A discussion on muscular contraction and relaxation: their physical and chemical basis - Introduction

Abstract

The elementary unit of contraction in skeletal muscle is a single twitch, evoked either by a single nerve impulse arriving at an end-plate or by a direct electric shock. It is a very rapid affair, requiring for completion a time varying from a few milliseconds to a fraction of a minute according to the muscle and its temperature. It is followed by a slow recovery (‘recharging’) process taking from a few minutes to an hour. The focus of the present discussion is the relation between ( a ) known mechanical, thermal, electrical and other physical changes accompanying contraction and relaxation and ( b ) the underlying chemical causes. The fundamental difficulty is that normal chemical methods are very slow and insensitive in face of the require­ments posed by the muscle twitch. To illustrate this, the heat production in a twitch, say 3 mcal./g., if derived (for example) from the breakdown of creatine phosphate, would require only 2.5 x 10 -7 g. mol. of phosphate, per gram of muscle, to be set free. To detect such a quantity at all is a formidable enough task; to resolve its appearance in times of the order of tenths or hundredths of a second is one of fantastic difficulty. Even in the very slow twitch of a toad’s muscle at 0° C the state of full activity is reached within 30 or 40 msec, after a stimulus, and relaxation is already obvious at 0.3 or 0.4 sec.