128 



PHYSIOLOGICAL TRIGGERS 



electrogenic activity would not excite a sufficient number of other elements to 

 cause explosive responsiveness. On the other hand, in tissues normally respond- 

 ing explosively, the electrical thresholds of the membrane transducer elements 

 would be more nearly uniform. However, differences in their recovery rate 

 after absolute refractoriness, or differences increased by chemicals, would 

 convert explosive to graded responsiveness, temporarily in the first case and 

 permanently in the second. Formally, this suggestion is equivalent to the 

 concept of inactivation of sodium 'carrier' in the Hodgkin-Huxley theory It 

 differs from the latter in assuming that the ionic valves of the membrane do 

 not have identical potential dependence and that the latter can be modified by 

 factors such as specific ion effects and drug action. Increasing the spread of 



Input 

 TriggerjL 



Fig. 3. Graded responsiveness from an electronic circuit model. Left: A modified monostable 

 circuit. The resistors between the cathodes of the ampHfier tubes (12 \\}^) change the ampli- 

 fication in the circuit. Right: Oscillographic records (superposed sweeps) of the 'responses' to 

 increasing trigger pulses. The lower set of records shows growth of the 'response' and a sudden 

 transition to maximum output. The amplification was decreased for the upper set, producing 

 continuous gradation in amplitude. Time in msec. 



'valve' thresholds is equivalent to decreasing the amplification in a monostable 

 electronic circuit analog of the spike generator. When the loop gain of this 

 circuit is decreased sufficiently, the normal triggered 'all-or-nothing' response 

 becomes fully graded (fig. 3). This behavior of the analog, of course, does not 

 demonstrate the correctness of the hypothesis for the biological system, but is 

 suggestive of further experiments. 



As pointed out earlier, (p. 125), a discontinuous event, the spike, may arise 

 from the operation of a series of continuous i)rocesses. The concept of a popula- 

 tion of 'electrogenic units' involves a fundamentally different mechanism, in 

 which each 'unit' (valve, carrier, etc.) itself behaves as a bistable device. Thus 

 in the immediate region of a 'sodium valve' of the membrane tlie j)otential 

 should change abrujjtly from the resting value ('potassium potential') to that 

 of the reverse 'sodium potential.' The actually recorded potential, however, 



