MECHANORECEPTION 29 



smoothly, and must attain some critical level before any impulses are 

 initiated. 



All hairs have a steady resting potential until a mechanical stimulus 

 is applied. The potential may be either positive or negative, depending 

 upon the type of hair. When the hair is stimulated an increase in 

 negativity, which varies smoothly as the stimulus is increased or 

 decreased, is seen at the recording electrode. Two types of receptor 

 potentials have been observed. In one kind of hair the potential per- 

 sists only during the motion of the hair (Fig. 17); in another kind, it 

 lasts as long as the hair is deformed (Fig. 18). In the former the return 

 of the hair to the unstrained position initiates another receptor 



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Fig. 18. Response of mechanosensory hair on outer clasper of male 

 Phormia to mechanical stimuli. A, B, and C are successive records taken 

 from the same hair, but the stimuli were not the same. Arrows indicate 

 onset and cessation of deformation. The deformation of the hair was 

 approximately proportional to the amplitude of the response for each 

 record. Positive at recording electrode is down. Time marks recur at 

 0-2-second intervals. (Courtesy of M. L. Wolbarsht.) 



potential. This kind of hair adapts very rapidly. The other type adapts 

 very slowly. Sometimes adaptation is still incomplete at the end of 

 twenty minutes. 



The impulses always have an initial positive phase in contrast to the 

 negative-going receptor potential. They are usually monophasic. 

 Impulses occur only after some threshold of receptor potential has 

 been reached. The threshold receptor potential for two types of fast- 

 adapting hairs which is accompanied by an impulse is plotted for a 

 series of test stimuli in Figs. 19 and 20. There is also a relation between 

 the frequency of discharge and the magnitude of the receptor potential. 

 Bernhard, Granit, and Skogland (1942) had shown that frequency was 

 determined by the refractory period of the spike-generating mech- 

 anism and the magnitude of the generator potential. Up to the point 

 where the increase in generator potential causes no further increase in 



