Fig. 6.
Subthalamic neurons fired rhythmically with minimal spike frequency adaptation in response to current injection and exhibited a sigmoidal f–I relationship.A, Example of driven rhythmic firing by a subthalamic neuron. B, The same neuron displayed a sigmoidalf–I relationship. Note the transition to secondary range firing occurred at ∼35 Hz. Subthalamic neurons exhibited a rapid speed-up in instantaneous firing frequency in the first few intervals of a driven spike train in the secondary and tertiary ranges. From the point of maximal firing frequency a small spike frequency adaptation developed slowly. C, D, Calcium-activated potassium current limited excitability of subthalamic neurons during high-frequency firing. Suppression of calcium-activated potassium currents with cadmium (C) or apamin (D) shifted the f–I relationship to the left and disrupted low-frequency firing associated with the primary range. Speed-up and spike frequency adaptation within driven trains of spikes were present when calcium and SKCacurrents were blocked.