Complex Internal Dynamics Between Place And Grid Cells Case Study

Complex internal dynamics between place and grid cells
Spatial learning and memory have been implicated in navigation and the formation of episodic memory (Pilly & Grossenberg, 2011). Episodic memory can be affected by neurological disorders, such as dementia and Alzheimer’s disease. A better understanding of the neural mechanisms underlying spatial behaviour could therefore help to understand such cognitive decline. An important contribution to this endeavour was the discovery of spatially modulated place and grid cells (Kiehn & Forssberg, 2014). Hippocampal place cells are neurons that fire whenever an animal is located in a particular region in the environment (Moser & Moser, 2008). These place cells remain to fire in a novel environment,
In an early study, Brun et al. (2008) demonstrated that the input from layer III mEC cells is necessary for precise location-specific firing in CA1 cells. In this study, they made a local lesion across the entire central part of the ipsilateral mEC, with variable additional damage to more dorsal and ventral regions in the mEC. Subsequently, they measured the spatial modulation of firing in both CA1 as CA3 pyramidal cells while the rats were running in a familiar and novel environment. Compared to control animals, the lesioned animals exhibited a more dispersed place field, less coherent fields and reduced spatial information when placed in a familiar environment. However, both experimental groups demonstrated a weaker spatial modulation, when placed in a novel environment. Furthermore, CA3 cells mostly receive their input from layer II mEC cells. Hence, no impairments were evident in spatial representation in CA3 subsequent to the lesions in layer III of the mEC. Therefore, the absence of impairments in spatial representation in CA3 pointed to the selective ipsilateral lesions in entorhinal layer III as a possible source of reduced spatial specificity of CA1
However, only a subset of the EC neurons send their projections to the hippocampus. In these studies it remained elusive which cell types contributed to these processes. Zheng et al. (2013) demonstrated that the vast majority of excitatory projections of the mEC to place cells are generated by a variety of entorhinal functional cell types, of which the grid cells were the most frequent spatial cell type. In this study, a combined optogenetic-electrophysiological experiment was conducted. With the use of a recombinant adeno-associated virus, Channelrhodopsin-2 transgenes were expressed in hippocampal-projecting cells in the EC. Cells that fired at a fixed minimal latency in response to photostimulation were identified as hippocampus-projecting mEC cells. Many light-responsive MEC cells were grid cells, but short-latency firing could also be induced in border cells and head-direction cells, as well as neurons with irregular firing fields or no fields at all. In each cell group, the majority of neurons discharged at minimal response latencies, suggesting that they had direct projections to the