INITIATION OF IMPULSES AT RECEPTORS 



'S' 



FIG. 4. Receptor potentials from different receptors. .1 and B. from frogs muscle spindle, pro- 

 cainized. Top: stretch. Bottom: receptor potential. Time, A, 500 cps; B, o.i sec. [From Katz (58).] 

 C: from cat's Pacinian corpuscle, procainized. Upper trace {at starty amplitude and duration of 

 displacement and time in msec. Note that this trace crosses the other trace during displacement. 

 Lower trace (at starty receptor potential record. [From Gray & Sato (37).] D: from crayfish 

 stretch receptor. Arrows mark duration of stretch. Time, i sec. [From Eyzaguirre & Kuffler (27).] 



depends only on the amplitude of the stretch. Figure 

 4B shows also the events occurring when the stretch is 

 released. It can be seen that there is a change of 

 potential in the opposite direction to the other deflec- 

 tions, that is the electrode near the receptor goes posi- 

 ti\e to the distant electrode. The tiine course of this 

 deflection tends to be slower than that of the initial 

 dynamic phase, but it must be remembered that 

 relaxation of the muscle depends on the restoring 

 forces in the tissue while the stretch is actively im- 

 posed. The three phases of the receptor potential 

 correspond to the initial burst, to the maintained 

 discharge and to the reduced discharge that follows 

 the end of a stretch. 



Figure 4C (37) shows a receptor potential from a 

 Pacinian corpuscle and with it the voltage pulse ap- 

 plied to the crystal transducer that was used to stimu- 

 late; impulse activity has been prevented with pro- 

 caine. This potential differs in several respects from 

 that found in the muscle spindle. There is no main- 

 tained plateau, the potential declining to zero once 

 the peak is past. The shape of the receptor potential 

 is nearly or completely the same whether excited by 

 a short pulse of say 0.3 msec, duration, Ijy the be- 



ginning of a long pulse (fig. 4C) or by the end of a 

 long pulse. As in the case of the muscle spindle these 

 results are consistent with the results of experiments 

 on the excitation of impulses by these receptors. These 

 two examples illustrate contrasting types of receptor 

 potential, the one associated with tonic behavior and 

 the other with phasic behavior. In particular it is 

 worth noting that a receptor potential, and with it an 

 impulse, is set up by decompression of a Pacinian cor- 

 puscle, while relaxation of a muscle spindle is asso- 

 ciated with a positive going receptor potential and 

 an inhibition of the impulse discharge. 



The receptor potential in figure 4Z) is that of a 

 slowly adapting stretch receptor from the crayfish 

 (27). This was recorded by means of a microelectrode 

 in the cell body of the neuron which, in this instance, 

 lies in the periphery close to the muscle; the receptor 

 part of the cell lies still further to the periphery in the 

 terminations that ramify in the receptor muscle. The 

 record shows a steady depolarization well maintained 

 throughout the stretch; there is no marked dynamic 

 phase, even when the early part of the potential is not 

 obscured by spikes, as in figure 4Z), though there is 

 soine initial decline in the level of the depolarization; 



