The shape and amplitude of action potentials in MSO principal neurons progressively change after hearing onset. Action potentials were evoked in 81 cells via suprathreshold 100 ms square current steps. A, At P14, action potentials were large, exceeding 0 mV. Over the next few days, action potential amplitudes and rates of rise decreased markedly. Current steps are shown below the action potential responses. B, Action potential amplitudes, measured with respect to the inflection point, declined with a single exponential time constant of 5.2 d. C, Maximum rates of rise decreased with a time constant of 3.1 d. D, The current required for action potential initiation increased with a time constant of 5.2 d (single exponential fit). AP, Action potential.