Fig. 4. Examples illustrating the spatial arrangement of myosin filaments and dense bodies that could be interpreted as a standard design of contractile unit in smooth muscle: a myosin filament spanning the distance between two dense bodies on opposite sides of the filament. The longitudinal EM sections were obtained from porcine trachealis fixed at their in situ length and in the relaxed state. Length of the thick filaments and the distance between the dense bodies are between 1.8-2.2 µm. Large arrowheads indicate the relevant dense bodies; small arrowheads follow the myosin thick filaments. Bar, 0.1 µm.