Fig. 1.

Variant firing patterns of CA1 PCs in low [Ca²⁺]_o. Intracellular recordings were made in five PCs in slices perfused with nominally Ca²⁺-free saline. The PCs were injected with long (200 msec) and brief (3–5 msec) positive current pulses of increasing intensity (in steps of 50–100 pA). The top panels(a) in A–Dillustrate the firing patterns of four different PCs evoked by two long current pulses. The bottom panels(b) depict the respective responses to brief stimuli. The injected current pulses are indicated below the voltage traces. A, A nonburster (NB). This neuron generated nonclustered spike trains in response to long current pulses (a) and a single spike in response to brief current pulses (b). B, A high-threshold burster (HTB). This neuron generated an initial burst only in response to strong long current pulses (a) and a single spike in response to brief current pulses (b). C, Grade I low-threshold burster (LTB I). This neuron generated bursts in response to threshold and suprathreshold long current pulses (a), but a single spike in response to brief current pulses (b).D, Grade II LTB (LTB II). This neuron fired bursts in response to both long (a) and brief (b) current pulses. E, Grade III LTB (LTB III). In addition to firing bursts in response to any threshold stimuli (data not shown), this neuron displayed spontaneous, rhythmic burst firing. The spontaneous burst marked with an asterisk in the top trace of E is shown in the bottom trace on an expanded time scale. In this and the following Figures, dashed lines indicate truncated spikes. Resting potentials are provided to the left of topmost panels.