The Contractile Apparatus of the Heart

In the present chapter, I will describe basic structural and functional features of the contractile apparatus of muscle cells of the heart, namely, cardiomyocytes and smooth muscle cells. Cardiomyocytes form the contractile myocardium of the heart, while smooth muscle cells form the contractile coronary vessels. Both muscle types have distinct properties and will be considered with respect to their cellular appearance (brick-like cross-striated versus spindle-like smooth), arrangement of contractile proteins (organized in sarcomeres versus non-sarcomeric organization), calcium activation mechanisms (thin-filament versus thick-filament regulation), contractile features (fast and phasic versus slow and tonic), energy metabolism (high oxygen versus low oxygen demand), chemomechanical energy conversion (high adenosine triphosphate (ATP) consumption and short duty ratio versus low ATP consumption and high duty ratio), excitation-contraction coupling (calcium-induced calcium release versus pharmacomechanical coupling), and molecular motors (type II myosin isoenzymes with high adenosine diphosphate (ADP)-release rate versus myosin isoenzymes with low ADP-release rates).