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Physiological synaptic signals initiate sequential spikes at soma of cortical pyramidal neurons

DOI: 10.1186/1756-6606-4-19

Keywords: action potential, soma, axon, refractory period, sodium channels

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The neurons are one of basic units to fulfill the brain functions, and their events are executed at different subcellular compartments, such as the reception of synaptic inputs, the integration of these synaptic signals, the production of action potentials and the secretion of neurotransmitters [1,2]. In terms of the sources for firing action potentials, the current belief is that action potentials are generated at axon hillock [3-11]. In these studies, short-time square pulses are given and a single spike is induced. However, the regulations and mechanisms for the physiological signals integrated from synaptic inputs to trigger the spikes remains unknown.The neurons integrate the signals from numerous synapses and produce sequential spikes as the digital codes to carry various messages under the physiological conditions [12,13]. These integrated signals in vivo are long-time in nature, and their depolarization pulses induce sequential spikes [14-18] and Figure 1). A source for these in vivo signals to initiate sequential spikes has not been documented, which we have investigated at cortical pyramidal neurons by dual- recording their soma and axonal bleb simultaneously.The physiological sources of firing action potentials are ideally identified by using in vivo signals, which has not been documented yet. In order to address this issue, we have analyzed these signals that were intracellularly recorded from cortical pyramidal neurons in living mice.In vivo signals including those inducing sequential spikes (Figure 1A) and subthreshold pulses (Figure 1B) appear long time. Figure 1C illustrates that these depolarization pulses integrated in vivo fall into a range of 50~1600 ms in their durations. These in vivo signals are generally classified into steady-state pulses (an extended waveform in left panel of Figure 1B) and fluctuation ones (in right). The former is similar to direct-current pulses used to induce spikes in the most of electrophysiological experiments, and t


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