Figure 6.

The CA3a region possesses more pacemaker cells A, five sequential multi unit records show population events are preceded by more spikes in the CA3a region (top) than in CA3b (bottom). The sixth trace for CA3a and CA3b is an averaged field potential showing a ramp deflection that emerges first in CA3a. All records made in 10 μm bicuculline. B–D, distinct patterns of single cell activity before spontaneously occurring epileptiform events. Intracellular record (top trace), extracellular field (bottom trace) and raster plots of spike timing for 12 sequential events (middle). In strong pacemaker cells (B) firing occurred more than 20 ms before the population field potential both at rest (spontaneous) and from potentials of −70 to −80 mV (hyperpolarized). In weak pacemaker cells (C) firing preceded the local field only at rest. In follower cells firing never preceded the local field. Spike timing was measured from more than 50 events at both potentials. E, more pacemaker cells were recorded from the CA3a region and more follower cells from the CA3b region. Strong pacemaker cells were only observed in CA3a.