Speed and temporal precision of action potential propagation in BC axons increases during postnatal development. A, Schematic illustration of somatic whole-cell recording from a BC combined with extracellular stimulation of BC axons in the GC layer (gcl) at a distance of 70–1000 μm from the soma. Eleven consecutively evoked antidromic action potentials recorded somatically in a young (P8; black traces) and a mature (P21; gray traces) BC. In 3 of 11 trials, extracellular stimulation of a young BC did not evoke action potentials (arrows). BCs were held at −70 mV before stimulation. B, Latency between stimulus artifact and action potential (see Materials and Methods) was plotted versus distance between stimulation and recording site for young (P6–P10, 4 cells; left) and mature (P18–P25, 4 cells; right) BCs. Conduction velocity was determined from the linear fit of the data obtained in young (black line) and mature (gray line) BCs. Average values (filled symbols) are superimposed with data from single experiments (open symbols). C, Superposition of the 11 consecutive traces shown in A. Note the less precise timing of action potentials in the young BC (black traces) compared with the mature BC (gray traces). D, Left, Bar graph summarizes the CV of the conduction velocity of antidromic action potentials in young (P6–P10, 4 cells) and mature (P18–P25, 4 cells) BCs. Superimposed circles represent data from single experiments. Right, Bar graph summarizes the reliability of antidromic action potentials (AP) recorded at the soma of BCs (holding potential, −70 mV) evoked by extracellular stimulation in the granule cell layer (gcl). **p < 0.01.