One brain system that is particularly vulnerable to age-related decline is the medial temporal lobe system (Barnes, 1979; Barnes and McNaughton, 1985; Gallagher et al., 1993). The anatomy of this system can be contrasted in human versus rat brain, seen shaded in light blue in Figure 32.1.
As can be observed in the circuit diagram shown in Figure 32.1, information flow in the system is multidirectional. Memory processing involves the hippocampal formation and entorhinal cortex, and information flows bi-directionally between these structures. Both of these brain regions receive direct projections from other brain areas, including the neocortical regions and subcortical structures. Communication with the hippocampus to other cortical regions is through the entorhinal cortex that receives input from and sends output to the para-hippocampal gyrus (which includes the perirhinal and postrhinal cortices). Cellular regions of the hippocampus can be observed in the lower diagram (Figure 32.1), which highlights circuitry in a coronal section of rat hippocampus. The entorhinal cortex provides input to the dentate gyrus granule cells via the perforant path. The subgranular zone is a site of new neuron generation throughout the lifespan, a phenomenon that is the focus of considerable current aging research (Bizon and Gallagher, 2003; Bizon et al., 2004; Merrill et al., 2003; Litchenwalner et al., 2001; Cameron and McKay, 1999). Granule cells project to the pyramidal neurons of CA3 and CA1 in turn. Finally, information is sent out of the hippocampus via projections through the subiculum to neocortical regions. This interaction of the medial temporal system with the neocortex underlies specific types of learning, consolidation, and retrieval of memories. This system is specifically important for processing explicit or declarative memory in humans, including conscious memories of events, people and places. Compromised function of the medial temporal system has been extensively studied across species, and contributes to specific memory deficits in normal aging, as well as in neurodegenerative conditions such as Alzheimer's disease.
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