Tag Archives: melanopsin

Brand-New Look at an Ancient System

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NASA Identifier: sts092-367-035 In the 21st century, for the first time, scientists are studying the workings of the eye’s ancient photoreceptor system, which evolved before vision. The origins of the nonvisual system possibly date back at least 500 million years. New research about the eye’s light-sensing system is driving high-precision light technology being designed for the International Space Station.

NASA Identifier: sts092-367-035
In the 21st century, for the first time, scientists are studying the workings of the eye’s ancient photoreceptor system, which evolved before vision. The origins of the nonvisual system possibly date back at least 500 million years. New research about the eye’s light-sensing system is driving high-precision LED light technology being designed to improve the health and safety of astronauts aboard the International Space Station.

Unlike the eye’s visual rods-and-cones system, which produces images, the nonvisual system provides a measure of environmental presence and intensity of light. It is composed of photosensitive retinal ganglion cells, which get their light-measurement abilities from a light-sensitive photopigment called melanopsin.

Melanopsin, not to be confused with melatonin or melanin (a pigment that gives color to skin and eyes), shows a peak sensitivity to short-wavelength blue light: the light that most readily activates the brain, suppressing melatonin — the chemical expression of darkness, as termed by pioneer melatonin researcher Russel J. Reiter — and preparing the body’s physiological and psychological systems for daytime activities.

In the 21st century, for the first time, scientists are studying the workings of this ancient photoreceptor system, which evolved before vision. Researchers believe the origins of the nonvisual system possibly date back at least 500 million years, to the branch of animal evolution featuring sea stars and sea urchins. But this photoreceptor system — discovered just over a decade ago, in 2002 — is so small that generations of scientists overlooked it during centuries of research on the eye’s visual processes.

“The discovery of a new sensory apparatus in the human eye after hundreds of years of careful research on the visual system serves as a reminder of how easy it is to miss critically important physiology,” neuroscientist George C. Brainard wrote in the 2005 research article “Photons, Clocks, and Consciousness” (Brainard, John P. Hanifin, Journal of Biological Rhythms).

Brainard, director of the Light Research Program at Thomas Jefferson University whose decades of research helped lead to the discovery of the eye’s nonvisual system, explained in the article that the science of human circadian phototransduction — the process in which light, via detection by the eye’s light-sensing system, is transformed into electrical signals for the brain — was still in its infancy. “Expanding the frontiers of this field will teach us how to better use light for the benefit of humanity,” Brainard wrote.

Brainard is playing a huge role in expanding those frontiers: The neuroscientist continues to work with NASA in developing light for long-duration space travel, including the International Space Station.

A Sci-fi-esque Light-Sensing System

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NASA Identifier: iss002-702-085 On Earth, we experience one sunrise and sunset in a 24-hour day. But in a flood of the full spectrum of light, International Space Station crews see about 16 sunrises and sunsets in that same time frame. This sunrise view was taken by the Expedition Two crew in 2010.

NASA Identifier: iss002-702-085
On Earth, we experience one sunrise and sunset in a 24-hour day. But in a flood of the full spectrum of light, International Space Station crews see about 16 sunrises and sunsets in a 24-hour period. This sunrise view was taken by the Expedition Two crew in 2010.

The light sensitivity of the eye’s daytime color vision system evolved later than nonvisual light detection systems and so is not optimized for detecting light to reset the brain’s master circadian clock. On the visible light spectrum, the sensitivity of the three-cone photopic visual system peaks in the mid-wavelength green range, at 555 nanometers. But the nonvisual system contains the photopigment called melanopsin, which has a peak sensitivity at 480 nanometers in the short-wavelength blue range.

In animal models, and studies of different types of blindness, if the visual rods-and-cones system was removed from the cell layers of the eye, or is non-functioning, the nonvisual system works perfectly fine on its own, sending clock-setting light to the brain.

While there is some interaction, circadian neuroscientist Steven W. Lockley explains, the visual and non-visual photoreceptor systems can function independently and have their own photoreceptors, neural pathways and effects on the brain.

It’s crucial that we understand human physiology and our complex relationship with light given how important light is to sleep, circadian rhythms, and health. The eye’s nonvisual retinal system — a sci-fi-esque mechanism of which ophthalmologists largely remain unaware 13 years after the system’s discovery — represents a distinct photoreceptor systems with its own photopigment, neural pathway and function from the photoreception system that we use to see.

Neuroscientist George C. Brainard, director of Thomas Jefferson University’s Light Research Program, was one of the first scientists to study the effects of light wavelength on circadian photoreception. In 2001, his was one of two laboratories that made the major finding about the human eye: the melatonin suppression response — one of the non-visual responses to light — had a peak sensitivity in the blue light range that did not match the light sensitivity pattern of the rods and cones used to see.

Earlier work had shown that total visual blindness did not change circadian light responses, but these papers (Brainard et al., 2001; Thapan et al., 2001) provided more formal functional evidence of a non-rod, non-cone photoreceptor in the human eye.

This discovery, coming on the heels of colleagues’ research showing that the mammalian eye contains a light sensor separate from the visual rods-and-cones system, set up neuroscientist David Berson’s monumental finding in 2002: the mystery photoreceptor and a light-sensitive molecule called melanopsin, discovered in 1998 in the camouflaging skin cells of the African clawed frog, were one and the same.

Unfortunately, the conversation about light wavelengths gets stuck on the short end of the spectrum. It is true that the eye’s nonvisual light-sensing photoreceptor system is most sensitive to short-wavelength blue light, but as Lockley explains, all visible light can affect circadian rhythms. Any light source after dusk can be considered unnatural, including the light necessary to do shiftwork but also the light inside our homes, such as from TVs, cellphones, computers, and other electronic devices, which keeps us awake at night and disrupts our sleep and circadian rhythms.

Light after dusk, as relayed by the eye, tells the brain it’s daytime. And if the brain thinks it’s day, not night, it will induce daytime physiology as it thinks that we are awake at the wrong time. Consequently, light at night shifts the clock, suppresses melatonin, increases heart rate and temperature and alerts the brain — all of which are associated with daytime in a day-active species like humans.

But short-wavelength blue light — a stimulant, as opposed to relaxing, long-wavelength yellow/red light at the far end of the visible light spectrum — is also medicinal light. It can be used to improve alertness, reset disordered clock rhythms, or alleviate seasonal depression.

It’s the natural color of a brilliant, midday sky pouring in through the windows of assisted-care facilities for the elderly, improving patients’ moods, sleep, and cognitive symptoms of dementia.

And it’s a crucial component of the LED lighting wavelength model being tested for the International Space Station.