Triggering of Contraction in Skeletal Muscle 



JEAN BOTTS 



Naval Medical Resrarch lustitiilc, Bethesda, Maryland 



IN NORMAL LIVING MUSCLE the iiiput of a Very small amount of energy, giving 

 rise to a wave of excitation, can rapidly precipitate a contraction capable of 

 performing many, many times the work equivalent of the input energy. The 

 complete chain of events leading to this sudden amplification is not known, 

 but many probable links in the chain have been described. Considerable atten- 

 tion has been directed toward the excitation events at the neuromuscular junc- 

 tion, the surface phenomena associated with the resting potential and the action 

 potential, and the properties of various anatomical units ranging from whole 

 muscle down to molecular sub-units of contractile protein. In recent years 

 more emphasis has also been given to the immediate recovery processes taking 

 place in muscle. The discovery of 'relaxing factors,' for example, has opened up 

 a new approach to the study of the contraction-relaxation cycle. However, 

 much is still unknown and the interpretation of much that is known remains 

 controversial. Several excellent papers and reviews have been concerned with 

 the link between excitation and contraction (34, 45, 81, 82, 89). The main 

 purpose of this chapter is to review the issues involved and call attention to 

 some of the more recent work bearing on this problem. 



INITIATION OF AN IMPULSE AT THE NEUROMUSCULAR JUNCTION 



Although we will be mainly concerned here with the events taking place in 

 the muscle iiber proper, it is pertinent to consider first some of the special 

 features of the nerve-muscle junction and the processes taking place which 

 initiate the wave of excitation in the skeletal muscle fiber. In the generally 

 accepted picture, the incoming nerv^e impulse induces at the nerve endings a 

 release of acetylcholine (A(^h) which is able to bring about an increase in ion- 

 permeability with a depolarization of the end-plate region of the muscle fiber. 

 Sodium ions pour in, accelerating the fall in potential in the end-plate and 

 adjacent regions; and this process ultimately gives rise to the propagated 

 action potential. 



Recent work by Fatt and Katz (32, 7^7^) and by del Castillo and Katz (21, 23) 

 suggests that ACh is released in discrete, uniformly-sized spurts from various 

 regions of the nerve endings. The evidence for this is based on the discovery of 

 'miniature end-plate potentials,' (32). With an internal micro-electrode located 

 in a muscle fiber at the end-plate region, it is found that even during quiescence 



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