¹ From the Department of Biology, Tufts University, Medford, Massachusetts 02155 and the Department of Anatomy, State University of New York, Stony Brook, New York 11790.

Dr. Hidaka's present address is Department of Physiology, Faculty of Medicine, Kyushu University, Fukuoka, Japan.

The individual muscle fibers of the anterior byssus retractor muscle (ABRM) of Mytilus edulis L. are uninucleate, 1.2–1.8 mm in length, 5 µm in diameter, and organized into bundles 100–200 µm in diameter, surrounded by connective tissue. Some bundles run the length of the whole muscle. Adjacent muscle cell membranes are interconnected by nexuses at frequent intervals. Specialized attachments exist between muscle fibers and connective tissue. Electrical constants of the resting muscle membrane were measured with intracellular recording electrodes and both extracellular and intracellular current-passing electrodes. With an intracellular current-passing electrode, the time constant , was 4.3 ± 1.5 ms. With current delivered via an extracellular electrode was 68.3 ± 15 ms. The space constant, , was 1.8 mm ± 0.4. The membrane input resistance, R_eff, ranged from 23 to 51 M. The observations that values of depend on the method of passing current, and that the value of is large relative to fiber length and diameter are considered evidence that the individual muscle fibers are electrically interconnected within bundles in a three-dimensional network. Estimations are made of the membrane resistance, R_m, to compare the values to fast and slow striated muscle fibers and mammalian smooth muscles. The implications of this study in reinterpreting previous mechanical and electrical studies are discussed.

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