192 - The Cell 



propagation is synchronous with the spread- 

 ing of the state of excitation. 



Resting potentials and action potentials 

 have been studied most extensively in nerve 

 cells, particularly the giant nerve cells of the 

 squid and other mollusks. These cells are 

 large enough to permit one of the electrodes 

 to the oscilloscope to be placed inside the 

 cytoplasm, while the other is kept on the 

 outer surface of the cell membrane (Fig. 

 11-3). Many other cells and tissues have also 

 been studied, however, and it is sale to con- 

 clude that the propagation of a definitive 

 action potential is a universal characteristic 

 of cellular excitation. 



The basic nature of these bioelectric po- 

 tentials will be discussed more fully later, in 

 a section dealing with the nervous svstem 

 (Chap. 25). Here, however, a few important 

 general factors will be considered. 



The maintenance of an adequately high 

 resting potential and the maintenance of 

 excitability depend upon the active ion 

 transport mechanisms of the cell (p. 121). 

 These mechanisms build up the concentra- 

 tion of potassium ions (K + ) inside the mem- 

 brane, frequently to a point where it is 30 

 times the outside concentration; and at the 

 same time the "sodium pump" tends to keep 

 the sodium (Na+) concentration about 10 

 times lower inside than outside. But several 

 other factors must also be kept in mind: (1) 

 the cell membrane is distinctly more perme- 

 able to potassium (K+) ions than to sodium 

 (Na+); (2) the mobility of K+ is greater than 

 that of Na+; (3) most of the negatively 

 charged ions inside the membrane are large 

 organic anions, such as protein, which have 

 very low mobility and virtually no pene- 

 trability; (4) most of the anions outside the 

 membrane, among which chloride (CI - ) is 

 heavily represented, are small inorganic 

 ions; and (5) the ion conductance of the 

 resting membrane is very low. 



Granting the foregoing conditions, which 

 tend to be universally present, a resting 

 membrane potential of the observed magni- 



tude is inevitable. Perhaps the most impor- 

 tant factor is the high concentration gradient 

 of the K+ ion. These mobile ions, driven by 

 the high gradient, tend to escape through 

 the membrane, but the large anions inside 

 the cell are not able to follow. Thus a 

 counter electrostatic force is built up, which 

 holds a layer ol K+ ions hovering close to 

 the outer surface of the membrane. Accord- 

 ingly the outer surface is always positive in 

 relation to the inner surface. Sodium ions, on 

 the other hand, experience great difficulty in 

 penetrating the resting membrane. More- 

 over, if such penetration should occur, chlo- 

 ride (CI - ) ions would acompany the Na + , 

 canceling out the charge. Sooner or later, 

 the sodium would be expelled by virtue of 

 the active transport mechanisms. 



The foregoing factors likewise play a 

 dominant role in the generation of the 

 action potential, which invariably accom- 

 panies successful excitation. A stimulus 

 changes the membrane structure at the point 

 of stimulation. In this small area, the mem- 

 brane momentarily becomes much more 

 permeable to ions, particularly sodium ions 

 (Fig. 11-3). In this sharply localized region, 

 therefore, sodium ions, driven by the high 

 gradient and electrical potential, enter the 

 cell very rapidly, canceling the negativit\ of 

 the inner membrane surface and allowing 

 potassium ions to escape from the outer 

 surface. This rapid and momentary exchange 

 of ions effects a localized depolarization of 

 the membrane. Locally the outer surface in 

 the excited area becomes distinctly negative 

 with reference to surrounding unexcited 

 areas. The influx of sodium is so vigorous, 

 in fact, that a reversal of polarity occurs in 

 the strictly localized excited area (Fig. 11-3). 

 Within a few milliseconds, however, when 

 the action potential is reaching its peak 

 (Fig. 11-3), normal structure and resistance 

 are regained by the membrane. Then quickly 

 the membrane potential returns to normal 

 sign and value. Meanwhile, however, the 

 excitation keeps spreading. A wave of nega- 



