INITIATION OF IMPULSES AT RECEPTORS 



129 



EXCITATION OF IMPULSES BY CONTROLLED PULSES 

 AND PHASIC RECEPTORS 



In the last section stimulus-frequency relations 

 were considered. Such relations give important in- 

 formation about units that signal the values of steady 

 states by indicating them as particular and repeatable 

 frequencies of impulses. That is to say these relations 

 are important for nonadapting or tonic units. On the 

 other hand, the response of phasic units, and the 

 adapting part of responses of tonic units, are de- 

 pendent on the time course of the stimulus; in partic- 

 ular the rates of change at the beginning and end of 

 the pulse are important. To investigate these phasic 

 units in detail, it is therefore important to use stimuli 

 of known time course. It is also important that the 

 stimulus should be adequately damped. The im- 

 portance of this can be shown by an example: Pacin- 

 ian corpuscles have thresholds of a few tenths of a 

 micron and, for the amplitude threshold to be mini- 

 mal, the displacement must be complete in less than 

 a millisecond (34); if large displacements of tens of 

 microns are used, it only requires a one per cent oscil- 

 lation to give rise to what appears to be a repetitive 

 response. Various techniques have been used for this 

 purpose. Thus for mechanical receptors, electro- 

 magnetic (6, 57) and crystal transducers (34, 35) 

 have been used. The former have bigger displace- 

 ments, but generally have a slower time course than 

 the latter which can have a damped rise time of 0.2 

 msec, and a displacement of 10 to 20 /j. It should be 

 noted that even 0.2 msec, is not very short compared 

 with the latency from the beginning of the stimulus 

 to the impulse. 



Quantitative Aspects of Excitation 



Using such methods, the latencies for impulse ini- 

 tiation in Pacinian corpuscles and frog skin receptors 

 have been measured (34, 35). In the Paciniaii cor- 

 puscle latencies after the onset of mechanical deforma- 

 tions of any duration are longer (i.e. 0.5 to 3.0 msec.) 

 than those following the beginning of a constant 

 current stimulus to the receptor's own nerve fiber 

 within a millimeter of the ending. After mechanical 

 stimulation of frog skin even longer latencies have 

 been observed. The latency observed in the Pacinian 

 corpuscle can be shown to be due to the time taken 

 for the receptor potential to develop (37); it seems 

 likely, therefore, that the longer latencies found with 

 frog skin receptors indicate even more prolonged 

 receptor proces.ses. Curves of recovery after the ini- 



tiation of an impulse by a short mechanical pulse to 

 a Pacinian corpuscle have been shown to be similar 

 to the curves of recovery obtained after electrical 

 excitation of the ending's own nerve fiber close to the 

 corpuscle and of nerves in general (34). Thus, in this 

 instance at least, there is direct evidence that the 

 time course of recovery at the site of impulse initiation 

 is not much different from that in other parts of 

 nerves. 



The change of amplitude threshold with change of 

 stimulus velocity has also been measured, and the 

 minimum velocity of stimulus necessary for excitation 

 found. Thus, just as there is a critical slope in the 

 excitation of nerve by a linearly increasing current, 

 so there is a critical slope in the excitation of phasic 

 receptors by linearly increasing displacements. Such 

 measurements give a quantitative measure of the 

 adaptation of such receptors. Thus the critical slope 

 for a Pacinian corpuscle is given as 1 200 rheobases 

 per sec. (36) and that for receptors in frog's skin 61 

 rheobases per sec (35). 



As a means of investigating the fundamental mech- 

 anisms of receptors, such measurements have been 

 superseded by direct recording of receptor potentials; 

 but they are still of use in certain types of quantitative 

 investigation (53). 



On and Off Responses 



At least some phasic receptors respond with one or 

 a few impulses to a change from one state to another; 

 this response is not qualitatively dependent on the 

 sign of this change. Thus many photoreceptors re- 

 spond when the intensity of illumination on them is 

 suddenly raised from one level to another and again 

 when the intensity is suddenly reduced (30). The same 

 type of response to change of state is seen in receptors 

 in toad and cat skin (73). Measurements of the 

 threshold amplitude for on and oft" responses to rec- 

 tangular displacements have been made for Pacinian 

 corpuscles and frog skin receptors; in the former the 

 threshold for a compression (the 'on response') is 

 usually slightly lower than that for a decompression 

 (the 'off response'), but not infrequently the reverse is 

 true (34); on the other hand the excitability of the 

 frog's cutaneous receptors to a compression is much 

 greater than the excitability to the decompression 

 (35). These difTerences may well be due to the 

 mechanics of the systems, for in these experiments 

 compression is a result of an externally applied force, 

 while decompression depends solely on the restoring 

 forces inherent in the tissue; it is likely that restoration 



