Supercontracting Striated Muscle in a Vertebrate

Abstract

THE capacity of vertebrate striated muscle fibres to shorten is much curtailed by their Z-disks. Skeletal muscle sarcomeres, for example, can be reversibly contracted to about 40% of their fully extended length¹, an observation that accords well with the sliding filament model of muscle contraction^2,3. Once the thick filaments slide up against the Z-disks at the ends of the sarcomere, further shortening merely causes crumpling of the ends of the thick filaments¹ and a rapid decrease in tension. In marked contrast many unstriated muscles can exert tension whilst shortening to less than 10% of their fully extended length, as they have no Z-disks to interrupt the sliding of their filaments⁴. The regimenting and compartmentalizing influence of Z-disks in striated muscle supports speed and power of contraction but it is at the expense of shortening capacity.