934 CENTRAL NERVOUS SYSTEM 



ulating object. This would serve to relieve the cat of pain, such as might 

 be occasioned if it stepped on a thorn. The receptors for these two 

 reflexes have their thresholds lowered each to a particular form of stimu- 

 lus, and the nature of the response is such that the leg moves in a way 

 which is appropriate to the conditions under which stimulation occurs in 

 nature. 



When nerve impulses are set up in the afferent neuron of a reflex 

 arc, the path over which they may travel is limited by the insulation of 

 the fibers of the nerve trunk to those neurons with which the afferent 

 fiber makes connection through synapses located in the grey matter of 

 the spinal cord. The characteristics of the synapse, which we have de- 

 scribed in Chapter XC determine the destiny of the nerve impulse and the 

 characteristics of the reflex response which results. 



Summation. It was pointed out that a single nerve impulse frequently 

 fails to pass across a synapse, over which a series of impulses may travel 

 if they follow one another in rapid succession, so that their effects are 

 summated. This characteristic of synaptic conduction is of importance in 

 regulating reflex activity, as may be realized when it is considered that 

 the organism is constantly in receipt of many unimportant stimuli. Be- 

 cause of the necessity for summation only those stimuli call forth a re- 

 sponse which are of some intensity and duration. Consequently mo- 

 mentary and hence insignificant stimuli do not affect its behavior. 



6. The Refractory Period. This has been well defined by Sherrington 

 as being "a state during which apart from fatigue the mechanism shows 

 less than its full excitability." We are already familiar with the re- 

 fractory period in the cases of the heart muscle and the musculature of 

 the esophagus and intestine. For example, the application of a stimu- 

 lus to the quiescent frog heart while it is contracting in response to an im- 

 mediately preceding stimulus fails to produce any further effect. The re- 

 fractory period is extremely brief (one thousandth of a second) in a 

 nerve trunk, but is much longer in a reflex arc, being probably longest 

 in the case of the scratch reflex, in which it is demonstrated by the 

 fact that, however frequently we apply suitable stimuli to the sensory 

 surface, the rhythm of response of the contracting limb is always the 

 same. After each stimulus, therefore, a refractory period must become 

 developed during which a repetition of the stimulus has no effect. It 

 is evident that the existence of the refractory period is the factor 

 responsible for the rhythm of the movements. 



It is interesting to consider what part of the reflex arc is responsible 

 for the existence of the refractory phase. It obviously can not be a 

 function of the motor neuron, for through the same motor neuron may 

 be discharged, at one time, impulses which bring about the scratching 

 movement and, at another, those causing a tonic flexion of the 



