PHYSIOLOGICAL PROPERTIES OF THE SIMPLE REFLEX ARC 811 



connections. It can be demonstrated by observing the action cur- 

 rent produced in the spinal cord by stimulating the anterior or posterior 

 spinal roots. In the former case no action current is observed, but it is 

 very evident in the latter case. 



6. The Refractory Period. This has been well denned 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 the exact structure of the reflex arc that 

 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 same muscles. Nor can the seat of the refractory period be in the 

 sensory area of the skin or the afferent neuron, for if a scratch move- 

 ment is elicited by stimulation at a point A in the proper skin area, 

 the rhythm of response which it calls forth will not in any 

 way be altered by the application of a second stimulus applied at B 

 at some distance from A and having a different frequency (Fig. 211). 

 There is evidently, therefore, some part of the reflex arc that is common to 

 impulses starting both at A and at B, for if in each of these spots a refrac- 

 tory phase occurred, then there would be interference before the two im- 

 pulses had reached the centers of the spinal cord. By exclusion, there- 

 fore, "the seat of the refractory phase seems to lie somewhere central 

 to the receptive neuron in the afferent arc "-(Sherrington 18 ). 



Many other types of reflex activity illustrate rhythm due to the re- 

 fractory phase. Two laboratory examples may be given: (1) When 

 the central end of an afferent root is stimulated in the lumbar region 

 of the spinal cord, the movement produced is distinctly rhythmic .in 

 character. (2) Upon stimulating the central end of the sciatic nerve 

 in a frog whose spinal cord has been cut some days previously, a clonic 



