58 The Physiology of Sense Organs 



These investigations indicated that the higher frequency of 

 impulse discharge seen at the onset of a mechanical stimulus, or 

 during a step-wise increase in stimulus strength, is due to phasic / 

 response characteristics of the receptor potential. Steady 

 depolarizing currents applied to the sense organ produced stable, 

 non-adapting trains of impulses, which lasted throughout the 

 duration of the electrical stimulus. Thus, strictly neural factors 

 (such as accommodation and increased refractoriness) are appar- 

 ently of small consequence in these organs. High-gain extra- 

 cellular recordings, obtained from electrodes placed close to the 



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Fig. 25. Depolarizing receptor potentials (upward deflections) 

 and impulses evoked in an associated sensory neurone by displacing 

 an insect cuticular hair. The arrows indicate the approximate time 

 of onset of each stimulus and the direction of movement with 

 respect to the preceding one. Time marks recur every 200 milli- 

 seconds. (From Wolbarsht,i°3 pig. 8.) 



insertion of the sensory nerve in the muscle, showed an unmistak- 

 able phasic (' dynamic ') component early in the waveform of the 

 receptor potential similar to those seen by Katz in the muscle 

 spindle of the frog (cf. fig. 15). Thus, there can be little doubt 

 that the major adaptive process in the muscle spindle of vertebrates 

 involves a decline in receptor potential amplitude, and this, in 

 turn, probably results from a mechanical uncoupling at the level 

 of the insertion of the sensory nerve-endings into the receptor 

 organs. Other examples also can be cited, such as the mechano- 

 sensory neurons associated with trichoid sensilla in various 

 insects. ^°^ As shown in figure 25, records of the receptor 

 potential waveform from these cells, obtained during mechanical 

 stimulation of the organ, indicate once again that, in rapidly 

 adapting cells (such as those which respond only to movement), 

 the first component of sensory adaptation is a decline in the 



