous and
photomechanical. In the animal's natural environment, the effects of
circadian efferent input and natural lighting are thus reciprocal: The
evening onset of efferent activity pushes retinal structure in one
direction; the morning onset of daylight pulls it in the other. The 2
processes, however, are not equivalent. In the absence of cyclic lighting,
circadian efferent input produces an attenuated structural rhythm, but
cyclic lighting alone produces no rhythm.
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