94 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



decreases with the logarithm of the external sodium 

 concentration, the proportionality constant being very 

 close to 58 mv which is the coefficient of Nernst's 

 equation (cf. p. 1 1 7). 



Based on this and other experimental facts, Hodgkin 

 & Huxley (59) formulated a hypothesis in which the 

 inward surge is interpreted as the consequence of an 

 increase in the membrane permeability specific to 

 sodium ions. We shall discuss this point later (p. 1 18). 



THRESHOLD AND SUBTHRESHOLD PHENOMENA 



In the early part of this century when physiolo- 

 gists had no way of directly observing the potential 

 difference across the excitable membrane, a great 

 number of articles were published dealing with the 

 problem of threshold excitation of the nerve or the 

 muscle. At first, physiologists were charmed by the 

 elegant physicomathematical scheme of the ionic 

 theory of nerve excitation formulated by Nernst (91). 

 He derived the relation between the threshold in- 

 tensity of current and its duration on the assuinption 

 that excitation took place when the concentration of 

 some ion reached a certain critical level near the 

 semipermeable membrane of the nerve. Nernst 

 argued that the passage of an electric current through 

 a uniform electrolytic conductor in the nerve cannot 

 bring about any electrochemical changes (except for 

 raising temperature) that might be responsible for 

 initiation of an impulse. His argument is ba.sed upon 

 the principles of electrolytic conductors and un- 

 doubtedly it is still valid at present. Nevertheless, 

 physiologists .soon abandoned Nernst's approach to 

 the problem and accepted more formal, physico- 

 chemically vague arguments which reached a climax 

 with Monnier-Rashevsky-Hill theory ol nerve excita- 

 tion (48, 88, 102). 



At present it is possible to pass rectangular pulses of 

 current uniformly through the excitable membrane of 

 the nerve fiber, and to determine how the membrane 

 potential behaves when the stimulus reaches thresh- 

 old. The assumptions adopted by previous investi- 

 gators can thus be subjected to direct tests. 



Threshold Membrane Potential 



The excitable membrane at the node of Ranvier of 

 the vertebrate myelinated nerve fiber is a narrow 

 ring-shaped band. Its width (0.5 to i m) is far smaller 

 than the diameter of the fiber at the node or than the 

 distance between neighboring nodes. It is possible to 



record potential changes across this membrane by the 

 use of a positive feed-back amplifier [e.g. McNichoI 

 & Wagner (87)]. 



At the top of figure 16 is shown the experimental 

 arrangement used to siud\ the behavior of the nodal 

 meiTibrane in threshold excitation. The fiber is 

 mounted across three pools of saline solution separated 

 by two air gaps. The large pool, where node Nn in the 

 figure is immersed, is filled with a dilute cocaine- 

 Ringer's solution. The pool in the middle, where the 

 node under study, N,, is located, is filled with normal 

 Ringer's solution. In the small pool, filled with 

 cocaine-Ringer's or an isosmotic pota.ssium chloride 

 solution, the small portion of the nerve fiber including 

 N; is immersed. The electrode in the large pool is 

 connected to a source of a .square voltage pulse. The 

 middle pool is grounded, and the smallest pool is 

 connected to the high impedance input of a positive 

 feed-back amplifier. .Since there is practicalK no 

 current in the portion of the fiber in the air gap 

 between Ni and No, the potential measured by the 

 amplifier approximates the potential drop across the 

 nodal membrane of Ni. A rectangular voltage pulse 



FIG. 16. Demonstration of the constancy of the threshold 

 membrane potential in stimulation of a single node of Ranvier 

 (Ni) with rectangular \oltage pulses (S). Nodes N,, and N2 

 are inexcitablc. V indicates the input of a positive feed-back 

 amplifier for recording the membrane potential. In each record 

 the stimulus intensity and duration used are given. [From 

 Tasaki (1^6).] 



