Low Rheobase cells are the dominant cell type in superficial retrosplenial granular cortex
LR neurons were the most commonly encountered cell type in L2/3 of RSG. To quantify the relative percentage of neurons in the superficial RSG, the recorded neurons were assigned to one of four groups based on their intrinsic physiological properties: FS, RS, LR, and unclassified. The unclassified group consisted of neurons whose intrinsic and/or firing properties did not fall under any of the three defined groups. Thus, this group includes other neuronal subtypes not investigated in this study as well as a few potential FS, RS, and LR neurons whose properties were difficult to classify. We found that LR cells are the dominant cell type in both layers 2 and 3, accounting for 51.9% of the neurons in layer 2 and 57.14% in layer 3 (Figure 3). However, 0 out of 25 recordings in layers 5 and 6 were of LR cells, suggesting that LR neurons are restricted to the superficial layers of RSG (data not shown). Surprisingly, the prevalence of RS cells in L2/3 of RSG was strikingly low, representing only 18.5% of all layer 2 neurons and 8.9% of layer 3 neurons. The FS probabilities are slightly skewed, as experiments detailed later in this manuscript specifically targeted FS interneurons. Thus, their probability reported here is likely slightly larger than their true representation in the cortex. Nonetheless, it is clear that LR cells are the most prevalent cell type within the superficial layers of the granular retrosplenial cortex.