Figure 4.

Grid cell firing is driven by sustained depolarizations. Whole-cell recordings from MEC of mice navigating on a spherical treadmill. (a) Membrane potential of an MEC neuron (black) during a run along a linear track. Two firing fields were crossed, as indicated at the bottom. Membrane potential was decomposed into a ramp (red) and a theta oscillation (grey). Note the sustained increase in membrane potential during field crossings. (b) The ramp voltage increased more than theta oscillation amplitude during firing field crossings, indicating that sustained depolarizations drive grid cell firing. ((a,b) Adapted with permission from Domnisoru et al. [4].) (c) Average firing rate (top), subthreshold membrane potential (middle) and theta MPO amplitude (bottom) were plotted against normalized position in a firing field of a stellate cell. Theta MPOs contributed only little to the depolarization in the field centre. (d) Normalized firing rates of stellate cells were plotted against deviations of theta MPO amplitudes (left) and subtheta membrane potential (right) from the mean. By contrast to subtheta membrane potential changes, changes in theta MPO amplitudes did not significantly correlate with firing rates, indicating that firing was primarily driven by slow depolarization. ((c,d) Adapted from Schmidt-Hieber & Häusser [26].)