8 The Physiology of Sense Organs 



across the membrane, and both tend to drive sodium into the cell. 

 The increased permeabihty to this ion thus results in an immediate 

 inward current, depolarizing the membrane in surrounding 

 regions. In fact, during the brief time that sodium ions are 

 actually flowing into the cell, the membrane potential actually 

 reverses to a value of about 45 millivolts more positive on the 

 inside than on the outside. This potential difference is close to 

 the electrochemical equilibrium potential of sodium ions in the 

 system, and presumably it would be maintained as long as the 

 selective permeability to sodium were to continue. However, the 

 depolarization itself triggers a membrane reaction which, after a 

 brief period, reverses the sodium activation and inactivates the 

 mechanism responsible for the increased permeability to this ion 

 species. At the same time, there is a slow increase in the perme- 

 ability of the membrane to potassium ions. It is, in fact, the 

 facilitated outward movement of these ions that constitutes the 

 falling phase of the nerve impulse. The driving force which 

 accounts for their outward movement is of course the highly 

 positive state on the inside of the cell during the action potential; 

 the increased membrane permeability merely increases the already 

 finite mobility of potassium through the membrane structure, and 

 this movement restores the membrane potential toward its resting 

 value. The electrochemical equilibrium potential for potassium 

 ions in the system is roughly 90 millivolts, the inside being 

 negative to the bathing medium. This explains the tendency, 

 seen with many nerve cells, for the falling phase of the action 

 potential to undershoot the resting level of membrane potential ; 

 the increased permeability of the membrane to potassium during 

 the falling phase means that the cell's membrane potential 

 is dominated by this ionic current to a greater extent than it 

 would be otherwise. However, the hyperpolarizing undershoot 

 at the end of the action potential inactivates the enhanced potas- 

 sium permeability, so that the membrane potential once again 

 returns to resting values. 



The rather complex series of events described above offers no 

 explanation for the propagated nature of the action potential, i.e. 

 its unique ability to re-establish itself in immediately adjacent 

 membrane regions and thus spread throughout the cell. A clue 

 to the operation of this mode of propagation is in the very nature 



