A transverse tubular system and neuromuscular junctions in a molluscan unstriated muscle

The buccal mass retractor muscles of Philine aperta are unusual in possessing a system of transverse tubules in what are in other respects typical unstriated muscle fibres. The tubules are formed from invaginations of the sarcolemma that penetrate up to 9 micron radially into the fibre. There is some indication that the tubules are zoned at certain points along the fibre. Physiologically, the outer membrane of the muscle fibre does not support an action potential, and the fibre contracts by summation of small twitches in response to individual excitatory junction potentials. Neuromuscular junctions occur in shallow grooves along the surface of the fibres. They contain 3 types of vesicles; clear, intermediate and dense-cored. A scheme suggesting how striated muscle may be derived from the unstriated condition is described.