Optical stimuli are required to be communicated to the brain from the eye. For this, necessary data is pre-processed by the eye.
Retinal ganglion cells that convey visual data to the brain through the optic nerve respond to light stimuli traveling in one direction. This selectivity is caused by inhibitory interneurons impacting the ganglion cell activity via their synapses.
A team at the Max Planck Institute for Medical Research in Heidelberg has demonstrated that the synapses dispersion between ganglion cells and interneurons conforms to specific rules. Dendrites reaching from the body of the amacrine cell in an opposite direction to the direction chosen by the ganglion cell come in contact with the ganglion cell.
The retinal sensory cells transform light to electrical signals. These signals are then conveyed via interneurons traveling in the opposite direction to the retinal cells, which transmit them to the brain. The interneurons are inter-connected to make single ganglion cells get visual data from a circular region of the visual or the receptive field. Some of the retinal cells work when the centre of the receptive fields comes in contact with light while the rim remains dark (ON cells). Some cells work when light beams across their receptive fields in a certain direction, moving in the opposite direction restrict their working.
The trajectory of the dendrites of the cells was measured along with those of amacrine cells with an electron microscope. This allowed the team to scan the outside of a tissue with the electron beam of a microscope to form an image. A slice measuring less than 25 nanometers is cut off following each scan, with a diamond knife. The 3D method allowed the team to detect the densely crowded branched dendrites of retinal neurons and detect the synapses between them. The automatic imaging captures information with millions of sections.
Source: http://www.mpg.de/