CONDUCTION OF THE NERVE IMPULSE 



77 



of individual fibers from observations on the nerve 

 trunk. 



COMPOUND CHARACTER OF PERIPHERAL NERVE 



Soon after the first World War, Forbes & Thacher 

 (34) introduced a condenser-coupled vacuum tube 

 amplifier into the field of electrophysiology. Aided 

 by the continued development of electronic engi- 

 neering, Gasser & Erlanger (38) in 1922 took the 

 first photograph of a ner\e response recorded with 

 an instrument ideal in being inertialess. They 

 started using a cathode ray oscillograph to register 

 the time course of responses of the nerve. 



The standard technique of recording electric signs 

 of activity of a whole nerve trunk is to kill (ordinarily 

 by crushing) one end of a nerve and to place one 

 of the recording electrodes on this killed end (see 

 fig. i^). The other electrode needed to measure the 

 potential difference is placed on the intact part of 

 the nerve near the killed end. Ordinarily, either lightly 

 chlorided siKcr wire (abbreviated as Ag-AgCl) 

 or calomel half cells (Hg-HgCl) are used for recording 

 for they are nonpolarizable. Stimulating electrodes 

 (S in fig. i) can be either the Ag-AgCl Ringer type 

 or a pair of plain platinum wires. A precaution has 

 to be taken to ' isolate' the stimulus from ground, 

 namely, to eliminate metallic connection of the 

 stimulating electrodes with ground. The main reason 

 for the necessity of stimulus isolation is to prevent 

 flow of stimulating (and other) currents between the 

 stimulating and ground electrodes. The electrodes 

 and the nerve are generally mounted in a moist 

 chamber to prevent evaporation of water from the 

 surface of the nerve. 



The arrangement of the recording electrodes just 

 described is called a 'monophasic lead' and a re- 

 sponse of the nerve recorded with this arrangement is 

 referred to as a 'monophasic action potential'. The 

 traditional picture illustrating the principle of this 

 method of recording action potentials is as follows. 

 The portion of nerve carrying an impulse is 'elec- 

 trically negative' to the portion at rest. When an 

 impulse started by a stimulus emerges in the region 

 of the recording electrode Ei, the potential differ- 

 ence between Ei and E2 undergoes a transient vari- 

 ation which makes the potential at E2 more positive 

 (or less negative) to that at Ei. Since the impulse does 

 not reach the region of E2, a potential variation 

 representing the ner\ous activity at Ei is recorded 

 monophasically. 



The modern picture illustrating the principle of 

 monophasic recording (83, 1 24) is slightly different 

 from the classical one. Attention is now focused 

 upon the nerve fibers and the intercellular space in 

 the nerve trunk. When a nerve fiber carries an im- 

 pulse, it generates a transient flow of current in the 

 surrounding fluid medium. In the region of Ei and 

 E2, this transient current in the intercellular space is 

 directed from E; to Ei, raising the potential at E2 

 relative to Ei for a short period of time. The currents 

 produced simultaneously by many fibers in the 

 nerve are superposed in the intercellular space and 

 give rise to a large coiTipound action potential. In 

 this modern picture, the 'electrical negati\ity' in 



40- 



20- 



O.Zmtce 



100 



129 



150-1 



FIG. I. A. Demonstration of the constant velocity of 

 propagation of the a- and /3-waves in the action potential of 

 the sciatic nerve of the bullfrog. S, the stimulating electrodes; 

 El and E», recording electrodes, the latter at the killed end 

 of the nerve. The distance from the site of stimulation to the 

 recording electrode Ei is indicated on the \ertical line. The 

 starting points of the oscillograph trace show the distances at 

 which the records were taken. Abscissa, time. [From Gasser & 

 Erlanger (38).] B. A similar observation made on a three- 

 fiber preparation of the toad. The diameters of the fibers were 

 13, 9 and 5 II. The strength of the stimulating shocks employed 

 was twice the threshold for the smallest fiber. [From Tasaki 

 (124)-] 



