It is suggested that, in the conditions which exist in vivo, one head of a myosin molecule interacts with another head of the opposite molecule, inside the backbone of the thick filament. The other head lies outside and can interact with actin. This model is based on the fact that a dimer of the myosin heads exists and that there is a close correlation between the properties of the dimer and those of the thick filament diameter. In natural filaments, there are myosin molecules in excess and it is suggested that these molecules have their two heads outside the backbone.
