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

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.