Correlation between Changes in Cross Striation and Mechanical Tension in Striated Muscle Fibre and their Structural Interpretation.
Authors: BUCHTHAL FRITZ , KNAPPEIS G. G. 
Published: December 2008

The mechanical reaction of the isolated cross striated muscle fibre and the time course of the structural changes are investigated by means of microcinematographing the changes in length occuring in the anisotropic and isotropic substance, and recording the mechanical tension simultaneously. A's part in the height of compartment (A/A + I) varies continuously between 64 per cent at rest, and 56 per cent at the apex of contraction. The apex of the single contraction is reached 50 ms after the last resting point, and after a further 50 ms A/A + I returns to the resting value. The change of A/A + I occurs more quickly immediately after rest than during the remainder of the time course. There is a striking conformity between the decrease and increase in the change of A/A + I in spite of the essential difference in the simultaneously registered course of tension.

The time course of the change in A/A + I during contraction and the size of the change in the range of stretch investigated (length 100–145), is independent of the stretch‐and amounts to about 100 ms.

In spite of the fact that A/A + I reaches its resting value after 100 ms an essential mechanical tension is still found. This difference may be explained by assuming a different time course of the contraction and relaxation process in A and I.

In accordance with previous investigations, no “all or none” reaction is found in the directly stimulated (curarised) fibre.

The change of A/A + I propagates over the fibre without decrement. In experiments comparing local stimulation of the one end of the fibre with stimulation of the entire fibre, the same tension and gradation is found with both methods of stimulation. Locally released contractions are therefore propagated over the fibre without decrement. The intensity of the propagated wave of contraction is thus graded with the strength of stimulation, and proceeds over the fibre without decrement. Thus in muscle, in contradistinction to nerve, we have a combination of propagation and gradation.

Simultaneous recording of the change in cross striation and tension at varying strength of stimulation shows that there is no measurable grading in A/A + I at equilibrium length and moderate degrees of stretch, whereas the mechanical tension varies in the ratio 1:3. To explain this apparent constancy in A/A + I at different degrees of contraction, one must either assume a very complicated mutual compensation between the changes in stiffness and equilibrium length in the A and I substance, or reckon with an “all or none” reaction of the minute structure elements. The latter possibility is a more simple explanation of the experimental observations. According to whether the contraction is weak or strong, a larger or smaller amount of resting submicroscopic elements shunts the contracting units.

A quantitative examination of this assumption shows that a grading of contraction cannot induce measurable changes in A/A + I as long as the resting tension is relatively small. Only with a high degree of stretch (length 170), can a measurable gradation be expected in A/A + I The theory is confirmed by experiments. On the basis of the molecule equivalent previously described, and the present observations, a structural interpretation of the contraction mechanism is attempted.

The authors are indebted to Dr. H. King, National Institute for Medical Research, Hampstead, for his kindness in supplying us with purified curarinechlorid, and to Mr. E. Kaiser, Engineer, for valuable advice concerning the treatment and interpretation of the material.

The present work has been supported by grants from the Michaelsen Foundation, and the Nordish Insulin Foundation.