Origins of the Receptor Potential 75 



Bennett has examined some sensory cells in Mormyrids which 

 appear to be implicated in the detection of potential field variations. 

 Briefly, he has found three types of cells, all of which are sensitive 

 to potential differences orientated along the transverse axis of the 

 animal. In all cases the receptor cells make functional contact 

 with second-order neurons at the periphery. It was of especial 

 interest to examine their mode of excitation, because at first glance 

 they would appear to violate the doctrine of electrical inexcit- 

 ability of the receptor cell membrane enunciated in Chapter 2. 

 From the results of his experiments, however, Bennett suggests 

 two types of sensory mechanisms which do not involve changes in 

 the resistance of the external (receptive) face of the cells involved. 

 In one type, very small potential gradients are envisaged, which 

 directly cause the release of a chemical transmitter at the peripheral 

 synapse by the sensory cell. With such a mechanism, the external 

 face of the cell is presumed to act purely as a resistive element to 

 inward current-flow generated by the potential gradients in the 

 external medium, as exemplified by the ampullary receptors of 

 figure 34. The resultant depolarization of the sensory cell would thus 

 directly effect the release of transmitter substance from its internal 

 face, just as in the nerve terminations in a presynaptic neuron. 22 

 The other type of sensory mechanism postulated is conceptually 

 more complex. The theory of its operation is exemplified by 

 the tuberous receptor and is shown in figure 34. According to this 

 hypothesis, the external face of the receptor cell consists of a mem- 

 brane with extremely high electrical resistance and a large capaci- 

 tance. Thus, current flow can occur through this membrane only 

 when there are changes in the external field strength, i.e. during 

 charging or discharging of the large capacitance associated with the 

 external face of the cell. The membrane comprising the inner 

 (synaptic) face of the cell is supposed to be electrically excitable in 

 the classical sense, so that with sufficient depolarization by an in- 

 ward capacitative current at the external face of the cell, regenera- 

 tive activity occurs at the inner face and is transmitted to second- 

 order neurons through either chemical or electrical synaptic struc- 

 tures. Bennett's hypothetical capacitative mechanisms is intrigu- 

 ing and revives an idea originally introduced by Katz^' to explain 

 the phenomenon of the receptor potential generated by mechanical 

 deformation of the muscle spindle. However, whilst recognizing 



