Increased excitability of SG neurons in the arthritis pain model. Increased action potential firing rates and decreased thresholds for action potentials were recorded in SG neurons in slices from arthritic rats compared to controls. A, B, Current-clamp recordings of action potentials (spikes) generated by direct intracellular injections of depolarizing current pulses of increasing magnitude (0 to 200 pA; 500 ms) in an SG neuron from a normal animal (A) and in an SG neuron from an arthritic animal (B). C, Analysis of the input-output relationships shows significantly increased spike frequency in arthritis (n = 13 neurons) compared to control (n = 25 neurons; P < 0.05; two-way ANOVA followed by Bonferroni posttests). D, Significantly decreased spike thresholds (membrane potentials at which action potential firing started) were recorded in SG neurons in arthritis (n = 11) compared to control neurons (n = 17; P < 0.01; unpaired t-test). * P < 0.05, ** P < 0.01.