FIGURE 2.

Schematic representation of the currents responsible for an action potential. For all panels: upper row, current flowing across the membrane of a neuron; lower rows: black traces, membrane potential of a neuron recorded by an intracellular electrode positioned in the soma and first derivative of the membrane potential; green trace: local field potential recorded by an extracellular electrode positioned near the AIS (LFP1); purple traces: local field potential recorded by an extracellular electrode positioned near an active and a passive dendrite (LFP2). (A) An action potential is generated at time 0 (t₀). The inward current triggered by the activation of Na⁺ channels at the AIS spreads actively in the axon (red arrows) and leaks out passively through the membrane (blue arrows). The intracellular electrode measures a depolarization of the soma at time 1 (t₁). The LFP electrode at the AIS detects a sink starting before the somatic depolarization. The dendritic LFP electrode does not record any change. (B) At time 2 (t₂), the active current has invaded the whole soma. The membrane potential reaches the peak of the action potential. The LFP recorded at the AIS terminates while the LFP recorded in dendrites starts. (C) At time 3 (t₃), the active current has reached the dendrite. The action potential recorded in the soma is finished. The dendritic LFP occurs as a positive deflection corresponding to the passive leakage of the current through the membrane. In case of an active dendrite, the positive event is immediately followed by a negative one caused by the presence of a sink.