72 The Physiology of Sense Organs 



to abolish such ionic currents by changing their electrochemical 

 gradient. In the case of the generator potential of the Limulus 

 eccentric cell, net current flow across the membrane theoretically 

 disappears at a membrane potential of —8 millivolts (figure 29), 

 indicating that at this level the e.m.f. which drives the generator 



Fig. 32. A graph of the amplitude of the generator potential 

 (receptor potential) in a crustacean stretch receptor neuron as a 

 function of the ambient resting membrane potential. In the two 

 curves shown, extrapolations indicate that the receptor potential 

 amplitude declines to zero when the ambient membrane potential 

 is at o and — 10 millivolts, respectively. It is concluded that the 

 equilibrium potential of the ionic current generators responsible 

 for the receptor potential is close to these levels of membrane 

 potential, since, net current flowing across the membrane is very 

 small or absent altogether. (From Terzuolo and Washizu,'® Fig. 4.) 



currents has been effectively removed. Similar results were 

 obtained in the crustacean stretch-receptor cell (fig. 32), where 

 the equilibrium level for the receptor potential is close to zero. 

 The extracellular ion most likely to be implicated in the generation 

 of the receptor potential would in fact seem to be sodium, since 

 it is highly concentrated in the extracellular environment and, 

 under certain conditions, is known to pass selectively across the 



