الفهرس 
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Abstract
Aging of the brain is an unavoidable process characterized by a large array of alterations in structure and function, eventually leading to brain dysfunction and cognitive decline. Numerous aging theories have been proposed such as neuron loss theory, loss of neuronal connectivity and trophic signaling, nuclear DNA damage, mitochondrial DNA damage, telomere shortening, loss of integrity of autophagosome / lysosome system, and the free radical theory of aging.
In the brain, both excitatory and inhibitory synapses are lost. However, no significant loss of neurons from the human cerebral cortex could be found with age. The structural changes of cortical pyramidal cells during aging are consistent in the form of increasing spine-loss and dendritic atrophy. The volume of white matter decreases with age due to loss of myelinated fibers. The common age-related alterations in myelin are the occurrence of myelin sheathes with dense cytoplasm, formation of balloons, dense cytoplasmic inclusions within the myelin sheath, increased frequency of appearance of sheaths with redundant myelin and the formation of circumferential splits in thick sheaths. An age-associated increase in the numbers of neuroglia, specially oligodendrocytes has been observed. Neuroglial cell types accumulate inclusions within their cytoplasm as they age.
In the spinal cord, a marked age-related reduction in the number of human anterior horn cells and significant decreases in their cross-sectional area and perimeter have been reported.