A new organisational design in skeletal muscle fibres.

In vertebrate skeletal muscles, the architecture of myofibrils is particularly well conserved throughout the taxa. It is composed of suites of repeating functional units called sarcomeres which give the muscle its striated structure. Here, we show that the skeletal sound producing muscles of the cusk eel Parophidion vassali have a different organisation, distinct from the classical type found in textbooks. Within sarcomeres, filaments are not straight lines but have a Y-shaped structure. This looks like chicken wire, with one branch connecting to a branch from the myofibril above and the other connecting to a branch from the myofibril below. This organisation seems to be an adaptation to counteract a trade-off between the speed and force. The low ratio of myofibrils within cell muscles and the high volume of sarcoplasmic reticulum strongly suggest that these muscles are capable of fast contractions. In parallel, the Z-bands are quite wide about 30% of the sarcomere length. This extraordinary long Z-band could smooth out the tension variations found in high-speed muscle contraction, helping to produce sounds with low variabilities in the sound features. Simultaneously, the Y-shaped structure allows having more cross-bridges, increasing the force in this high-speed muscle.