From The Eyes Of Darkness

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Birth of a hypothesis

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Chapter Analysis of The Eyes of Darkness

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The eyes of mammals reveal a dark past

Donec scelerisque, urna id tincidunt ultrices, nisi nisl lacinia mi, at pellentesque enim mi eu felis. Nullam malesuada egestas tincidunt. Pellentesque nec risus dui. Both cones and rods participate in dark adaptation, slowly increasing their sensitivity to light in a dim environment. Cones adapt faster, so the first few minutes of adaptation reflect cone-mediated vision.

Rods work slower, but since they can perform at much lower levels of illumination, they take over after the initial cone-mediated adaptation period.

"Eyes of Darkness"

This is actually a general feature of many sensory systems: if a sensation relies on stimulation of more than one type of receptor cell, the most sensitive receptor type at any given time is the one that mediates sensation. So, what happens in the cones and rods during dark adaptation?

The Eyes of Darkness

To attempt to answer this question we need to first consider the mechanism underlying cone and rod function. The only light-mediated event in vision is the interaction of visible light photons with protein molecules in the photoreceptors known as cone or rod opsins, which are also known as "visual pigments. Rods, on the other hand, have a single form of opsin called rhodopsin.

In vertebrates, all photoreceptor opsins contain a molecule called retinal, or retinaldehyde. The ultimate source of retinal is dietary vitamin A; this is the reason why an early sign of vitamin A deficiency is night blindness.

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The absorption of a photon by a molecule of retinal induces a change in the molecular configuration of its hydrocarbon chain—a process known as photoisomerization. After photoisomerization, opsin becomes chemically active and is able to initiate a series of biochemical events in the cones and rods that ultimately lead to a change in the number of glutamate molecules released by the photoreceptor. Glutamate, an amino acid and neurotransmitter, acts as a messenger that conveys to other retinal cells information about light stimulation of photoreceptors.

Following its activation by light, an opsin molecule releases its transformed retinal molecule. Free opsin—an opsin that has released its retinal molecule—is likely to be the molecule responsible for the retina's reduced sensitivity to light.

The Eyes of Darkness Book Summary and Study Guide

Dark adaptation is required for the recovery of this sensitivity. It is accomplished through a restoration of the original biochemical configuration of visual pigments. This involves a recombination of free opsin with an untransformed retinal—which results in a regeneration of cone opsins and rhodopsin. The rate of delivery of retinal to the photoreceptors is the probable reason for the relatively slow rate of dark adaptation.

Since this process evolved to adapt to the slow changes in illumination that occur during the transition from day to night, the rate of change in sensitivity is quite adequate to compensate for changes in natural lighting. Many diseases that interfere with the complex molecular mechanism underlying dark adaptation lead to night blindness. In addition to vitamin A deficiency, which is the most common cause of night blindness in the nonindustrialized world, inherited eye diseases can cause this condition. Many of these diseases, such as retinitis pigmentosa, are caused by mutations in the genes that code for the many proteins that drive the elegant molecular machinery involved in light detection.

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