STRUCTURE OF NERVE CELL MEMBRANES 129 



other hand, there are a number of K + in the channel and these are 

 equally spaced across the membrane, the strong interaction between the ions 

 and their solvation (the channel walls) prevents any fluctuation in channel 

 size along its length. Through such a pore, ions can move without the barrier 

 occasioned by random changes in channel size. The information presently avail- 

 able is inadequate for determining whether the membrane channel is comparable 

 to a homogeneous dielectric, or whether there are ionized sites that can be occu- 

 pied by an ion. In this latter case, the sites need not be permanently ionized 

 but only ionizable upon the close approach of an ion. The actual situation is 

 hot important for the present discussion except to note that for a homogeneous 

 dielectric, ions of like charge could not approach each other more closely than 

 about 25 A., so that for a 100 A. membrane, the maximum number of ions per 

 channel would be four. Potassium ion distributions resulting in low and high 

 mobility are shown below. The lower line represents a "through channel" or 

 one of high ion mobility and is only compatible with a diminished membrane 



Inside K + - - Outside 



K +-_ - K+ - - K+ - K+ 



potential, while the upper line represents a channel with low ion mobility and 

 is the probable arrangement at high membrane potentials. 



Since membrane potential and number of through channels are apparently 

 inversely related, the passage of an electric current outward through the mem- 

 brane may transform a number of K + channels into through channels; a similar 

 effect is to be expected for Cl~ moving in the opposite direction. The change to 

 a through channel is equivalent to the imposition of a new interspace distribu- 

 tion pattern upon the membrane; the local and transient changes that are 

 involved are of considerable interest. Since the through channel is a rigid struc- 

 ture, its creation may perturb the interspace size distribution for those channels 

 immediately surrounding it. Some 12 interspaces have in common at least one 

 of the macromolecules immediately surrounding a given interspace, but the 

 disturbance of creating the through channel is likely to extend to at least 

 another layer of membrane molecules. The general reaction to the rapid filling 

 of a K + channel may be expected to be a momentary compression of these 

 surrounding interspaces. Some will be transformed to (Na + )i size, thus per- 

 mitting an influx of these ions; a considerable multiplication is to be expected 

 if the creation of a K+ through channel extends in effect over only two layers 

 of membrane molecules as 36 new Xa + channels may be created. As the perme- 

 ability of the resting membrane to K + is considered as 20 times that of Na + , 

 activity can result in a 720 X increase in Xa + if the mechanism for interspace 

 size change is efficient. The influx of Xa + will be terminated by the 'flow' or 

 diffusion of membrane molecules forming the Na + channels to more probable 



