Figure 5.
Efficient initiation of dendritic Na+ spikes in CA3 pyramidal neurons. (a) Examples of dendritic spikes in CA3 pyramidal neurons. Top traces, 5-ms current pulses (intensity: 600 pA, 960 pA and 1,180 pA). Upper middle traces, subthreshold response. Lower middle traces, isolated dendritic spike. Bottom traces, dendritic spike followed by an axosomatic spike. Black traces represent somatic signals and red traces represent dendritic signals. (b) Dendritic spikes evoked by EPSC-like current waveforms in CA3 pyramidal neurons. Top, EPSC-like currents used as stimuli (rise time constant = 0.25 ms, decay time constant = 5 ms). The peak current was increased from 300 pA to 2,700 pA in 600-pA steps. Bottom, corresponding EPSP-like voltage waveforms. Black traces represent somatic signals and red traces represent dendritic signals. (c) All-or-none characteristics of dendritic spikes. Plot of peak amplitude of the dendritic signal against intensity of the current pulse. Amplitude was measured after subtraction of scaled subthreshold responses. The stepwise increase at ~700 pA corresponds to the initiation of the dendritic spike, whereas the second increase at ~1.2 nA reflects initiation and backpropagation of the axosomatic action potential (bAP). Data shown in a and c were taken from the same cell; data shown in b were taken from a different cell. The recording sites on apical dendrite are 133 μm (a,c) and 142 μm (b) from the soma. (d) Histogram of number of recordings with (gray bars) or without dendritic spikes (open bars) at different distances from the soma. Note that dendritic action potential initiation robustly occurred at distances >100 μm. Stimulation intensity = 100 pA to 3 nA. The blue curve shows the corresponding probability of dendritic spike initiation (right axis), as obtained by fitting with a Boltzmann function. (e,f) Plot of initiation threshold for axosomatic spikes (e) and dendritic spikes (f) as a function of distance. Data in e and f are from the same set of recordings. In distal recordings, current stimuli easily evoked dendritic spikes, but occasionally failed to trigger axosomatic spikes even at high intensity (no corresponding data points in e). Open circles indicate threshold values for 5-ms current pulses. Filled circles indicate threshold values for EPSC-like current waveforms.