328 



HARRY GRUNDFEST 



Depolarizing 



'rVr^r'ri 

 Hyperpolarizing 



Resting Potential 



Fig. 2. Interaction of generator potential and polarizing voltages on electrically 

 excitable membrane. Eccentric cell of Limiilus eye was impaled with a micro- 

 electrode, used both for recording and for polarizing the cell. Magnitude of the 

 applied currents are given on left. During polarization a constant light intensity 

 was also delivered for about 1 • 5 sec. This stimulus gave rise to a depolarizing 

 generator potential. The latter decreased with increasing depolarization (left) 

 while the spike frequency increased. The strongest depolarization itself evoked 

 spikes which recommenced after a silent period following the light stimulus. 

 With increasing hyperpolarization of the cell (right) the generator potential 

 increased, but the spike frequency decreased. The diagram indicates how these 

 interactive effects occur. The smaller generator potential during depolarization 

 was carried higher above the critical firing level. For the strongest hyperpolarizing 

 current the generator potential barely attained the critical firing level. The 

 changes in membrane potential from the resting level produced by the polarizing 

 currents are drawn proportional to the applied currents, but the amplitudes 

 of the generator potential exaggerated. (Modified from Fuortes, 1958; full 

 spike traces lost in original figure have been retouched. They were about 50 mV 

 high and probably were generated not in the cell body, but in the axon.) 



inexcitable input membrane and the other in an electrically excitable conduc- 

 tile membrane (Grundfest, 1957b, c, 1959a, c). Wherever this has been studied 

 it is now fairly general knowledge that the input element does not "support 

 spikes" and that the potential which it does produce is local and non-pro- 

 pagating (cf. Edwards and Ottoson, 1958; Eyzaguirre and Kuffler, 1955; 

 Gray, 1959; Loewenstein, 1959; Paintal, 1959). Direct evidence that this 

 membrane component of the Limulus eccentric cell does not react to electrical 



