A new look at light

You might know that your retina senses light primarily through its rods and cones which are sensory cells specialized in converting photons into electrical signals.


Pisa at Sunset (I took this picture)


What you might not know is that there is a third light-sensitive cell in the mammalian eye. These cells are retinal ganglion cells (RGCs), but not all RGCs are directly sensitive to light.

But what you really probably don't know is that these RGCs sense light using the same protein that allows a toad's (Xenopus Laevis) skin to sense light (melanopsin). 


"It's true, I tell ya!"

These cells (the non-rod, non-cone light sensors) react to light directly, but they aren't exactly good at it. Their sensitivity is lower than the rods and cones, and they don't seem to transmit shape or color information.  So what is their purpose? Why have a secondary set of cells that sense light in a poor and unfocused way when you already have highly specialized rods and cones? 

To make it even more confusing, the rods and cones actually connect to these cells, adding their light-sensing information to theirs. 

weird, right? In a recent review paper, Pickard and Sollars (2012) explain that these cells likely play a role in controlling the sleep-wake cycle (circadian rhythm). Rats and mice with strongly degenerated rods and cones still set their circadian clock by the light cycle they are exposed to.  These cells send strong projections to the hypothalamus which controls everything sleep-wake cycle.


In addition, these cells or at least the melanopsin gene, may play a role in Seasonal Affective Disorder (SAD) by modulating the light-dependent cycles of the suprachiasmatic nucleus (a part of the hypothalamus).



SAD (source)

Their vague ability to sense 'brightness' makes these cells nicely suited to regulating the body's response to daily and seasonal changes in light. But whether these cells need to be light-sensitive to perform these functions or whether their sensitivity to light is just an evolutionary remnant is unclear. 



© TheCellularScale



 Pickard GE, & Sollars PJ (2012). Intrinsically photosensitive retinal ganglion cells. Reviews of physiology, biochemistry and pharmacology, 162, 59-90 PMID: 22160822