CHARACTERISTICS OF REFLEX ACTIONS 305 



feebler and finally disappears altogether long before any signs of fatigue 

 in the motor apparatus can be detected by stimulation of the motor nerve 

 itself. The fatigue is produced equally well if the reaction be excited by 

 stimulating a sensory nerve directly, and since we know that it is practically 

 impossible to fatigue nerve fibres, we must conclude that the seat of fatigue 

 is in the grey matter of the spinal cord itself. 



(5) ' BLOCK ' OR RESISTANCE. In the central nervous system 

 there is an absolute block to the passage of an impulse backwards through 

 a synapse, i.e. from a nerve-cell or its dendrites into the end ramifications 

 of an axon. The phenomena of fatigue show that there is a certain degree 

 of resistance at the synapse to the passage of an impulse in the normal 

 direction, and that this resistance is rapidly increased under the conditions 

 which produce fatigue. When we study the structure of the central nervous 

 system more fully, we find that although there are certain shortest possible 

 paths, i.e. ones involving few neurons, for every impulse arriving at the 

 central nervous system, yet so extensive is the branching of the entering 

 nerve fibres and so complex are the neuron systems with which they come 

 in connection that an impulse entering along one given fibre could spread 

 to practically every neuron in the spinal cord and brain. Such a result is 

 indeed observed in animals poisoned by strychnine. In such animals the 

 slightest stimulus applied to any part of the skin excites strong tonic spasms 

 in the whole musculature of the body. Every single nerve fibre, that is to 

 say, can discharge into every motor neuron of the cord. That this result 

 does not ensue on localised stimulation in a normal animal is dependent on 

 the varying resistance to the passage of an impulse into the several neurons 

 with which the entrant fibre comes in relation. A small stimulus will dis- 

 charge only along the few neurons where the resistance is lowest. Increase 

 of the stimulus, either by increase of its strength or by summation of weak 

 stimuli, will enable the impulse to spread along more neurons and therefore 

 will elicit a more widespread response. Only when the * blocks ' are entirely 

 removed- by the administration of strychnine, or when the stimuli are 

 abnormally powerful and long continued, will the impulse spread to all 

 regions of the central nervous system, so that response becomes general 

 and inco-ordinate instead of local and adapted to the stimulus. 



(6) FACILITATION OR < BAHNUNG.' The passage of a nervous 

 impulse across a synapse or series of synapses in the central nervous system 

 has a twofold effect. If the passage be too often repeated, phenomena of 

 fatigue are produced and there is an increase of the block at each synapse. 

 If however the stimulus be not excessive and the reaction not too frequently 

 evoked, the effect of passage of an impulse once is to diminish the resistance, 

 so that a second application of the stimulus evokes the reaction more easily. 

 The process of summation in fact is chiefly in the direction of removal of 

 block. We have a close analogy to this process of facilitation in the ' stair- 

 case phenomenon ' observed in cardiac and unstriated muscle. In these 

 tissues the repetition of a sub-minimal stimulus renders it in time effective, 

 and then repetition of the now effective stimulus causes a gradually 



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