SUMMATION OF INADEQUATE STIMULI 223 



after-potentials run a course characteristic of the fiber. The spike 

 potential in normal mammalian nerve fibers (group A) does not drop 

 vertically back to the resting potential, but is followed by another nega- 

 tive potential (after the spike has passed) lasting about 15 milliseconds. 

 This potential has a rising phase of its own and begins before the spike 

 potential has returned to normality. 



The negative after-potential is a separate phenomenon. The present 

 evidence shows it to be depressed by lack of oxygen. It is abnormally 

 prolonged by veratrine and depressed by carbon monoxide poisoning; 

 its duration is decreased by monovalent ions (K + ) or increased by diva- 

 lent cations such as Ca. Its basic significance, however, is still in doubt. 

 The overall action potential drops below the resting potential, becomes 

 positive, and returns to its normal value in about 50 milliseconds. The 

 positive after-potential is probably intimately related to the spike poten- 

 tial, which is unquestionably the sign of the nerve impulse itself. 



As an impulse travels along a nerve, the region of breakdown is in an 

 absolute refractory state and the region immediately behind this is in a 

 relatively refractory state. The importance of all these phenomena lies 

 in the fact that they determine the excitability of the nerve. 



Excitability is defined as the reciprocal of the threshold magnitude 

 necessary to excite the tissue. 



Summation of Inadequate Stimuli 



If the stimulus is subliminal, no nerve impulse is developed. How- 

 ever, if a second subliminal stimulus follows the first within 1 millisecond 

 or less and a nerve impulse is developed, the impulse is said to be due to 

 the summation of inadequate stimuli. To account for this it has been 

 suggested that any stimulus gives rise in a nerve fiber to a local excitatory 

 process which does one of two things: if it is sufficiently great, it initiates 

 a nerve impulse; if not, it reverses itself and subsides. If, during the 

 reversal, a second subliminal stimulus is applied to bring the intensity of 

 the process up to or above threshold, it will give rise to an impulse. 

 Hence a stimulus, however weak or briefly applied, leaves the point of 

 application in the nerve fiber in an altered state of excitability after the 

 stimulus has ceased to act, and a time interval must elapse before com- 

 plete recovery has set in. 



If the stimulus is threshold so that a nerve impulse is initiated by the 

 excitatory process, the excitability drops to zero immediately after the 

 response has occurred: the cell or organ passes through an unexcitable 

 state called the completely refractory stage. This absolute refractory 

 period during which excitation is impossible is followed by a relatively 



