All retinal nervous cells have numerous lysosomes in their cytoplasm. In the NFL the axons of the glial cells contain dense bodies, formed from incompletely digested myelin, which causes the axon's swelling. In some cases, in elderly retinas, astrocytes of large dimensions are found that have very elevated cellular activity and a higher density of intermediate filaments; these are called reactive astrocytes. The function of the reactive astrocytes is to protect the neurons (in this case, the ganglion cells) from ischemia producing neurotrophic factors, increasing the expression of antioxidant substances (glutathione, vitamin C) and increasing the production and transport of glucose. However, it has been observed that astrocytes are more vulnerable to oxidative damage during aging. In fact, as the years rollby, the reactive astrocytes cause changes in the geometry and volume of the extracellular space, which slows the diffusion of neuroactive substances.
The extracellular space is not only the microenvironment of the nerve cells but is also an important channel of communication between the neurons and astrocytes. The changes to the diffusion parameters, which arrive with aging, can bring on a disappearance of the transmission signals and increase the sensitivity of the nervous tissue to ischemia. The ischemia is due to an increase in the extracellular acidity with accumulation of potassium and other toxic substances (similar to glutamate) that damage the neurons.
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