CHAPTER III 



Conduction of the nerve impulse 



ICHIJ I TASAKI 



Laboratory of Neurophysiology, National Institute oj Neurological Diseases and Blindness, 

 National Institutes of Health, Bethesda, Maryland 



CHAPTER CONTENTS 



Introduction 



Compound Character of Peripheral Nerve 

 General Character of the Nerve Impulse 

 Cable Properties of the Invertebrate Axon 

 Cable Properties of the Myelinated Nerve Fiber 

 Conductance of the Membrane During Activity 

 Threshold and Subthreshold Phenomena 



Threshold Membrane Potential 



Strength-Duration Relation 



Subthreshold Response 



Measurement of Excitability by Using Test Shocks 

 Abolition of the Action Potential 

 Nervous Conduction Along Uniform Axon 



Nervous Conduction in Myelinated Nerve Fiber (Saltatory 

 Conduction) 



Effect of Increase of External Resistance 



Safety Factor 



Does the Nerve Impulse Jump from Node to Node? 



Field of Potential Produced by a Nerve Impulse 



Conduction in a Polarized Nerve Fiber 



Pfliiger's Law of Contraction 



Effect of Narcosis upon Nervous Conduction 

 After-Potentials and Rhythmical Activity 



After-Potentials 



Rhythmical Activity 

 Current Theories of the Resting and Action Potentials 



Resting Potential 



Action Potential 



THE MODERN DEVELOPMENT of the coiicept of thc neivc 

 impulse may be said to have started with the measure- 

 ment of the velocity of the nerve impulse by von 

 Helmholtz (141) in 1850. He measured the time 

 interval between delivery of an electric shock to the 

 nerve of a nerve-muscle preparation and the start 

 of contraction of the muscle by two different methods. 

 The first method used was to start a constant current 

 throuE;h a ballistic galvanometer at the lime of 



delivery of the shock and to interrupt the current 

 automatically by a switch opened by the twitch of 

 the muscle. The second method he used was based 

 on graphical registration of the muscular contraction 

 on a moving surface. He compared the time intervals 

 measured by stimulating the nerve near its two ex- 

 treme ends. 



Helmholtz's finding and the subsequent con- 

 firmation and expansion of his observation by a 

 number of investigators established the fact that a 

 nerve impulse travels along the nerve at a rate far 

 slower than that of light or sound in a similar medium 

 but substantially faster than the process of transporta- 

 tion of substances by streaming or diffusion in a slender 

 tube like a nerve fiber. Later, in 1908, Lucas (80) 

 found that the velocity of the nerve impulse doubles 

 with a rise in temperature of about 10 degrees. The 

 question of whether or not the nerve impulse is 

 associated with any chemical reacdons, however, 

 was not solved until Tashiro (138), Parker (98), 

 Fenn (32) and Gerard (40) established the increase 

 in production of carbon dioxide and consumption of 

 oxygen related to nervous activity. The demonstra- 

 tion of heat production associated with propagation 

 of nerve impulses by Downing et al. (24) gave a 

 further strong support to the view that chemical 

 reactions underlie the process of nervous conduction.' 



An entirely different line of approach to the study 

 of the processes underlying nervous conduction 

 originates with Hermann (47). He worked on a 

 'core-conductor model' of nerve which is the prede- 

 cessor of the passive iron model (75). The basic idea 



' Some investigators are of the opinion that all the chemical 

 reactions take place late in the recovery phase and not during 

 the period in which electrical signs of activity can be observed 

 [e.g. Hodgkin & Huxley (59)]. 



75 



