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Structural Dynamics of Synaptic Junctional Areas in Aging and Alzheimer's Disease

In contrast to previously accepted dogmas, during the last decades it has been clearly demonstrated that the nervous system has a great capability to modify its structures in response to different environmental stimuli.’~2 This adaptive ability is retained throughout the organism’s life span and involves changes occurring at nerve cell soma, dendritic networks, and synaptic terminal regions. Because of the peculiar functions of the synaptic junctional areas (i.e., the transmission of the nervous impulse), the changes occurring in these discrete, peripheral, and specialized zones of the nerve cells have attracted much of the attention of researchers and, nowadays the term “synaptic plasticity” serves to define such a dynamic condition of the junctional areas between neurones. With the advancement of technology, sophisticated methodologies have been introduced to study different aspects of the possible synaptic modifications, and the meaning of synaptic plasticity has become more specific according to the type of studies carried out. Thus the biochemical plasticity of the synapses indicates changes in molecules (i.e., neurotransmitters), enzyme activities, receptor binding, and so forth. The physiologist, when speaking of plasticity at synaptic regions, refers to variations in spike amplitudes or electric postsynaptic potentials that are due to defined stimuli during neurotransmitter-induced discharge. From a morphological standpoint, synaptic plasticity is used with regard to modifications in the ultrastructure of synaptic regions purported to adapt their architecture to the changing requirements. We took into account this last aspect of synaptic plasticity, and the results we obtained in the human central nervous system (CNS) during physiological aging and in Alzheimer’s disease are discussed in this paper.