The most well studied of these are the somatically-targeting cholecystekinin (CCK)- and parvalbumin (PV)-expressing interneurons ( Freund 2003).
#Ca1 hippocampus anatomy full
Although a full discussion is beyond the scope of this review and is covered by others ( Klausberger and Somogyi, 2008), there are a myriad of interneurons types that can be defined by protein markers as well as by the neuronal compartment they target: basal dendrite, soma, axon, proximal apical dendrite, and distal apical dendrite. In addition to the above excitatory pathways, inhibitory inputs play an important role in shaping excitability and in vivo function. Each pathway can elicit inhibition in feedforward fashion. Indirect EC input arrives at the proximal apical dendrite, via DG and CA3, or at the basal dendrite, from CA2. Distal apical dendrite receives direct input from entorhinal cortex (EC) and nucleus reuniens of thalamus (nRT). CA1 pyramidal neuron excitatory (triangle) and inhibitory (circle) inputs to different parts of its apical dendrite in SR, SLM and basal dendrite in SO. CA1 pyramidal neuron is indicated with labeled strata stratum oriens (SO), stratum radiatum (SR) and stratuma lacunosum moleculare (SLM). Genetic or chemical silencing of each of these three pathways has distinct effects on different types of memory ( Nakashiba et al., 2008 Brun et al., 2008 Suh et al., 2011 Xu and Sudhof, 2013 Hitti and Siegelbaum, 2014), highlighting that they carry unique information to the CA1 PN.Ī Hippocampal CA1 in relation to other subregions: subiculum (S), CA2, CA3, dentate gyrus (DG). Direct inputs from CA2 mainly target the basal dendrite in stratum oriens, which also receives a minority of SC input. Another input to this compartment is the nucleus reuniens of thalamus (nRT), which forms a relay between prefrontal cortex and CA1. The distal apical dendrite receives synapses from the “direct pathway”, monosynaptic input from layer III of entorhinal cortex (EC), in stratum lacunosum moleculare. This completes the classical “trisynaptic pathway” from entorhinal cortex to CA1. In this manner, processed information from dentate gyrus is carried forward to CA1 via its mossy fibers input to CA3. The Schaffer collateral (SC) pathway, originating from CA3, primarily targets the proximal apical dendrite in stratum radiatum. CA1 PNs have a long apical dendrite and a shorter basal dendrite upon which major temporal lobe pathways synapse in a compartmentalized fashion ( Figure 1). Hippocampal circuitry from the viewpoint of the CA1 pyramidal neuronĬA1 pyramidal neurons (PN) carry the primary output of the hippocampus to other brain regions, and thus an analysis of its inputs elegantly summarizes overall hippocampal information processing ( van Strien et al 2009 Basu and Siegelbaum, 2015). Thus, in this review I use CA1 as the focal point for discussions of heterogeneity at the cellular and circuit level, how this evolves across its primary anatomical axes, and relevance to AD. However, CA1 constitutes the primary output of the hippocampus and, along with subiculum, are the first hippocampal areas affected in Alzheimer’s disease. These regions are interconnected but play distinct roles in memory and are differentially affected by disease ( van Strien et al., 2009 Small et al., 2011 Cheveleyre and Piskorowski, 2016). The hippocampus is well known to be comprised of subregions, namely dentate gyrus (DG), CA1, CA2, CA3, and subiculum. Within this region, neuropathological staging of tau pathology has highlighted the involvement of the transentorhinal and entorhinal cortices first, followed by progression to the hippocampus ( Braak and Braak, 1991). The medial temporal lobe has been a major focus of Alzheimer’s disease (AD) research due to the onset of amnestic symptoms at early stages.