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Journal of Experimental Biology 58,91-103 (1973)
Published by Company of Biologists 1973

Two Rates of Relaxation in the Dorsal Longitudinal Muscle of a Leech

JENNIFER B. MILLER ¹ and D. J. AIDLEY ¹

¹ School of Biological Sciences, University of East Anglia, Norwich

1. Experiments were performed to measure the mechanical properties of a preparation of the dorsal body wall of the leech Haemopts sanguisuga.

2. A quantitative histological description of this preparation is provided. It is concluded that the fibres of the dorsal longitudinal muscle are almost entirely responsible for the contractile properties of the preparation, as measured along its longitudinal axis.

3. The preparation was subjected to two types of electrical stimulation. Pulsed stimulation produced a fairly rapid contraction and relaxation. Contraction after a d.c. stimulus was slightly slower, and the relaxation consisted of an initial rapid phase followed by a much slower phase. The extent of the residual tension in this second phase was greater at higher stimulus intensities.

4. The muscle showed some degree of fatigue when subjected to a series of stimuli. Each type of electrical stimulus caused fatigue of both responses.

5. Curare had no effect on relaxation rates. Both phases of relaxation were somewhat slower in the presence of a high concentration of magnesium ion or -amino butyric acid, and faster in the presence of 5-hydroxytryptamine. It was thus not possible to separate the two types of response by pharmacological means.

6. The mechanism whereby two rates of relaxation are produced is discussed. It is suggested that d.c. stimulation affects excitation-contraction coupling in some way.