Differences in maximum velocity of shortening along single muscle fibres of the frog.

Abstract

SUMMARY1. The velocity of 'unloaded' shortening (VO) and the force-velocity relation were studied during fused tetani (0-5-2-0 TC) in short successive segments along the entire length of single fibres isolated from the tibialis anterior muscle of Rana temporaria. The segments were defined by opaque markers of hair that were placed on the fibre surface, 0-5-0 8 mm apart, from one tendon insertion to the other. The change in distance between two adjacent markers (one segment) was monitored by means of a photoelectric recording system, while the fibre was released to shorten isotonically between 2X2 and 2-0 gsm sarcomere lengths. The accuracy of the V0 measurement was better than 4 % in all parts of the fibre.2. V0 varied along the length of the fibre, each fibre having a unique velocity pattern that remained constant throughout the experiment. The difference between the highest and lowest values of V0 within the fibre varied between 11 and 45 % of the fibre mean in thirty-two preparations (mean difference 23 + 2 %, S.E. of mean).3. An attempt was made to relate the V0 pattern to the fibre's orientation in the body in fourteen complete experiments. The highest values of VI were obtained near the proximal end ofthe fibre, and there was a clear trend for V0 to assume lower values towards the distal end.4. The V1 pattern along the fibre did not correlate with the segments' capacities to produce force nor with the passive viscoelastic properties of the segments.5. Force-velocity data obtained from individual segments provided a good fit to Hill's (1938) 6. The results support the view that the kinetic properties of the myofilament system differ from one region to another along the length of a muscle fibre.

Synchronous oscillations of length and stiffness during loaded shortening of frog muscle fibres

“…The records shown in Fig. 4 also confirm that the overall rate of loaded shortening, like the maximum speed of shortening, varies along the length of a muscle fibre (Edman et al 1985.…”

Contractile Performance of Striated Muscle

“…23 B). It is noteworthy that the segmental differences in V 0 did not bear any relation to the passive elastic properties of the muscle fibres (see further Edman et al, 1985) supporting the view that the observed differences in shortening velocity do reflect true differences in the kinetic properties of the myofilament system along the fibre.…”

“…The absence of a real plateau of the length-tension relation can be explained by the fact that the thick and thin filaments are not constant in length and, furthermore, that there is normally some staggering (longitudinal misalignment) of the filaments within the A and I bands in a given fibre cross-section (See further Edman & Reggiani, 1987). To this may be added that the force producing capacity may differ by several (up to 10) % along the length of the muscle fibre (Edman et al, 1985). Taken together these findings imply that one or more regions of the fibre are actually capable of producing a somewhat greater force than that recorded from the fibre as a whole within the "plateau" of the length-tension relation (see Edman & Reggiani, 1987).…”

“…The orientation of the fibre in the body does indeed seem to have some influence upon the V 0 pattern. Thus, there was a clear trend towards higher velocities in the proximal part of the fibre and towards lower velocities in the most distal part (For details concerning the statistical treatment, see Edman et al 1985).…”

“…23. In these experiments (Edman et al, 1985) opaque markers were placed on the fibre surface, 0.5-0.8 mm apart, along the length of the preparation. The change in distance between two adjacent markers (one segment) was monitored by means of a photoelectric recording system, while the fibre was released to shorten isotonically against a load close to zero during tetanic stimulation.…”