Figure 6. Thick filament stress controls the conformation of the myosin motors in skeletal muscle.

The thick filament (red/yellow/green) is shown connected to the Z disk (vertical black bar) by visco-elastic titin links (black spring and dashpot) and partly overlaps with thin filaments (pink, light green). (a) In the absence of stress, the thick filament is OFF (red), with a short axial periodicity (inward pointing arrows) and helical tracks of myosin motors (grey ellipses). A few myosin motors are more perpendicular (constitutively ON; green). (b) Active contraction at high external load; both the thick filament (green) and thin filament (light green) are ON; the axial periodicity of the thick filament has increased (outward pointing arrows). (c) Elastic (phase 1) response to a force step in relaxing conditions; titin is elastically strained and passive force is transmitted to the thick filament which remains OFF. (d) Visco-elastic lengthening of titin in phase 2 is accompanied by the transition to a partially ON (yellow) thick filament with more myosin motors in perpendicular orientations.