The Thermodynamics of elasticity in resting striated muscle

Abstract

Many biological structures are highly extensible. Their elasticity may be of the 'normal' type, as in fibrous tissue (e.g. tendon) which lengthens on warming; or of the 'rubber-like' type, as in so-called 'elastic' tissue which shortens on warming. Resting striated muscle is of the second kind. Various authors have studied the mechanical properties of resting muscle from the standpoint of the thermokinetic theory of long-range elasticity, their experimental evidence being derived chiefly from measurements of the thermal coefficient of tension at constant length. Such methods, depending on the assumption of reversible equilibria, cannot throw direct light on the mechanism and time-course of the processes involved in mechanical change. The thermal changes produced in resting muscle by stretch or release at finite speed have been examined experimentally. They are not simultaneous with the mechanical changes that cause them, but lag behind, starting slowly and continuing after the stretch or release is complete. The work done on a muscle during a stretch (or by a muscle during release) is much less than the heat produced (or absorbed). Thus the internal energy decreases as the result of extension and increases with shortening. A reversible physical, or physico-chemical, change is caused by stretch or release and requires time to reach a new equilibrium. The thermoelastic (heat/tension) ratio is defined, for a small stretch or release. It varies with length, being zero or negative at very short lengths, reaching a maximum at moderate extension, falling to zero again at a greater extension (the 'inversion point') and remaining negative thereafter. The heat change becomes diphasic as the 'inversion point' is approached, remaining diphasic at all greater lengths. The diphasic character of the heat begins to appear at the length at which deviation first occurs from the linear relation existing between log (tension) and length. At shorter lengths one elastic component only (of the 'rubber-like' type) is involved; at greater lengths another component with 'normal' elasticity also comes in. The nature of these two components is considered. Thermoelastic temperature changes have been recorded during a steady condition of alternating stretch and release. At 0 degrees C the thermoelastic (heat/tension) ratio regularly becomes negative at very short lengths. At room temperature (18 degrees C) it falls to zero but seldom below. The maximum value of the thermoelastic ratio at 0 degrees C permits the calculation of a temperature coefficient of tension at constant length which agrees well with direct measurements by other authors. At 18 degrees C very high values of the thermoelastic ratio are sometimes found; the variability observed at the higher temperature must be attributed to critical conditions (such as the presence or formation of nuclei) occurring inside the muscle fibres. It is possible that many of these effects are due to 'crystallization', produced by stretch and reversed by release, between neighbouring protein molecular threads. Another possibility, not necessarily incompatible with this, is that a reversible change in the ionization of protein threads is produced by stretch, resulting in increased alkalinity and a liberation of heat. The thermodynamics of these processes is considered, including that of possible volume changes occurring during stretch. The eventuality is discussed that the processes resulting from stretch and release do not proceed to positions of true equilibrium.