Unique intrinsic properties of Low Rheobase (LR) cells
LR cells in the RSG have intrinsic properties that differ substantially from those of neighboring RS cells. Most importantly, and the reason we have suggested their name, is that they fire in response to very little current input, i.e. they have a low rheobase. This is dependent on their large input resistance, which in turn is most likely related to their small soma size and relatively sparse dendritic tree. LR cells also demonstrate a late-spiking phenotype in response to near-threshold current injections (Kurotani et al, 2013). Based on our findings, we argue that the late-spiking behavior of these neurons is not their most unique or defining feature. FS cells and many RS cells in the RSG also show late-spiking behavior in response to near-threshold stimuli (Figure 1). Additionally, the name “late-spiking (LS) neuron” is already most often attributed to neurogliaform cells, inhibitory interneurons found in the superficial layers of the neocortex, including layer 1 (Cruikshank et al., 2012). Instead, the intrinsic excitability of LR neurons, facilitated mostly by their high input resistance and low rheobase, is their most distinct feature and is critical to their potential computational functions. In addition, their lack of spike frequency adaptation suggests an ability to respond to high frequency inputs far more reliably than RS cells can. This, coupled with their low rheobase, makes them a rare and unique class of neurons that can respond to both weak inputs as well as sustained or high frequency inputs, thereby possessing the ability to transfer the incoming synaptic inputs to their post-synaptic targets with a high degree of precision. Thus, the name Low-Rheobase (LR) is indicative of both their unique properties compared to other cell types in this region as well as the computational functions that they are likely to perform in this circuit.