Exercise XVIII 



THE NERVE IMPULSE 95 



Recording 

 leads 



with time. To study this, the signals are applied 

 to the electron beam while it is moving hori- 

 zontally across the screen at some prescribed 

 rate. The horizontal sweeping of the electron 

 beam is controlled by the waveform generator, 

 which sets high voltage potentials across a pair 

 of plates that deflect the beam in the horizontal 

 direction. We control the rate of the horizontal 

 sweep by the waveform duration dial. The 

 operating mode dial lets us select how the sweep 

 is to be controlled, whether automatically or 

 manually. (Ignore the "gated" and "triggered" 

 selections on the equipment; these are for special 

 applications.) 



These instruments and the power supply com- 

 plete the recording setup. However, to study 

 nerve impulses, we must stimulate the nerve to 

 fire. This can be done artificially by applying a 

 brief electrical shock. (Next time we shall study 



the biological initiation of nerve impulses by 

 light striking a photoreceptor.) 



The pulse generator provides controlled elec- 

 trical stimuli. It, like the indicator sweep, is 

 activated by the waveform generator. Thus for 

 every sweep of the indicator beam, one stimulus 

 pulse is delivered. The pulse generator has 

 three controls: the pulse delay knob sets the 

 point during the horizontal sweep at which the 

 stimulus pulse is delivered; the pulse width 

 knobs control the duration of the stimulus pulse; 

 and, finally, the pulse amplitude controls its 

 voltage. 



That is all there is to the equipment. To 

 familiarize yourself with the controls, connect 

 the lead from the "pulse out" terminal of the 

 pulse generator to the "vertical input" of the 

 indicator. (The pulse generator provides enough 

 voltage so that the preamplification can be 



