Origins of the Receptor Potential 73 



membrane and depolarize it. However, the equilibrium potential 

 of this ion is about +45 millivolts in most preparations, and this 

 is a much greater level of depolarization (actually a reversal of 

 membrane polarity) than those quoted above. Presumably, there- 

 fore, some other ion species must also move with sodium and limit 

 its influence upon the membrane potential. It has been suggested 

 that an outward movement of potassium ions would be consistent 

 with these findings, and in fact experiments with synaptic struc- 

 tures indicate that excitatory potentials produced following release 

 of chemical transmitter from a presynaptic neuron do involve 

 potassium ions.^^ Experiments with sensory structures have been 

 confined to observations on the effects following the removal of 

 sodium ions from the extracellular medium. Following such 

 treatment, the amplitude of the receptor potential obtained from 

 a standard stimulus in the Pacinian corpuscle has been found to 

 undergo a reduction by up to 90 per cent. 2''' ^* There still remains 

 a residual depolarization, however, which is not reversibly 

 dependent upon the presence of sodium ions (fig. 33). Similar 

 results to these were obtained following application of sodium-free 

 solution to the frog muscle spindle.^'* ^*' ^* In these experiments, 

 the receptor potential was reduced, within several minutes, by 

 70 to 80 per cent., but was never completely abolished in a 

 reversible manner. It may therefore be concluded that, while 

 sodium ions are certainly involved in the ionic current responsible 

 for generating the receptor potential, other ion species also take 

 part. Moreover, these must be in addition to the suspected 

 involvement of potassium ions. This supposition is based on the 

 fact that while the potassium equilibrium potential is presumably 

 at a greater membrane potential than occurs at rest, the residual 

 receptor potential remaining after removal of sodium from the 

 external medium is still depolarizing in direction. In the 

 report mentioned above, ^' Calma found that reduction of the 

 normal calcium ion concentration was effective in reducing the 

 size of the receptor potential, and indeed it has now been shown 

 that calcium can take part in the inward excitatory current in 

 several different excitable tissues.***** In Calma's experiments, 

 however, these effects were countered by raising the magnesium 

 ion content of the medium, and in view of the antagonistic effects 

 of these two ions with respect to the excitability of nerve cell 

 S.O.— F 



