Active supercontraction in rolling-up muscles of Glomeris marginata.

The lateroventral muscles of Glomeris marginata keep the animal rolled up and are able to develop and maintain great tension. Their fibers are not equipped with a particularly strong contractile apparatus but can supercontract. The sarcomere shortens its resting length by up 60% and in a typical supercontraction the thick filaments pass through the Z-line into adjacent sarcomeres. The Z-line structure changes according to the contraction state: It passes from a homogeneous, dense zig-zag line in decontracted fibers to a rarified, vaguely outlined Z-band in supercontracted fibers, in which it is possible to see actin and myosin filaments. The Z-line is thus involved in an active expanding process and is functionally very different from the fragmented and discontinuous Z-line of 'classical' supercontracting muscles. The different meaning of the two cases of supercontraction is discussed.