ELECTRICAL STIMULATION 273 



Stimulating current 5% above minimal stimulus 



Summation interval ' Excitation time 

 sec. sec. 



Frog's nerve . . '0005 .. -003 



sartorius .... -0015 . . " -017 

 heart '0080 .. 2-000 



REFRACTORY PERIOD. The phenomenon of a refractory period 

 has long been known in connection with the heart muscle and has often 

 been regarded as characteristic of this muscle. If, in the isolated ventricle, 

 a beat be evoked by a single minimal stimulus, subsequent repetition of 

 the stimulus during the course of the contraction is ineffective, and becomes 

 effective only when the contraction has passed away. The heart is said to 

 be refractory to stimuli during this period. The duration of the refractory 

 period is a question of the strength of the stimulus used. With strong 

 stimuli the heart may be made to contract when the relaxation has only 

 progressed half way, and with very strong stimuli one contraction may 

 be made to follow the last at such a short interval that hardly any trace 

 of relaxation is observable between the beats. The phenomenon seems to 

 be common to all excitable tissues. Thus if two stimuli are applied to a 

 nerve within a sufficiently brief interval, the second stimulus is ineffective, 

 so far as can be determined by the response of an attached muscle or by 

 means of a capillary electrometer. The period is longer the lower the 

 temperature and varies from -0006 sec. at 40C. to -002 sec. at 12C. This 

 critical interval is lengthened if the irritability of the nerve is depressed 

 by narcotics. We may ascribe it 'to the second stimulus being applied 

 before the excitatory change due to the first stimulus has reached its 

 culminating point. 



THE EFFECT OF TEMPERATURE ON EXCITABILITY. It was 

 found by Gotch that the excitability of a nerve within certain limits was 

 increased by cooling the nerve and diminished by raising its temperature 

 (Fig. 121). Thus, if a frog be cooled to 2C. or 3C. for a day, it will be 

 found that simple section of the sciatic nerve may suffice to send the gastroc- 

 nemius into continued contraction, and under these circumstances ' closing 

 tetanus ' may be obtained with the greatest ease. This increase of excita- 

 bility does not apply to all kinds of stimuli. In the case of nerve its irri- 

 tability was found to be increased by warming, and diminished by cooling 

 for induction shocks and for all galvanic currents of less duration than 

 005 sec. In skeletal muscle Gotch found the excitability for all forms of 

 stimuli increased by cooling. Lucas has shown that these paradoxical 

 effects in nerve, namely, increase of excitability towards currents of long 

 duration and the simultaneous decrease towards currents of short duration, 

 are conditioned by two opposed changes in the tissue. The fall of tempera- 

 ture (1) delays the subsidence of the excitatory process, (2) renders more 

 difficult the initiation of a propagated disturbance. The first of these 

 effects reduces the current required for excitation in a ratio which varies 

 with the duration of the current. The second' increases the current 



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