Moving around in the half-light is difficult but not impossible. To help us in this undertaking we have the rods, a type of light-sensitive cells (photoreceptors) present in the retina of vertebrates, capable of detecting very low lights which allow to move about even in poorly lit cellars or caves. They are biological wonders capable of detecting even a single quantum of light, but they need continuous maintenance. They are the protagonists of the new study published in PNAS by a team of researchers of SISSA – Scuola Internazionale Superiore di Studi Avanzati and the Istituto officina dei materiali of the National research council CNR-Iom which reveals new and essential details of how the retina works and in particular photoreceptors.
These consist of two segments: the outer segment (OS) and the inner segment (IS). The OS of the rods is the one where the biological machine capable of capturing the light is located, while the IS is responsible for the information to be transferred to the brain. “We have understood that the outer segment is more fragile than what was thought”, comments Vincent Torre, neuroscientist of SISSA leading the team that conducted the research, adding “The OS consists of a stack of lipid discs containing the proteins responsible for phototransduction. New discs are generated at the base of the OS while used discs are removed at the tip of the OS. Traditionally, it was thought that in a stack of about 1000 discs there was almost perfect uniformity. However, our work shows that only the first 200 or 300 discs at the base of the OS are those effectively capable of detecting the single photon of light, characteristic from which comes the great sensitivity of the rods. The other discs positioned close to the tip gradually lose effectiveness and sensitivity and for this reason they must be disposed of and replaced with new discs in perfect condition”.
It was the Calcium, an ion present in large numbers in biological processes that allowed the understanding of this mechanism. Its concentration in the OS is an excellent indicator of the functionality and integrity of phototransduction, the process with which the photoreceptors convert light into nerve signals.