Figure 3. Relationship between sarcomere structure and A-band position.

(A) SHG-2PF linescan imaging of the myocyte shows a Ca²⁺ transient seen in the Fluo-4 signal (top left panel of images on right) and the triggered contraction in the SHG signal (top right panel). (B) L_M is the distance between two adjacent A-bands within the same sarcomere. L_Z is the distance between the two A-bands across the adjacent sarcomeres. The sarcomere length L_SL is equal to the sum of L_M and L_Z. L_Z shortens during cell contraction due to the sliding of the thick filament against the thin filament. (C) The histogram of A-bands distances is calculated using the kernel density method described in the online methods. L_M is determined by the thick filament structure within the sarcomere and does not change during cell contraction. L_Z shortens as described. The numerical values of L_SL during systole and diastole calculated in this way match with sarcomere length calculated using a Fourier transform based method in a similar myocyte (Online Figure I).