Fig. 3.

Ca²⁺ transients induced by action potentials (APs) backpropagating to the proximal processes of hippocampal neurons. A: image of a cultured hippocampal neuron filled with fluo-4 (20 μM) and stimulated with a prolonged depolarization showing the Ca²⁺ accumulation within the cell. Two neuronal processes (p1 and p2) are indicated by arrows. Scale bar: 10 μm. B: short (10 ms) somatic depolarizing current injections elicited 2 APs (top) at 20 Hz. Line scans (middle) were recorded from the 2 processes (p1 and p2), at the position indicated by the dashed line in A. Line scans show an increase in fluorescence as a consequence of intracellular Ca²⁺ elevation. Scale bar: 250 ms. The corresponding relative changes in fluorescence (bottom) show a faster transient for the thinner process (p1) simply due to the different surface-to-volume ratio. For display purposes transients were digitally filtered off-line (adjacent-averaging routine, smoothing factor n = 10, Origin 7). C: effect of K⁺ channel blocker tetraethylammonium (TEA, 10 mM) on repolarization of the APs that induced the fluorescence transients displayed in D. Black, APs before TEA application (control); red, APs in the presence of TEA; blue, APs after washout of TEA. D: line scans recorded under control conditions, during application of 10 mM TEA, and after washout (top) and corresponding relative changes in fluorescence (bottom). Scale bar: 250 ms.