Fig. 6. Population responses to repeated electrical stimuli. Symbol and line plots (A and B ) show the mean (and SE) C-fiber responses of cells in wild-type (open symbols in A , left ) and neurokinin-1 knockout mice (filled symbols in B , right ) to a train of 16 electrical stimuli (5 mA, 2 ms) at different frequencies (1.0, 0.2, and 0.1 Hz;B , top right ). Responses of each neuron were normalized as a percentage of the mean response to the first electrical stimulus in each series. The bar graphs (C and D ) show mean total responses (and SE) at C-fiber (C ) and A-fiber latencies (D ) for cells in wild-type (open bars) and neurokinin-1 knockout (filled bars) mice. +P < 0.05, ++P < 0.01 compared with 0.1-Hz stimulation; * P < 0.05, ** P < 0.01, wild-type versus knockout mice.