The Structure of Mytilus Smooth Muscle and the Electrical Constants of the Resting Muscle

Abstract

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 lim in diameter, and organized into bundles 100-200 Mm 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 r, was 4.3 1.5 ms. With current delivered via an extracellular electrode r was 68.3 15 ms. The space constant, X, was 1.8 mm 4 0.4. The membrane input resistance, Ref, ranged from 23 to 51 M. The observations that values of r depend on the method of passing current, and that the value of X 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, 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.

Localization of calcium‐accumulating structures in the anterior byssal retractor muscle of Mytilus edulis and their role in the regulation of active and catch contractions.

“…1 shows electron micrographs of the resting ABRM fibres. In agreement with the previous reports (Twarog, 1967a;Heumann & Zebe, 1968;Twarog, Dewey & Hidaka, 1973), the ABRM consists of spindle-shaped individual fibres of 5-7 #um diameter containing the vesicles and the mitochondria in close apposition to the fibre surface (P1. 1 A).…”

Tension‐length behaviour of a molluscan smooth muscle related to filament organisation

“…( to us that the whole muscle behaves functionally as a single cell, that the length of the thick and thin filaments is constant, and that at any particular muscle length the 'sarcomeres' in each cell have the same amount of filament overlap. The idea that under suitable activation conditions the ABRM could respond as a single cell is supported by recent work which shows that, as in vertebrate smooth muscle, the cells in the ABRM are electrically connected via nexal junctions (Twarog, Dewey and Hidaka 1973). Electron microscopy (Sobiezsek 1973) indicates that the filaments in the ABRM are organised in an arrangement comparable to a sarcomere, and the available evidence does not exclude the idea that both the thick filaments and the thin filaments have uniform lengths (Sobiezsek 1973).…”

Paramyosin in invertebrate muscles. II. Content in relation to structure and function.

“…In such molluscan smooth "catch" muscles as lamellibranch opaque adductors (6-8, 10-12, 20, 27) and Mytilus anterior byssus retractor muscle (ABRM) (18,20,21,26,28,32), thick filaments range from 500 to 1,500 /~ in diameter and from 10 to 40 tzm in length. They are oriented parallel to the cells, but are not organized into identifiable ordered, repeating sarcomeric units (33). The cross-striated adductors of the lamellibranch Aequipecten, on the other hand, resemble vertebrate striated muscle with respect to both filament dimensions and sarcomere organization (24).…”