COUPLING THE PREPARATION TO THE STIMULATOR 



uses a cathode follower to produce a positive-going output. Both stimulators 

 have facilities for disconnecting the flip-flop from the multi-vibrator and 

 driving it instead from an external pulse so that single-shock operation is 

 obtained. Bernstein's is shown in Figure 38.6; it has repetition frequencies 

 from 0-2 to 1,000 per sec and pulse durations from 100 ^sec to 50 msec. 

 Ead's has frequencies from 0-05 to 1,000 per sec and durations from 10 

 fj.sec to 100 msec; it is shown in Figure 38.7. Notice in these circuits the 

 use of catching diodes to define the Hmits of grid potential applied to the 

 output valves. 



In recent years a rather more elaborate pattern of stimulator has emerged, 

 having the block diagram of Figure 38.8. At each stroke of the oscillator 



Relaxation 

 oscillator 



External °- 

 trigger 



in 



-o 



Variable delay 

 No.1 



Pulse 

 forming circuit 

 No.i I 



Output 

 stage 



Variable delay 

 No. 2 



No.1 



— o 



Output 



No.2 

 Output 



-^ Trigger pulse 

 to CRT. time 

 base generator 



Figure 38.8 



the-time base sweep on the cathode ray tube is initiated, and one or two 

 shocks are delivered to the preparation as required, after a delay. Apart 

 from the value in research of being able to investigate the effect of two 

 stimuli delivered at different times, the use of delays ensures that nothing 

 of physiological interest can happen until the time base is 'on its way', and 

 there is no risk of phenomena being missed at the beginning of the trace. 

 The first stimulator of this kind was described by Attree^ and another was 

 published later by Perkins^. Attree uses a Miller transitron oscillator, a 

 transitron delay circuit, a pentode flip-flop pulse-forming circuit and a 

 cathode follower output. In addition there are a number of other refine- 

 ments. Perkins uses a multi-vibrator, a flip-flop delay circuit, flip-flop 

 pulse-forming circuit and cathode follower output. The author has also 

 designed a stimulator of this type, which is shown in Figure 38.9, but before 

 discussing it, it is necessary to digress into an important problem of technique : 

 how best to couple the preparation to the stimulator. 



COUPLING THE PREPARATION TO THE STIMULATOR 



If one is using electrical stimulation and, say, observing the rate of secretion 

 of a gland or the contraction of a muscle, no problem arises. The output 

 connections are applied to the preparation via electrodes as dictated by the 

 experiment. The difficulty arises when one is using electrical recording as 

 well, where stimulus pulses of the order of volts are being applied to the 

 preparation, and perhaps, only a few cm away, recording electrodes — which, 



607 



