Figure 7.
Dendritic spikes increase the efficacy of axosomatic action potential initiation. (a) Schematic illustration of the recording configuration. Encircled characters indicate correspondence between scheme and subsequent figure panels. (b) Dendritic spikes evoked by EPSC-like current waveforms in CA3 pyramidal neurons. Top, EPSC-like currents were used as stimuli. The peak currents were 150, 600, 1,200, 1,800 and 2,400 pA. Center, corresponding EPSP-like waveforms. Bottom, expanded view of the somatic EPSP-like waveforms. Black traces represent somatic signals and red traces represent dendritic signals. The recording site on the apical dendrite is 142 μm from the soma. (c) Local application of 1 μM TTX to the dendrite prolonged the rising phase of the somatic EPSP-like voltage waveform and increased the initiation threshold of axosomatic spikes. EPSC-like current waveform; peak current was 150 pA and 300–2,100 pA in 300-pA increments. The recording site on the apical dendrite is 294 μm from the soma. Data in b and c were from different cells. Arrows indicate acceleration of rising phase produced by dendritic spikes. (d) Dendrosomatic propagation of dendritic spikes. Amplitude of dendritic spikes (DS) at the soma, normalized to that at the dendrite, was plotted against distance. The amplitude of the dendritic spike was measured after subtraction of scaled subthreshold responses. The attenuation of the backpropagated action potential (bAP) is replotted for comparison. (e) Summary graph of the average 20–80% rise time of EPSP-like voltage waveforms recorded at the soma without (−) or with dendritic spikes (+; *P < 0.05). (f) Summary graph of the average current threshold for the initiation of axosomatic spikes under control conditions and after local application of 1 μM TTX to the dendrite (*P < 0.05). In e and f, circles and lines represent data from individual experiments and bars indicate mean ± s.e.m.