Receptors, photoreception and brain perception. New insights.

Once photons have activated photosensitive cell receptors, a biochemical process mediated by G-proteins transforms the initial signal into nerve potentials. The generated impulses transmit the information through ganglion cells, after a complex interaction with other neurons by means of different neurotransmitters. Since visual function is processed in parallel, ganglion cells are divided into M-neurons which are in charge of capturing large objects, P-neurons capable of analyzing fine details and colors, and non-M, non-P neurons which are sensitive to changes in light intensity. Retina, bipolar and ganglion cells share circular receptive fields with an antagonistic surround whereas the lateral geniculate nucleus possesses rectangular receptive fields. Thus, when central cones are stimulated, ON-center cells depolarize, while OFF-center cells hyperpolarize. At the brain cortex, the magnocellular layers lead to orientation and achromatic perception, the parvocellular layers perform color vision in the blobs and achromatic contrast and orientation in the interblobs, and eventually, binocular perception is the result of multiple disparities phenomenon. On these bases, patients with agnosia for form and pattern or for depth and movement have been described. Likewise, color blindness is another disease that could be the result of photoreceptor dysfunctions or brain perception defects.

Cellular receptors, acceptors and clinical implications.

Evidence of the existence of a special decodifier on the effector cells, with which inductor substances must interact in order to trigger their characteristic responses, has been found since the beginning of the century. These cytoplasmic sites are able to recognize, accept and receive the message transmitted by the inductor substance, giving rise to a specific cell response. Thus, the inductor-receptor binding complex interaction at the cellular level may alter permeability to ions as happens with neuro-transmitters, or can trigger pinocytosis in the case of apolipoproteins. Cellular receptors have been involved in the mechanism of action of several drugs and its dysfunction explains the physiopathology of a number of diseases. This may involve diminished clathrin protein synthesis triggering a defect in affinity, such as seen in malnutrition. It is also applicable in genetic disturbances, which are capable of precluding receptor development as in cases of familiar hypercholesterolemia.