14. Depolarization of the Cell Membrane 



The cell membrane divides the world in two: the "inside" and 

 the "outside." It receives all the messages coming from its sur- 

 roundings; with its specific permeability and pumping action it 

 decides the composition of the intracellular environment on which 

 function depends. It has an electric gradient, being mostly nega- 

 tive inside and positive outside, and is thus the seat of an electric 

 double layer with its potential energy, which it spends in "excita- 

 tion." To build up this potential it needs energy and so does it need 

 energy to restore this potential after excitation. It needs energy to 

 perform its pumping action, to move ions or molecules against 

 electric or concentration gradients. Last, but not least, it needs 

 energy for maintaining its structure which, like all living struc- 

 tures, is from a physical point of view an improbable one. 



For the study of the cell membrane, the muscle fiber offers a 

 propitious material, because the function of muscle is contraction, 

 a function which entails violent changes in energy, chemistry, and 

 shape, and can thus be registered by means of crude instruments 

 and observed even with the naked eye. These macroscopic changes, 

 summed up as "contraction," are changes in the physical state of 

 the actomyosin system. In living muscle these are elicited by 

 the collapse of the normal membrane potential. Whatever the 

 link between membrane potential and actomyosin may be, the fact 

 stands that the contraction of actomyosin indicates in the intact 

 muscle the collapse of the membrane potential, making, so to say, 

 this collapse visible. Ling and Gerard found that the resting po- 

 tential of the membrane could be decreased to a certain critical 

 value without eliciting contraction. In the muscles studied this 

 potential was around 55 MV. If this critical potential was reached 



99 



