212 THE BIOPHYSICAL PROBLEM OF NERVE CONDUCTION 



The threshold value or minimal stimulus of a particular kind has a 

 magnitude which is just sufficient to excite. A stimulus greater than 

 threshold value will not produce a greater response. The magnitude of 

 the response is independent of the magnitude of the stimulus, provided 

 that it is not smaller than threshold. This general relation is known as 

 the all-or-none-law. The implication is that the response is characteristic 

 of the reacting structure and not of the energy employed to disturb the 

 state of that structure. 



Most living cells can be used as reacting structures in which the proper 

 stimulus will set up a disturbance, or nervous impulse. With modern 

 technique it is even possible to study the impulses set up in sensory nerve 

 fibers by the appropriate stimulation of the sense organs so as to deter- 

 mine the frequency, the form, and the speed with which the messages 

 are transmitted to the brain. 



Various forms of energy can be employed as stimuli. In biophysical 

 investigations of the excitability of nerve fibers it has become general 

 practice to use a brief electric current as stimulus, for it is highly probable 

 that the process of excitation itself is electrical. 



Electrical Stimuli 



Physiologists agree that an impulsive electrical discharge is by far the 

 most effective and convenient means of artificially stimulating nerve 

 fibers to set upa" nerve impulse," which travels over the whole length 

 of the nerve fiber in both directions from the point of stimulation. Be- 

 cause the intensity and duration of an electric current can be very accu- 

 rately controlled, it has become common practice to study the activity of 

 an isolated nerve when stimulated by a brief electric shock. It has the 

 advantage that such stimulation can be repeated without producing 

 appreciable damage in the nerve fiber. 



In most experimental work on the response of nerves to electric stimu- 

 lation, current taken off the secondary of a mechanically interrupted 

 induction coil has been used. A commercial 60-cycle alternating current 

 has also been employed. For precision work, impulsive currents should 

 be used with pulses timed by means of a mechanical regulator or more 

 accurately by a variable-frequency stimulator. 



Induction Coil Discharge 



In every properly designed induction coil with mechanical interrupter 

 a condenser is connected across the interrupter. Its capacitance must 

 be so chosen as to quench the spark as quickly as possible when the 

 mechanical interrupter opens the primary circuit. As a result, the break 

 of the primary current is much quicker than the make. For a given 



