(A) image of a neonatal motoneuron during intracellular recording and stained with a fluorescent marker (Alexa 568 Hydrazide). (B) 2-photon confocal image of the same motoneuron after withdrawal of the intracellular micropipette. The dotted box indicates the area shown in the lower panels (C1-C4). (C) the motoneuron was also injected with a calcium-sensitive dye (Oregon Green BAPTA-1). An averaged image of the intracellular fluorescence is illustrated together with 3 regions of interest (ROI) (blue: soma, green and red: primary dendrites). C1-4, raw 2-photon confocal images showing the changes in fluorescence of the calcium dye during spontaneous activity (C1), quiescence (C2 and C4) and evoked bursting (C3). (D) optical signals measured from the 3 ROIs shown in C (blue, red and green), are displayed together with the intracellular membrane potential (Vm motoneuron) and the slow potential recorded extracellularly from the ventral root of the same spinal segment (Right vr-L6). The box in green highlights the increased somatic and dendritic optical signal corresponding to the burst of action potentials recorded intracellularly. Electrical stimulation of an adjacent ventral root (denoted by the horizontal violet bar) evoked a similar episode of bursting and optical activity.