Vulnerability of cortical neurons to Alzheimer's and Parkinson's diseases

Alzheimer's disease (AD) and sporadic Parkinson's disease (PD) are the most frequently occurring degenerative illnesses of the human nervous system. Both involve multiple neuronal systems, but only a few types of nerve cells are prone to develop the disease-associated intraneuronal alterations. In AD affected neurons produce neurofibrillary tangles and neuropil threads, while in PD they develop Lewy bodies and Lewy neurites. In both illnesses select types of projection cells that generate long, unmyelinated or sparsely myelinated axons are particularly susceptible. This kind of selective vulnerability induces a distinctive lesional pattern which evolves slowly over time and remains remarkably consistent across cases. In the present review, lesions developing in the cerebral cortex are described against the backdrop of the internal organisation and interconnectivities linking involved cortical areas and subcortical nuclei. In AD, six and in PD, three stages can be distinguished, reflecting the predictable manner in which the proteinaceous intraneuronal inclusions spread through the cerebral cortex. In AD stages I-II and in PD stage 4, the pathological process makes inroads into the anteromedial temporal mesocortex, entorhinal allocortex, and Ammon's horn; thereafter, in AD stages III-IV and in PD stage 5, it proceeds into the adjoining high order association areas of the basal temporal neocortex. In AD stages V-VI and in PD stage 6, the damage affects additional neocortical association areas including first order association areas and eventually extends into the primary areas of the neocortex. The gradually evolving lesional pattern in AD and PD mirrors the ground plan of the cerebral cortex. The highest densities of lesions occur in the anterior mesocortical transitional zone between allo- and neocortex. From there, the involvement diminishes by degrees and extends into both the hippocampal formation and the neocortex. The severity of the neocortical lesions decreases in inverse proportion to the trajectories of increasing cortical differentiation and hierarchical refinement.