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.