Journal of Neurophysiology, Vol 62, Issue 5 997-1005, Copyright © 1989 by APS
Mechanics of stretch in activated crustacean slow muscle. I. Factors affecting peak force
W. D. Chapple
Department of Physiology and Neurobiology, University of Connecticut, Storrs 06268.
1. The active stiffness of ventral superficial abdominal muscle (VSM) of
the hermit crab, Pagurus pollicarus, was measured with ramp stretches of
different amplitudes and velocities. Active stiffness was calculated by
subtracting the peak force produced by passive stretch and the isometric
force just before stretch from the peak force produced by stretching active
muscle. The result was then divided by stretch length to give stiffness. 2.
The relationship between force just before stretch (the level of
activation) and active stiffness was curvilinear and was found to apply
under a variety of experiment conditions. For pooled data from eight
experiments, active stiffness (GN.m-2.m-1) = 3.2*stress
(MN/m2)-7.6*stress2. Decreasing the number of motor units or activating the
inhibitor did not alter this relationship nor did the addition of
proctolin, octopamine, or 5-HT to the bath. The relationship also applied
during the rising phase of isometric tension. However, stiffness declined
more rapidly than predicted by this relationship after the end of tetanus.
3. Active stiffness varied inversely with stretch amplitude for fast
stretches, and the slope of this relationship increased with increasing
muscle activation. At lower stretch velocities, the slope was much less
than at rapid stretch velocities, so at low levels of activation and
stretch velocity, active stiffness was essentially independent of stretch
length. 4. Active stiffness covaried with muscle force as both were sampled
at shorter and shorter lengths on the ascending limb of the length-tension
curve.(ABSTRACT TRUNCATED AT 250 WORDS)
