Various challenges in understanding the thick filaments, within and outside skeletal and cardiac muscles.

Thick filaments isolated from various sources, most frequently skeletal and cardiac muscles, have been studied, but several aspects of their behavior remain to be clarified. Myosin II is the principal component of these filaments. A "traditional" interacting-heads motif (IHM) has been observed in isolated thick filaments. In this motif, the two heads of the myosin II molecule interact and are stuck to the backbone of the filaments. Another aspect, the super-relaxed state (SRX state), has been described in situ, in relaxed demembranated muscle fibers and myofibrils. It has frequently been claimed that the IHM and the SRX state are closely related. Some authors still consider this relationship valid, but this view is now broadly called into question. These two phenomena occur in very different conditions, making it difficult to determine if and how they are related. For example, macromolecular crowding is a characteristic feature in situ (regardless of interfilament spacing), but not in the conditions in which the "traditional" IHM has been observed. Recent studies in situ have attempted to resolve this problem, but some of the reported findings conflict. Moreover, the association of other proteins with the myosin filaments in situ increases thick filament complexity. Experimental conditions may affect the results obtained but the consideration of long-overlooked data would help to prevent erroneous interpretations. For instance, neither the absence (EM studies) or presence (in situ studies) of cell-associated water nor electrical charges are taken into account in any of the published studies in this domain and the omission of these two parameters could lead to contradictory conclusions. My principal objective here is to provide a brief overview (with a limited number of illustrative references) of the increasing complexity of our understanding of thick filaments over the years, particularly as concerns the weak coupling or absence of coupling between the IHM and the SRX state (recent findings that may be difficult to interpret).