REFRACTORY PERIOD 187 



in conductivity. These two factors go hand-in-hand, i.e. the 

 nerve is non-irritable and offers a resistance to the passage of the 

 impulse sufficient to swamp it during the absolute refractory 

 period ; during the relative refractory period the nerve steadily 

 recovers its irritability and its conducting power ; while the last 

 period is one of supernormal irritability and conductivity. 



8. Summation. If a second stimulus be applied to the nerve 

 during the third refractory period, it will give rise to an impulse 

 which will meet with less resistance in its passage along the nerve. 

 Now, if the first impulse be subminimal, i.e. insufficient to cause a 

 manifestation of energy in the motor mechanism which the nerve 

 supplies, then the second impulse if it be propagated along a 

 nerve during the supernormal period may cause the motor end- 

 organ to act. Such a phenomenon is called summation. 



9. Fatigue. Nerve fibres can apparently act as conductors of 

 the nervous impulses for very long periods without showing any 

 signs of fatigue. It is generally said that nerves cannot be 

 fatigued. While this is true of the conducting power of the fibre 

 it is not applicable to the neuron as a whole. (1) The nerve cell 

 loses something in the process. Granules which are apparent while 

 the cell is at rest diminish slowly during activity. Then (2) 

 changes take place at the synapses, the junction between neuron 

 and neuron, and also at the " end plate " or junction between 

 nerve fibre and organ. These potential junctions lose their power 

 to cause the impulse of one neuron to act as stimulus to the next 

 neuron or to the end organ. They become fatigued. 



10. Metabolism. This leads one to infer that the energy 

 exchanges during the conduction of impulses are small. The 

 amount of oxygen used is negligible and it is doubtful whether 

 a measurable amount of CO 2 is produced. There is no doubt of 

 the need for oxygen for the metabolic changes of the nerve cell, 

 but the extra amount necessitated by the passage of a nervous 

 impulse has not been estimated. It must be very small indeed. 

 Ingenious methods have been devised by Waller and by Tashiro 

 for the measurement of the CO 2 evolved during activity, but so 

 many uncontrolled sources of error enter into the experiment 

 that many physiologists hesitate to accept their figures as repre- 

 senting the actual amount of oxidation undergone. One must 

 bear in mind that an increased output of CO 2 does not necessarily 

 mean a simultaneous absorption of oxygen or a corresponding 

 liberation of energy. The CO 2 may be due to liberation of CO 2 

 from solution in the surrounding tissue by the heat of the stimulus 



