588 



Comparative Animal Physiology 



velocity (Ch. 23). At the end-plate a nerve impulse causes a local depolar- 

 ization which is manifest as an end-plate potential (e.p.p.)- The e.p.p. may 

 be associated with an increase in acetylcholine at the end-plate, either from 

 the nerve endings^«' '''■ '*' or from the muscle sole plate.^^-^^ The local 

 end-plate potential is graded in size and spreads electrotonically for a few 



Fig. 219. Action potentials trom the region of the end-plate in frog muscle: a, before 

 curarization; b to d, increasing degrees of curarization; e, curarization complete— e.e.p. 

 alone. AU-or-none propagated spike arises from e.p.p. in ad. From Kuffler.^^^ 



millimeters around the end-plate, where it sets off the muscle action poten- 

 tial (Fig. 219). The e.p.p. can be detected only at or near the neuromuscu- 

 lar junction, and under increasing curarization it becomes gradually re- 

 duced (Fig. 219). When the end-plate potential is reduced by curare to 

 below about 30 per cent of its maximum the muscle impulse can no longer 

 be initiated.^'*'* With two or more nerve impulses the graded e.p.p.'s can 



