Fig. 2.

Collective behavior in the axonal plexus depends on rate of ectopic spike generation: relationship to field gamma-frequency oscillations. (ai) Simulated gamma oscillation in the “intact” network (cf. ref. 7) with a mean ectopic rate of 1 per s per pyramidal axon. The “field” is the average somatic potential of 224 nearby pyramidal cells, and the “local axonal average” is the potential in midaxon of nine nearby pyramidal cell axons. Note the high-frequency components in the axonal average. (Right) Spectrogram taken from local axon average data. (aii) Simulated “isolated axonal plexus” (see Materials and Methods) with the same ectopic spike rate as shown in ai. (Right) Spectrogram from data. Compare with Fig. 1b. [Scale bars: ai, 10 mV; aii, 40 mV and 100 ms.] (b) Concentration dependence of high-frequency activity in the isolated oriens minisection and the model isolated plexus. (Left) Kainate (200 nM) fails to generate gamma-frequency oscillations in the intact CA1 minisection (data not shown) but does generate sporadic periods of high-frequency activity in these sections and in stratum oriens minisections (illustrated). In contrast, 1 μM kainate does generate gamma-frequency field oscillations in the intact CA1 section (see Fig. 1) and generates near-continuous high-frequency oscillations in the oriens minisection (illustrated). Plots of spike incidence in the stratum oriens field recordings revealed an increase in the number of spikes occurring with instantaneous frequencies of >100 Hz. The data shown are an example of spike incidence in a 2-s epoch of data from minisections in the presence of 200 nM (gray line) and 1 μM kainate (black line). (Right) Occurrence of random ectopic spikes with a modal incidence of 1 spike per axon per 20 s failed to generate field gamma-frequency oscillations in the intact model (data not shown) and induced sporadic high-frequency activity in the axonal plexus (illustrated as local axon average as shown in a). In contrast, an increased rate of ectopic spike generation (1 per axon per s) did generate field gamma oscillations (see a) and also generated a near-continuous presence of high-frequency activity in the isolated plexus. As with the experiment, this is accompanied by an increase in the incidence of spikes occurring with instantaneous frequencies >100 Hz. [Scale bars: Left, 0.2 mV and 200 ms; Right, 40 mV and 200 ms.]