Mar 10, 2011 15:08 GMT  ·  By

Though investigated constantly, the human eye remains one of the most complex constructs in the natural world. Recently, researchers in Germany managed to clear the mystery surrounding the way it works a bit, when they looked at how various neurons communicate with each other.

Scientists have known the basic mechanism underlying the functionality of the eye for a long time.

They know that light enters the organ through the front, is reversed, and then reaches the retina. Receptors on this ocular component convert it into electrical signals, and then relay it via neurons to the visual cortex at the back of the head.

But the way these nerve cells work together to relay the signals has been somewhat mysterious. Experts with the Max Planck Institute recently tackled the problem head-on, Science Blog reports.

The next step after retinal sensory cells are downstream interneurons, which relay the data to structures called retinal ganglion cells. Each individual ganglion receives information from a specific, circular area of the visual field known as the receptive field.

The structures also exhibit a behavior called direction selectivity, in which some of them are only activated if light falls on retinal sensory cells from a specific direction. If the angle changes, then the ganglion cells are inhibited.

This ability is controlled, the German team learned, by starburst amacrine cells (SAC), which connect to the ganglion cells via inhibitory synaptic connections. The researchers also believe that they managed to shed light on another mystery as well.

Until now, it was unclear what enabled direction selectivity. Some experts sad that it was a type of signal yet to be discovered, while others argued that it was the orientation of SAC dendrites that underlies this ability.

“Ganglion cells prefer amacrine-cell dendrites that run along the null-direction,” says MPI expert Winfried Denk, who conducted the work with colleagues Kevin Briggman and Moritz Helmstaedter.

The most important finding in the study was that the synapses (neural connections) between retinal ganglion cells and SAC are distributed asymmetrically, even though the cells themselves are symmetrical in their orientation.

In order to investigation correlation at the neural level, the MPI group used two-photon fluorescence microscopy (TPFM) and a form of electron microscopy method called serial block face electron microscopy (SBFEM).

“For the first time, minute cell structures can now be viewed at a high resolution in larger chunks of tissue. This procedure will also play an indispensable role in the clarification of the circuit patterns of all regions of the nervous system in the future,” Denk explains.