Responsiveness in Single Cells - 191 



Fig. 11-2. The electric potentials developed in a 

 beating heart are strong enough to stimulate a 

 nerve, if it is brought into contact with the heart. 

 In the experiment above, the frog muscle twitches 

 with each beat of the dog's heart. Note the wave 

 of electrical negativity that accompanies the 

 wave of activity (contraction) in the heart tissue. 

 (Redrawn from The Machinery of the Body, by 

 Carlson and Johnson. University of Chicago Press.) 



FROG 

 MUSCLE 



INACTIVE AREA 



ACTIVE AREA 

 NERVE 



heart, and these measurements prove helpful 

 in diagnosing cardiac function. 



Bioelectric Potentials. Each cell, of course, 

 is bounded by a plasma membrane that, in 

 the living state, displays a very distinct 

 electrical polarization. The outer surface is 

 positive in relation to the negatively charged 

 inner surface. In the resting, or unexcited, 

 cell this difference of potential is called the 

 resting potential; it usually amounts to some 



50 to 100 millivolts (Fig. 11-3). When a cell 

 is excited, however, a localized and momen- 

 tary depolarization of the membrane occurs 

 and, in fact, there is even a reversal of the 

 normal polarity. This rapid shifting of the 

 membrane potential is called the action po- 

 tential (Fig. 11-3). The action potential is 

 not static, however. It rapidly propagates it- 

 self over the entire cell surface, starting at 

 the point of stimulation. Moreover, the 



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INSIDE 

 ELECTRODE 



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OUTSIDE 

 ELECTRODE 



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Fig. 11-3. Bioelectric potentials as recorded in the axon of a giant nerve cell (squid). The 

 velocity of the action potential, as it speeds along the nerve fiber, is in exact synchrony with 

 the transmission of the excitation, or nerve impulse. The velocity specified here, however, is 

 derived from a mammalian nerve. (Modified from Bernhard Katz.) 



