Heaven help us all! -jessica The contractile unit of the myocardial cell is the Sarcomere: - runs Z line to Z line - contains thick filaments (myosin) and thin filaments (actin, troponin, and tropomyosin) - shortening occurs according to a sliding filament model (thin filaments slide along adjacent thick filaments by forming and breaking cross bridges between actin and myosin) Sarcoplamsic reticulum (SR): - are the site of storage and release of calcium for excitation- contraction coupling Steps in excitation- contraction coupling: 1. action potentials spread from the cell membranes into the T tubules 2. during the plateau of the action potential, calcium conductance is increased and calcium enters the cell from the extracellular fluid (inward calcium current) 3. This calcium entry triggers the release of more calcium from the SR (Calcium induced calcium release) 4. As a result- intracellular calcium increases 5. Calcium binds to troponin C, and tropomyosinn is moved out of the w...