Changes of volume in striated muscle.

Abstract

The record of change in volume, obtained from a frog sartorius muscle mounted isometrically at reference length shows a characteristic two-phased change. Five msec alter the start of stimulus pulse of 4 msec an increase in volume begins. This increase peaks at about 7 msec and a decrease in volume begins. This decrease is at a minimum at about 23 msec, and the volume then returns to its pre-twitch level. (The temperature is 24°C.) A small variation, averaging less than 10%, is seen in the above parameters when records from different sartorius muscles are compared. (The time to the minimum of the decrease is slightly more variable, showing as much as 40% variation among different sartorius muscles.) A rise in tension begins approximately at the start of the increase in volume. Tension reaches a peak at about 33 msec after the start of the stimulus. (This is well alter the minimum of the volume-decrease.) The record of change in volume obtained from muscles contracting isometrically at lengths well below reference length is characteristically different from that obtained from muscles contracting isometrically at reference length. “Slack” muscles, upon stimulation, show a large, rapid decrease in volume. This is probably similar to the decrease described by Ernst in his studies on “tensionless” muscles. This large, rapid decrease in volume is not seen if the muscle is stimulated isometrically at reference length. It is likely that this effect of length of muscle on the record of volumechange explains the difference in the results obtained by Ernst and co-workers on one hand and Meyerhof and his co-workers on the other. (Meyerhof's work appears to have been done with the muscle held at or near reference length.) The magnitude of the initial decrease in volume varies with length of the muscle (below reference length). It is maximal at “slack” or “end-free” lengths and decreases as the muscle is held near reference length. The initial decrease in volume at these lengths is abruptly terminated by a rapid volume-increase. The magnitude of this increase is also a function of the muscle length. Following this increase, the volume-record returns to its initial level.