An Electronic Controller of Volleys in Electrical Stimulation *

In neurophysiological investigation there is often need for a device which can precisely control the duration and repetition rate of a volley of electrical stimuli delivered to the tissue under observation. An electronic circuit (Fig. 1 ) was designed and built to meet that need. The circuit has three main components-the gating, univibrator, and pacemaker tubes. The gating tube (6SN7) is a cathode follower which is operated in a cut-off condition and which is activated for the desired time interval by a positive square wave generated in a univibrator tube (6SN7). The width of the positive pulse so generated determines the duration of the time interval that the stimulating current is allowed to pass to the tissues and is controlled by a resistor-capacitor combination. The bank of 0.02, 0.04, and 0.08 mfd. capacitors used here produces maximum intervals of approximately 30, 60, and 100 milliseconds, respectively. The 1 meg. variable resistor allows finely graded intermediate values to be obtained as desired. The rate at which the univibrator pulse is generated is determined by the pacemaker circuit. The 884 tube employed is a thyratron relaxation oscillator. A positive pulse is generated in the cathode circuit of this tube, and the pulse serves to activate the univibrator. The rate of discharge of the pacemaker is controlled by a variable resistor. The 5 meg. variable resistor depicted here can vary the repetition rate from 2 per second to 1 every 4 seconds. If a single stimulus is desired, the plate supply to the pacemaker is removed by opening switch SI, whereupon the univibrator is activated each time the switch S2 is closed manually, thus discharging the 0.001 mfd. capacitor in the grid circuit of the univibrator. The circuit, as used by the authors, controlled a 60-cycle per second sine wave stimulating current. It was desirable that the sine waves passing the gating tube should start consistently on their rising phase.