MECHANORECEPTION 49 



because of *over stretch', i.e., extension beyond the physiological 

 range. The slowly adapting receptors clearly serve a static function. 



The corresponding organs in Crustacea possess two neurons, a 

 *fast' and a *slow' one; that is, a phasic and a static receptor. It turned 

 out when the insect organs were examined with phasic stimulation 

 that the single neurons were in fact dual-purpose receptors (Lowen- 

 stein and Finlayson, 1960). At rest the discharge activity is, within 

 certain limits, a linear function of absolute length or total displace- 



so 



ii 40 



3 



mm. 



Fig. 39. Peak discharge frequencies plotted against intensity of stretch. 

 A, larva of A. pernyi; b, longitudinal receptor of larva of A. juncea. 

 (Redrawn from Finlayson and Lowenstein, 1958.) 



ment. With plastic alternation of stretch and relaxation the impulse 

 activity is a combined function of displacement and velocity. At low 

 stimulus frequencies the phase relation of maximum response is 

 approximately constant, and maximum activity coincides with maxi- 

 mum slope (= max velocity) (Fig. 40). Impulses begin to drop out as 

 higher stimulus frequencies (1-5-5 c/s) occur (Fig. 41). Above 5 c/s 

 the organ fails to signal phasic stimuli accurately, although the res- 

 ponse is still in phase with the stimulus. Up to this point the stimulus- 

 response relationship is linear (Fig. 42). 



This receptor is thus seen to respond to displacement at low stretch 

 velocities, but from 3 to 4 c/s onward there is no response activity at 

 maximum displacement and during relaxation. Lowenstein and Fin- 

 layson (1960) were convinced that the decline of activity after the 

 velocity peak of the stretch is passed is accentuated by post-excitatory 



