#The long dark light up the night series#

Glutamate, an amino acid and neurotransmitter, acts as a messenger that conveys to other retinal cells information about light stimulation of photoreceptors.

#The long dark light up the night series#

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.

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. (The ultimate source of retinal is dietary vitamin A this is the reason why an early sign of vitamin A deficiency is night blindness.) In vertebrates, all photoreceptor opsins contain a molecule called retinal, or retinaldehyde. Rods, on the other hand, have a single form of opsin called rhodopsin. 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.' Human cones have one of three types of opsin, each with a slightly different sensitivity to the spectrum of light, which is relevant for color vision. So, what happens in the cones and rods during dark adaptation? To attempt to answer this question we need to first consider the mechanism underlying cone and rod function. 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.

Rods work slower, but since they can perform at much lower levels of illumination, they take over after the initial cone-mediated adaptation period. Cones adapt faster, so the first few minutes of adaptation reflect cone-mediated vision. Remarkably, rods can respond reliably to a single visible light photon, so they operate at the physical limit of light detection.Д«oth cones and rods participate in dark adaptation, slowly increasing their sensitivity to light in a dim environment. Rods can act as light detectors even in extremely low levels of illumination but are ineffective-they are known to 'saturate'-in bright light. Photoreceptors for night vision are called rods. (Cones are unable to respond to light reliably in dim illumination, however.) The first, the cones, evolved for day vision and can respond to changes in brightness even in extremely high levels of illumination. The human retina can perform its light-detection function in an astounding range of light intensities, from bright sunlight to dim starlight, by relying on two types of light-sensitive cells, or photoreceptors.

This phenomenon is known as 'dark adaptation,' and it typically takes between 20 and 30 minutes to reach its maximum, depending on the intensity of light exposure in the previous surroundings. As time goes by, however, we gradually become able to detect the room's contents. If we go from the outdoors on a bright sunny day into a very dimly lit room, we are hardly able to see our surroundings at first. Rafael Caruso, an investigator in the National Eye Institute's Ophthalmic Genetics & Visual Function Branch in Bethesda, Md., guides us to an answer.