Ref ID : 1022
Pierre Clement and Jacqueline Amsellem; The skeletal muscles of rotifers and their innervation. Hydrobiologia 186/187:255-278, 1989
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The skeletal muscles of rotifers are monocellular or occasionally bicellular. They display great diversity of cytological features correlated to their functional differentiation. The cross-striated fibers of some retractors are fast contracting and relaxing, with A-band lengths 0.7 um to 1.6 um, abundant sarcoplasmic reticulum and dyads. Other retractors and the circular muscles are tonic fibers (A band >3 um), stronger (large volume of myoplasm) or with greater endurance (superior volume of mitochondria/myoplasm). All of these retractor muscles are coupled by gap junctions and are innervated at two symmetrical points; they constitute two motor unit implicated in withdrawal behaviour. The muscles inserted on the ciliary roots of the cingulum control swimming. They are multi-innervated and each of them constitute one motor unit. They have characteristics of very fast fibers; the shortest A-band length is 0.5 um in Asplanchna. All the skeletal muscles of bdelloids are smooth or obliquely striated as are some skeletal muscles of monogononts. These muscles are well suited for maximum shortening and are either phasic or tonic fibers. All rotifer skeletal muscles originate from ectoderm and contain thin and thick myofilaments whose diameters are identical to those of actin and myosin filaments in vertebrate fast muscles or in insert flight muscles. These are no paramyosinic features in the thick myofilaments. The insertion, innervation, coupling by gap-junctions and other cytological differentiations of rotifer skeletal muscles are reviewed and their phylogeny discussed.