Muscle and Electric Organs 



589 



summate. Eserine, which inactivates choHnesterase, enhances the e.p.p., 

 but has no effect on the muscle impulse. The e.p.p., then, is graded, is 

 local, can summate, decreases rapidly with distance from the end-plate, and 

 when of sufficient size sets up a muscle impulse which is propagated along 

 the muscle fiber, is all-or-none in height, and is conducted at a measurable 

 velocity. Propagation in frog muscle at room temperature is 1-2 m./sec.^-^^ 

 (Table 71). 



In most striated muscle fibers there is a marked negative after-poten- 

 jJ3|204, 241 (Fig. 220). Veratrine enhances the negative after-potential; it 

 also may delay relaxation by causing repetitive discharge. ^^"^ "'^ Negative 

 after-potentials can summate and fuse in multifiber preparations, owing to 

 asynchrony.^" 



The contractile response to a propagated all-or-none muscle impulse is a 

 twitch. The muscle membrane recovers quickly, more quickly than the 

 muscle can relax, hence the membrane can conduct impulses at high fre- 

 quencies, keeping the muscle in a contracted state (fused tetanus). An 



Fig. 220. Action potential (solid line) and myogram (broken line) for sartorius of cat. 

 Rapid propagated spike followed by slow negative after-potential. From Rosenblueth 

 and Hoagland.^ 



isolated entire muscle fiber can be stimulated to give all-or-none twitches;^^'^ 

 a piece of fiber or intact fibers in poor condition, however, give local con- 

 tractions which are not propagated. ^^^' ^^ In all-or-none twitches the muscle 

 impulse activates the contractile elements over which it passes; in graded 

 local responses the contractile elements are stimulated directly without a 

 muscle impulse. The local contractions are graded, summate, and show no 

 refractory period; hence the all-or-none nature of a muscle contraction re- 

 sides in the mechanism of conduction rather than in that of contraction.-^^ 

 Under conditions of fatigue, etc., the propagated response may disappear and 

 local contractions occur in the region of a neuromuscular junction'*'"^ or under 

 the stimulating electrodes. In frog muscles small motor nerve fibers may 

 cause only the graded local type of contraction, large motor nerve fibers the 

 all-or-none propagated twitch (Fig. 229). ^'*'^' ^^^ How the muscle impulse 

 activates the contractile elements is a major mystery. 



