CONDCCTION OF THE NERVE IMPULSE 



is not a direct one, but in some cases they are clearly 

 related to each other. 



After-Piilentials 



The term ' after-potential' was introduced by Gasser 

 and his associates [cf. Gasser & Erlanger (38); Gas- 

 ser & Graham (39)] to describe the small, slowly 

 declinina; potential change that follows the large, 

 short 'spike-potential' in the monophasic action po- 

 tential of a nerve trunk. The records furnished in the 

 left column of figure 34 show monophasic action 

 potentials of the nerve trunk taken at slow sweep 

 speeds. The action potential of A-fibers (top) shows 

 very little after-potentials, but the responses of B- 

 and C-fibers manifest large after-potentials following 

 the sharp spike-potentials. These potentials were re- 

 corded with extracellular electrodes (fig. i) from three 

 different nerve trunks of the cat. 



Let us ne.xi discuss the after-potentials recorded 

 from single fiber preparations. In the right-hand 

 column of figure 34 are shown the time courses of 

 the action potentials of three different kinds of e.\- 

 citable elements. An upward deflection in these 

 records represents a rise in the intracellular potential 

 (referred to the potential of the surrounding fluid 

 medium). The 'retention' of a higher potential level 



Mf 



I... I.. 



.I...I...I. 



Tiiiec 



50 

 msec 



200 msec 



FIG. 34. After-potentials in nerve trunks Qejl) and in single 

 fibers (right). A. Response of mammalian .-X-fibers. B. Re- 

 sponse of mammalian B-fibers. C. Response of mammalian 

 C-fibers. [The three records on the left are from Grundfest 

 (43)-] ^J' Action potential of a toad muscle fiber, recorded 

 intracellularly. Nf. Response of a toad nerve fiber poisoned 

 with veratrine. Sf. .Action potential of a squid giant axon. 

 Time marks on the right in msec. 



in the upper two records is often called a ' negative' 

 after-potential, because an action potential was con- 

 sidered in the classical physiology as a " negative' 

 variation of the potential of the nerve surface (cf. p. 

 105). Evidently, the term 'negative' after-potential is 

 at present confusing and inadequate. 



The after-potential of the frog (or toad) muscle 

 fiber (fig. 34, right top) seems to decay roughly at 

 the time constant of the membrane (30). This after- 

 potential is not associated with any measurable change 

 in the membrane resistance. These facts suggest that, 

 following one whole cycle of activity of the muscle 

 fiber membrane, there is an excessive charge of elec- 

 tricity remaining in the large capacity of the mem- 

 brane and this charge is dissipated through the mem- 

 brane resistance. In the nodal membrane of the toad 

 (or frog) nerve fiber, the time constant of the mem- 

 brane is far shorter than the duration of the spike 

 potential (table i, p. 89); therefore, an after-poten- 

 tial of this type does not exist in the amphibian nerve 

 fiber. 



The after-potential of the frog nerve fiber shown in 

 figure 34, right center, was induced by poisoning the 

 fiber with veratrine, an alkaloid which is known to 

 cause rhythmical activity in the mu.scle and nerve. 

 Gasser & Graham (39) have shown that this chemical 

 greatly enhances the (negative) after-potential of the 

 nerve trunk. The after-potential of this type is asso- 

 ciated with a concomitant decrease in the membrane 

 resistance (108, 133). 



The after-potential in the squid giant axon i.fig. 34, 

 right bottom) is often referred to as an 'undershoot': 

 the membrane potential stays, after the end of the 

 main spike potential, below the initial level of the 

 resting potential. As we have seen in the record of 

 figure 12, this after-potential is associated with a 

 pronounced decrease in the membrane resistance. 

 Grundfest el al. (45) found that there is a phase of 

 slightly increased membrane impedance following the 

 period of decreased membrane impedance. In the 

 sodium theory (p. 118), the undershoot in the squid 

 giant axon is attributed to an increase in the potas- 

 sium permeability of the membrane. 



The nature of the after-potentials in B- and C- 

 fibers in the vertebrate nerve is not clear. Further 

 discussions on the after-potentials of the nerve trunk 

 are found in the monograph bv Gasser & Erlanger 

 (38). 



Rhythmical Activitj! 



In excitable tissues in living organisms, action po- 

 tentials appear, as a rule, in more-or-less rapid sue- 



