December 30, 2003 | David F. Coppedge

Your Accelerated Eyes

When a beam of light hits your eye, a chain of events is set off that is really quite amazing.  Kendall J. Blumer (Washington University School of Medicine) describes a little of it in the Jan. 1 issue of Nature.1  You don’t have to understand the following description; just be glad you don’t have to operate your retina in manual mode:

Light streaming into the eye is detected by specialized neurons (photoreceptors) in the retina.  In response to light, a coordinated series of molecular events � the so-called phototransduction cascade � is triggered in these cells (Fig. 1).  Photons excite pigment-containing proteins called rhodopsins, which then switch on the protein transducin by loading it with the small molecule guanosine triphosphate (GTP).  When bound to GTP, transducin turns on a phosphodiesterase, an enzyme that breaks down cyclic guanosine monophosphate (cGMP � another small molecule).  High concentrations of cGMP open specialized ion channels in the outer cell membrane.  Thus, by reducing the concentration of cGMP, light changes the flow of ions across the membrane of photoreceptive neurons, producing an electrical signal that is necessary for communicating with the brain. (Emphasis added in quotes.)

Now that’s just to turn the signal on.  When the light stops, it needs to be turned off quickly.  Normally, it would take too long for this process to reverse, but the retina has a standard procedure that takes care of it:

But this presents a problem.  Photoreceptor cells can turn off in less than a second in response to a brief flash of light.  In contrast, the hydrolysis of GTP by transducin requires tens of seconds to complete, making it difficult to understand how such a mechanism could account for the rapid turn-off of photoreceptor cells.  To get around this problem, photoreceptor cells possess a protein called regulator of G-protein signalling 9 (RGS9) that accelerates transducin’s ability to hydrolyse GTP.

Blumer describes what happens when a person has a defect in this accelerator protein.  It can take tens of seconds to adjust to a bright room when walking out of a theater.  It can take tens of seconds to see when driving into a dark tunnel.  And perhaps the worst of all (for Rose Bowl fans): “Moreover, people with this problem also suffer from difficulties in seeing certain moving objects (such as balls thrown during a sporting event).”
    Having one such accelerator protein would be amazing enough, but now – the rest of the story: “RGS9 is one of nearly 30 such RGS proteins, which regulate signalling by hundreds of receptors coupled to transducin-like G proteins in cell networks of the nervous, cardiovascular, sensory and immune systems.”

Kendall J. Blumer, “Vision: the need for speed,” Nature 427, 20 – 21 (01 January 2004); doi:10.1038/427020a.

We need to know things like this to avoid taking our bodies for granted.  This one deserves a little pondering.  Do some simple experiments; see how quickly your eye adjusts to different light levels, and think about all those little protein machines knowing just what to do on cue.
    Poor Charlie.  The eye as he knew it was enough to give him cold shudders.  In 1859, biochemistry was not even a science yet.  Charlie must be approaching absolute zero by now.  A book preceding The Origin of Species by about 2900 years, by a wiser man (Solomon), makes a lot more sense after reading the above description:  “The hearing ear and the seeing eye, The LORD has made them both” (Proverbs 20:12).

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Categories: Amazing Facts, Human Body

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