In neocortical layer-5 pyramidal neurons, the action potential (AP) is generated in the axon initial segment (AIS) when the membrane potential (Vm) reaches the threshold for activation of the voltage-gated Na+ channels (VGNCs) Nav1.2 and Nav1.6. Yet, whereas these VGNCs are known to differ in spatial distribution along the AIS and in biophysical properties, our understanding of the functional differences between the two channels remains elusive. Here, using ultrafast Na+, Vm and Ca2+ imaging in combination with partial block of Nav1.2 by the peptide G1G4-huwentoxin-IV, we demonstrate an exclusive role of Nav1.2 in shaping the generating AP. Precisely, we show that selective block of ∼30% of Nav1.2 widens the AP in the distal part of the AIS and we demonstrate that this effect is due to a loss of activation of BK Ca2+-activated K+ channels (CAKCs). Indeed, Ca2+ influx via Nav1.2 activates BK CAKCs, determining the amplitude and the early phase of repolarization of the AP in the AIS. By using control experiments using 4,9-anhydrotetrodotoxin, a moderately selective inhibitor of Nav1.6, we concluded that the Ca2+ influx shaping the early phase of the AP is exclusive of Nav1.2. Hence, we mimicked this result with a neuron model in which the role of the different ion channels tested reproduced the experimental evidence. The exclusive role of Nav1.2 reported here is important for understanding the physiology and pathology of neuronal excitability.