CHARACTERISTICS OF REFLEX ACTIONS 343 



(4) FATIGUE. In the muscle- nerve preparation the weakest 

 point and that which soonest suffers from fatigue is the end-plate, or 

 rather the field of conjunction of nerve fibre and muscle fibre. In 

 the central nervous system the synapses of the different neurons are 

 equally susceptible, and since several of such synapses are involved 

 in every reflex action, we should expect to find that the central nervous 

 system would show signs of fatigue before the peripheral structures. 

 If a given reaction be repeatedly elicited by applying a stimulus to a 

 certain area of the surface, the reaction becomes 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. 

 This 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 discharge 

 therefore 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 



