Alzheimer's disease as a presumptive threshold phenomenon.

With the example of the basal forebrain cholinergic system as a site of primary importance in AD, the existence of a critical neuron loss as a threshold between normal aging and AD is suggested. If the extent of degeneration exceeds this threshold the system decompensates and the clinical picture becomes apparent. The fully developed stage of AD might, therefore, represent a condition beyond the capacity of compensation where plastic adaptive changes are still present but functionally insufficient.