780 



Comparative Animal Physiology 



of 0.4 msec). The negative after-potential is increased by veratrine, by the 

 alkahne earths, and by tetanization of a nerve trunk. In crab nerve fibers 

 the negative after-potential persists for 15 minutes or more.^-'^^- -^^ The after- 

 negativity in crab fibers, as in vertebrate fibers, is prolonged by veratrine^" 

 and by repetitive stimulation. '^° 



(5) A positive after-potential follovi's the negative after-potential (Fig. 

 293, A). It may persist for 100 msec, in small mammafian fibers and for 

 several seconds in frog fibers. It may last for many minutes in crab fibers.*^^'' 

 After-positivity is enhanced by yohimbine, by low calcium, and by a tetanus. 



A nerve fiber recovers its excitability rapidly so that after the spike its 

 membrane is again ready to conduct another impulse. The after-potentials 

 are related in some unknown way to oxidative recovery and to ionic unbalance. 

 In addition, the excitability of a nerve is enhanced during the negative after- 

 potential (catelectrotonus) and is diminished during the positive after-poten- 

 tial (anelectrotonus) (Fig. 294). 



Cervical sympathetic 

 after-potential 



I I I I I I ! I 



>140r . 20 



XI 



:§ 100 



i 



i 140r 



'. I I I I 

 60 80-, 



Cervical sympathetic ^ 



I 



100 



60 



Sapherjous 



msec. 20 



40 



60 



80 



Fig. 294. After-potentials and excitability cycle following a response, a, after-potential, 

 and b, excitability cycle in cervical sympathetic fibers (cat), c, excitability cycle in 

 saphenous nerve. From Gasser.^°^ 



Ionic Basis of Nerve Action Potentials. There are few fields of cellular 

 physiology in which it is so difficult to fit the varied chemical and physical 

 observations into a reasonable pattern as in nervous propagation. There is 

 little doubt that oxidations supply the recovery energy, probably by restor- 

 ing and maintaining the molecular structure of the plasma membrane. Thia- 

 mine is involved in activity. ""^^^ Energy appears to be made available by 

 phosphate transfer, as in muscle. 



A part of the action potential spike consists of reduction in the resting 

 potential, and this probably results from a net increased movement of po- 

 tassium outward, diminishing the ratio of potassium inside to that outside. 

 It has been demonstrated with thin-sheathed nerves (crustacean) that dur- 

 ing propagation the rate of exchange of potassium is greatly increased, with 

 the net effect of an outward movement of potassium. ^'^■''' ^^'''' ~'^^' ^^^ The 

 overshoot whereby the outside becomes negative to the inside may result 

 from siftiultaneous inward movement of sodium. ^^^ 



