A no-brainer? Mouse eyes constrict to light without direct link to the brain

A no-brainer? Mouse eyes constrict to light without direct link to the brain

Experimenting with mice, neuroscientists at Johns Hopkins Medicine report new evidence that the eye's iris in many lower mammals directly senses light and causes the pupil to constrict without involving the brain.

In a report in the June 19 issue of the journal Current Biology, the researchers detail how the pupils in a mouse's eyes get smaller when the animal is moved from a dark to a lit room even when the nerve connections between the animal's brain and eyes are severed.

Their findings prove that mouse eyes have a photosensitive function built directly into the ring of sphincter muscle surrounding the pupil.

The mammalian eye adapts to changing light conditions by constricting or enlarging the pupil. That action, referred to as pupillary light reflex, is controlled by opposing dilator and sphincter muscles in the iris. "The traditional view of this reflex is that light triggers nerve signals traveling from the eye's retina to the brain, thereby activating returning nerve signals, relayed by the neurotransmitter acetylcholine, that make the sphincter muscle contract and constrict the pupil," says King-Wai Yau, Ph.D., a neuroscientist at the Johns Hopkins University School of Medicine and an author of the report.

But research by Yau and his team several years ago, building on earlier work by others, showed that nocturnal mammals and mammals active at dawn and dusk, such as mice, rabbits, cats and dogs, don't need the brain for this reflex to work. Instead, Yau's lab found that even when isolated, the sphincter muscle contracts in response to light, employing a light-sensitive pigment called melanopsin.

The simplest explanation for that observation would be that the muscle is itself sensitive to light. However, other researchers offered the alternative explanation that there might be light-sensitive nerve fibers containing melanopsin present on the sphincter muscle that make the muscle contract by piggybacking on the brain's pupillary light reflex circuitry involving acetylcholine.



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