I30 



HANDBOOK OF PHYSIOLOGY 



NELROPHYSIOLOGY I 



is a much more rapid process in a Pacinian corpuscle 

 than in frog's skin. 



Summatiim 



Two subthreshold short pulses applied to a phasic 

 receptor within a suitable interval of each other can 

 summate and set up an impulse; the essential point 

 in this experiment is that the first pulse is over before 

 the beginning of the second and the summation takes 

 place in the receptor. Further discussion of this point 

 will be left to the ne.xt section where receptor poten- 

 tials are discussed. One particular case can, however, 

 be discussed here. It is possible to observe summation 

 between the subthreshold activity evoked by a small 

 short mechanical pulse and a brief electrical test 

 shock. .Such a test shock can be used to measure the 

 excitability of the receptor at different times after the 

 application of the mechanical pulse; in this way in- 

 direct evidence of the time course of a receptor poten- 

 tial has been obtained (34)- 



RECEPTOR POTENTIALS AND OTHER 

 GENERATOR POTENTIALS 



It is now widely held that the immediate cause of 

 impulse initiation in receptors and sense organs is the 

 development of an electrical potential change which is 

 graded according to certain characteristics of the 

 stimulus and which is confined to the region of the 

 receptor or organ. .Such potentials have now been 

 found in a number of situations of different types and 

 these findings, together with supporting evidence such 

 as summation results from other sites, form the justi- 

 fication for such a generalization. 



In this .section I shall use the term 'generator po- 

 tential' to describe any graded potential change oc- 

 curring in a sensory receptor or in a complex sense 

 organ that can reasonably be supposed to be a cause 

 of the initiation of an impulse. The term 'receptor 

 potentials' I will confine to those generator potentials 

 occurring in a single receptor. Thus the cochlea 

 microphonic is a generator potential but not a re- 

 ceptor potential. 



Generator Potentials in Complex Organs 



These lie outside the scope of this particular chapter 

 but are included briefly for completeness. The cochlea 

 is the best example of this group. In this organ there is 

 a potential difference maintained between the endo- 



lymph and the perilymph (96). During the application 

 of a sound wave, an alternating potential can be re- 

 corded and shown to have its greatest intensity at the 

 point on the basilar membrane at which the hair cells 

 are situated (16). This potential is directly related to 

 the sound pressure wave (100). There is reason to 

 suppose that this microphonic potential, as it is called, 

 is the cause of impulse initiation (16). 'Microphonic' 

 potentials have also been found in other sites, e.g. 

 the lateral line organs (54) and sacculus (105). These 

 potentials serve a similar function to the receptor 

 potentials of neurons but, in the cochlea at least, 

 they represent changes of potential between multi- 

 cellular compartments instead of across cell mem- 

 branes. It is not improbable that there are common 

 factors in the development of these two types of po- 

 tential, but we cannot expect to find close parallels. 



Receptor Potentials Generated in Nerve Terminals 



.Such potentials have been recorded from certain 

 mechanically excitable receptors (6, 27, 37, 58}, from 

 photoreceptors (42, 79) and from olfactory receptors 

 (78). In all these instances the receptor potential has 

 been recorded at a distance from its source, and in no 

 case has the membrane potential of the receptor 

 region been recorded directly. In each of the three 

 mechanical examples on which we have information 

 at present, the records were obtained by recording 

 the currents flowing along the nerve fiber, the nerve 

 fiber behaving as a pair of passive concentric conduc- 

 tors. The changes in these currents must have been 

 related to changes in potential across the membrane 

 of the terminal portions of the afferent nerve fiber, 

 since all currents recorded must have crossed the 

 membrane peripheral to the recording region; this 

 does not prove of course that the changes are actively 

 generated across the terminal membrane. Reasons for 

 believing that the receptor potentials are in fact 

 actively generated at this site are given in the last 

 section of this chapter. 



Examples of receptor potentials are shown in 

 figure 4. Figure 4.-1 and B are records from muscle 

 spindles from the frog (58); in both experiments the 

 preparations had been procainized to prevent impulse 

 activity. Figure ^A shows the changes that occur at 

 the beginning of a maintained stretch; it can be seen 

 that there is a relatively large initial change of poten- 

 tial and that this is foOowed by a small but main- 

 tained potential change. The earlier phase, called the 

 dynamic phase, is related to the velocity of the 

 stretch. The smaller maintained change of potential 



