The sliding filament theory explains muscle contraction based on how muscle fibers (actin and myosin) slide against each other to generate tension in the overall muscle.

Step 1: A muscle contraction starts in the brain, where signals are sent along the motor neuron (a). Color the motor neuron yellow . Within the motor neuron are vesicles that contain the neurotransmitter, acetylcholine. Color vesicles gray and the triangles that represent the acetylcholine orange . Acetylcholine reaches the receptors (b) on the muscle sarcolemma which causes an impulse.

Step 2: The impulse travels down the membrance and into the transverse tubules (c) where it causes calcium to be released from the sarcoplasmic reticulum . Color the t-tubule green and the circles that represent calcium dark blue . The sarcoplasmic reticulum is only partially pictured, shade this structure pink.

Step 3: Calcium binds to a structure on the actin that causes it to change shape. Color the actin myofilament (e) red.

Step 4: The change in shape allows myosin heads to form cross-bridges between the actin and the myosin. Color the myosin (g) blue. Color the cross bridges (f) purple.

Step 5: Energy from ATP is used to create a "power stroke" between the two filaments. Color the ATP bright orange . The actin filament then slides inward and shortens, or contracts, the whole muscle.