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Ultrastructural organization of the transverse tubules and the sarcoplasmic reticulum in a fish sound-producing muscle.

The ultrastructural basis for the extremely rapid contraction-relaxation cycle (up to 300 s(-1)) in the swim-bladder muscle (SBM) of a scorpionfish (Sebastiscus marmoratus), producing characteristic sounds for communication, was investigated by electron microscopy. The SBM fibres contained well-developed sarcoplasmic reticulum (SR) showing triadic contacts with well-organized transverse tubules (T tubules). It was newly found that different types of triadic contacts were present within the single SBM fibre. In the middle region of the fibre (approximately 54% of the fibre length), the triadic contacts were located around the level of boundary between the A- and I-bands (AI-type triad). However in the two end regions of the fibre (approximately 21% and approximately 12% of the fibre length), the triadic contacts were seen around the level of the Z-band (Z-type triad). Between the middle and end regions of the fibre, T tubule-SR contacts exhibited the form of pentads composed of a pair of T tubules and three SR elements, and newly found heptads composed of three T tubules and four SR elements. The fractional volume of SR relative to the fibre volume was estimated to be approximately 26% in the middle region of the fibre with the AI-type triads and approximately 15% in the fibre ends with the Z-type triads. These results are discussed in connection with the mechanism, by which the mechanical activity of the SBM muscle is neurally controlled.