ABSTRACT
A whole muscle is built up from fibres, composed of myofibrils, which are frequently seen as the minimal contractile units retaining all the properties of a whole fibre. Each myofibril contains sarcomeres, which are generally considered, at the ‘microscopic level’, to be the ‘elementary contractile units’. However, at the ‘molecular level’, the ‘crystallographic unit cell’ is defined by specialists in x-ray diffraction (see, e.g. Figure 7 in the paper by Matsubara et al. 1984). In this book, I refer to ‘unit cells’, which are the genuine ‘contractile units’, composed of ~90%–95% contractile proteins (mostly myosin and actin; some recent papers indicate that titin may play a role in contraction). In an ‘intact unit cell’, there is only one thick myosin filament, surrounded by the nearest thin actin filaments (the problem of the number of thin actin filaments per thick myosin filament is discussed on p. 35 in Section 3.2). In Section 3.7, it is demonstrated that there are various types of unit cell in an intact fibre. Their ‘mechanical behaviour’ depends strongly on their location within the myofibrils, the ‘mechanical performance’ of the myofibrils also being strongly dependent on their location within the fibre. The intact unit cells are located in the centre of myofibrils, themselves located in the centre of an intact fibre. This notion of intact unit cell is impossible
to define in demembranated fibres. Indeed, from pp. 97-98 in Section 3.8.1 and from Sections 3.8.2, 4.4.2.1 and 8.7, it appears that demembranated fibres are extremely complex and profoundly different from intact fibres.
