The_Function_of_the_Retina_in_the_Healthy_Eye
In the healthy retina, light is captured by the outer segments of photoreceptors and converted via the phototransduction cascade into a change in membrane potential, which is transmitted through bipolar and retinal ganglion cells to the brain. The signal is passed between cells via synapses using the neurotransmitter glutamate.
Photoreceptors are subjected to intense stresses. Exposure to intense light, combined with extremely high oxygen consumption, causes the production of reactive oxygen species, which damage proteins and lipids in the outer segments, and so 10% of this structure is phagocytose daily by the retinal pigment epithelium, or RPE. The RPE is a single cell layer that degrades the phagocytosed contents, recycles key elements of the light capturing pigment, and controls the supply of oxygen and nutrients to the photoreceptors so they can regenerate their outer segments. Photoreceptors are amongst the most metabolically active cells in the body, and insufficient nutrients can trigger metabolic stress.
A homeostatic molecular pathway called mTOR, detects low nutrient levels and reduces metabolic activity until sufficient nutrients become available.
Photoreceptors are also under endoplasmic reticulum or ER stress. The ER directs lipid manufacture as well as protein production and folding required for outer segment renewal and for neurotrophic factor secretion. Precise folding is essential to protein function, but errors can lead to misfolded protein overload in the ER. When mistakes occur, the unfolded protein response reduces protein synthesis and removes misfolded proteins until normal synthesis can be resumed.
Müller glia provide nutrients to photoreceptors and can phagocytose them if damaged.
They also recycle glutamate, maintaining the fidelity of signal transmission and preventing retinal neurotoxicity. Furthermore, the RPE Muller glia and photoreceptors secrete an array of endogenous neurotrophic factors absolutely critical in reducing the stresses on the different cells, maintaining retinal homeostasis and health. The cells of the retina constitute a self equilibrating biological system. When each element is healthy, homeostasis is maintained. But if any element is unable to perform its role, the entire retina can suffer.
