Figure 1. Low rheobase cells represent a highly excitable cell type in the superficial retrosplenial cortex. A. Intrinsic physiological properties of an FS neuron in the superficial layers of the granular retrosplenial cortex. Top trace, Ability to fire sustained high frequency trains of action potentials with little or no spike frequency adaptation. Middle trace, A substantial delay to first spike after current onset during a near-threshold current step. This late-spiking feature was consistent across all 42 recordings of FS cells in these layers. Right inset is a zoomed-in view of the first spike in the middle trace. It shows a narrow spike width followed by a large afterhyperpolarization. These features are distinctive of FS cells are were also consistent across all 42 recordings of FS cells in these layers. Bottom, injected current amplitudes for the voltage responses shown above. B. Intrinsic physiological properties of an RS neuron in the superficial layers of the granular retrosplenial cortex. Top trace, Presence of spike frequency adaptation when firing at higher frequencies in response to large suprathreshold current steps. This feature was consistent across all 17 recordings of RS cells in these layers. Middle trace, Minor delay to first spike after current onset during an at-threshold current step. Right inset is a zoomed-in view of the first spike in the middle trace. It shows a relatively wide spike width followed by a gradual return to baseline membrane potential. Bottom, injected current amplitudes for the voltage responses shown above. C. Intrinsic physiological properties of an LR neuron in the superficial layers of the granular retrosplenial cortex. Top trace, Ability to fire sustained high frequency trains of action potentials with little spike frequency adaptation. Middle trace, Moderate delay to first spike after current onset during an at-threshold current step. These features were consistent across the 108 recordings of LR cells in these layers. Right inset is a zoomed-in view of the first spike in the middle trace. It shows a moderate spike width followed by a clear afterdepolarization before returning to baseline membrane potential. This afterdepolarization was present in 66% of LRs recorded in these layers (n=61/93). Bottom, injected current amplitudes for the voltage responses shown above. D. Representative traces from FS, RS, and LR cell action potentials overlaid to show differences in spike width. Bar graph of the average spike widths for FS, RS, and LR cells showing a clear distinction between the three (error bars are standard error). E. Bar graph of the average rheobase for FS, RS, and LR cells showing a markedly low rheobase for LR cells compared to that of FS and RS (error bars are standard error). F. Bar graph of the average IR for FS, RS, and LR cells showing a uniquely high IR for LR cells (error bars are standard error). G. Bar graph of the average IC for FS, RS, and LR cells showing a markedly low IC for LR cells compared to FS and RS (error bars are standard error). H. Bar graph of the average membrane time constant (tau) for FS, RS, and LR cells (error bars are standard error). I. Bar graph of the average latency to first spike after onset of an at-threshold current injection for FS, RS, and LR cells showing a substantial latency to first spike for both FS and LR cells in these layers (error bars are standard error). J. Bar graph showing the average spike frequency adaptation ratio for FS, RS, and LR cells showing lack of adaptation in FS and LR cells (error bars are standard error).