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Journal of Experimental Biology 107,367-383 (1983)
Published by Company of Biologists 1983

Mechanical Responses of a Crustacean Slow Muscle

WILLIAM D. CHAPPLE ¹

¹ Physiology Section, The Biological Sciences Group, University of Connecticut, Storrs, CT 06268, U.S.A.

1. Mechanical properties of the abdominal ventral superficial muscle of the hermit crab, Pagurus pollicarus, were examined under isometric and iso-velocity conditions. The muscle was activated by stimulating its motor nerve at different frequencies.

2. Length-isometric tension relations were measured. Peak tension, P₀, was 0.16–0.2MNm^-2 and the sarcomere length of the muscle at the optimum length, L₀, was 10.8+1.0 µm. Passive tension was high at L₀. Correlated measurements of the operating length of the muscle and L₀ indicate that the operating length is at a point on the ascending limb of the length-tension curve approximately 0.77 L₀.

3. The relationship between activation level of the muscle and the length-tension relation indicates that the curve is not substantially displaced along the length axis by increasing activation level; increased force is primarily due to an increase in the slope of the ascending limb of the curve.

4. The force-velocity relation was obtained by measuring the force at a reference length during iso-velocity shortening of an active muscle. Hill constants of a/P₀ = 0.11 + 0.02 and b = 1.07 = 0.24 mm s^-1 were obtained. The maximum velocity of shortening per half sarcomere was approximately 4.2µms^-1.

5. Stretch of an active muscle did not produce an abrupt short range yield but a gradual transition between short range and terminal stiffness. This behaviour is shown to be due not to differences in cross bridge stiffness between VSM and other muscle but to a non cross bridge stiffness with a value that is one-fifth that of vertebrate muscle.

6. Such a low stiffness may provide an intrinsic mechanism for simplifying load compensation in the absence of rapid proprioceptive reflexes for the control of muscle stiffness.

Key words: Muscle mechanics, Crustacea, slow muscle

Submitted on February 15, 1983
Accepted on June 7, 1983

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