'32 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



at the end of the stretch the potential simply returns 

 to its resting level. This difference from the muscle 

 spindle potentials may be due to the effective velocity 

 of the stretch. In this record, unlike those that are 

 illustrated in figure ^A, B and C, the impulse dis- 

 charge has not been interfered with and five spikes 

 are shown arising from the receptor potential. It can 

 be seen that each impulse is preceded by a relatively 

 slow decrease in membrane potential (the prepoten- 

 tial) and that when this decrease reaches a critical 

 value the impulse is discharged. 



Receptor potentials are not confined to mechani- 

 cally excited receptors. It has long been known that 

 slow potentials could be obtained from the retina and 

 from compound eyes (31). There has been reason to 

 suppose that part at least of these potentials repre- 

 sented activity of the receptors themselves. Direct 

 evidence that single ommatidia produce receptor 

 potentials has now been obtained. In the single 

 isolated ommatidium of Limulus (42) a receptor 

 potential builds up rapidly when the ommatidium 

 is illuminated. The potential then dies away, but 

 with suitable recording conditions some depolari- 

 zation appears to remain as long as the receptor is 

 illuminated. The cessation of illumination is not 

 accompanied by hyperpolarization. The olfactory 

 mucosa of the frog produces slow potential changes 

 when excited by air containing a suitable agent (78). 

 The distribution in area and in depth of these 

 potentials and their relative insensitivity to cocaine 

 suggest that they are due to synchronous activity of 

 the olfactory receptors. 



Relation of Receptor Potentials to Impulse Initiation 



There can be little doubt that receptor potentials 

 are the immediate cause of the initiation of impulses. 

 They always precede the impulse and the impulses 

 appear when a critical potential has been reached. 

 In the crustacean stretch receptor this critical level 

 remains constant under a variety of conditions. With 

 the frog's muscle spindle the critical potential depends 

 on the frequency of the discharge, an observation 

 which has been discussed above. In this preparation 

 the frequency of discharge is linearly related to the 

 amplitude of the receptor potential, a fact which sug- 

 gests that the receptor potential is causally related to 

 the impulse discharge. That such a relationship is not 

 immediately visible in the results obtained from the 

 crustacean stretch receptor does not mean, of course, 

 that the frequency of the impulse discharge is not 

 related to the amplitude of the receptor potential. 



This is perhaps best explained by considering the 

 steps involved in the initiation of the impulse. There 

 are reasons, which will be considered below, for sup- 

 posing that the impulses are initiated at a point which 

 is near but not identical with that at which the re- 

 ceptor potential is generated. Currents due to the 

 receptor potential will then flow through and dis- 

 charge the membrane of the neighboring parts of the 

 nerve fiber; this part of the membrane will develop 

 local responses (48, 56), and if the membrane poten- 

 tial falls to the critical level an impulse will he 

 discharged. This sequence of events is essentially the 

 same as that found during the repetitive firing of a 

 carcinus axon in response to an externally applied 

 constant current (49). The slowly rising prepotentials 

 of the crayfish stretch receptor (fig. 4Z)) are similar 

 to those of the current excited carcinus axon (fig. 3). 

 In both these examples the recording conditions are 

 such that what is recorded is related to the membrane 

 potential at the point of impulse initiation and not 

 to the intensity of the charging current or the po- 

 tential of the source supplying this current. In conse- 

 quence what is seen is the passive discharging and 

 local response of the membrane at the site of initiation 

 followed by the impulse if and when the memlDrane 

 potential falls to a critical level; this part of the 

 membrane is then repolarized and the cycle starts 

 again. The rate of discharging of the membrane and 

 hence the frequency of the impulses depends on the 

 intensity of the discharging current which in turn 

 depends on the size of the receptor potential; this, 

 however, is masked during a train of impulses. If the 

 amplitude of the receptor potential could be measured 

 during the impulse discharge a relationship between 

 receptor potential amplitude and frequency would 

 no doubt be found, and this might be similar to the 

 relation Ijetween applied current and frequency in 

 the carcinus axon. A relation was found in the case of 

 the frog's muscle spindle because, between impulses, 

 conditions were such that the full amplitude of the 

 receptor potential was recorded; this was proved by 

 subsequent procainization. Possible reasons for this 

 behavior have already been considered. 



Qjiantitatwe Relations Between Stimulus and 

 Receptor Potential 



The amplitudes of the receptor potentials of the 

 muscle spindle and Pacinian corpuscle increase with 

 the amplitude of the displacement up to a certain 

 point and then level off to a maximum. An example is 

 shown in figure 5. This particular example was ob- 



