Spatial dynamics of action potentials estimated by dendritic Ca2+ signals in insect projection neurons

The spatial dynamics of action potentials, including their propagation and the location of spike initiation zone (SIZ), are crucial for the computation of a single neuron. Compared with mammalian central neurons, the spike dynamics of invertebrate neurons remain relatively unknown. Thus, we examined the spike dynamics based on single spike-induced Ca2+ signals in the dendrites of cricket mechanosensory projection neurons, known as giant interneurons (GIs). The Ca2+ transients induced by a synaptically evoked single spike were larger than those induced by an antidromic spike, whereas subthreshold synaptic potentials caused no elevation of Ca2+. These results indicate that synaptic activity enhances the dendritic Ca2+ influx through voltage-gated Ca2+ channels. Stimulation of the presynaptic sensory afferents ipsilateral to the recording site evoked a dendritic spike with higher amplitude than contralateral stimulation, thereby suggesting that alteration of the spike waveform resulted in synaptic enhancement of the dendritic Ca2+ transients. The SIZ estimated from the spatial distribution of the difference in the Ca2+ amplitude was distributed throughout the right and left dendritic branches across the primary neurite connecting them in GIs.