The sliding filament theory of muscular contraction is the theory of how the protein filaments (myosin and actin) within the sarcomere (functional unit of the muscle), interact (connect and slide past each other) to produce muscular tension - and therefore movement. Although this is a very complicated and intricate process, what follows will provide anyone who is interested a better understanding concerning the process of muscular contractions.

Under normal resting conditions little calcium is present in the myofibrils (contain the contracile protein filiments - myosin and actin). Without calcium present very few of the mysosin cross-bridges are bound to the actin, so no tension is developed and the muscle is said to be at rest.

Now we start to get some action....

Before any movement can begin, the myosin filaments must attach to the actin filaments. The myosin filaments have cross-bridges that resemble small paddles - the attachment of the myosin to the actin must happen for a contraction to occur.

When the sarcoplasmic reticulum is stimulated to release calcium ions, the myosin cross bridges attach much more rapidly to the actin filaments - thus allowing the cross-bridge flexion to occur. This calcium release is triggered from the action potential or electrical impulse from the motor neuron.

Now that calcium is present and the myosin filament is attached to the actin filament, contration, or shorting of the muscle can occur.

The energy for this flexion comes from the breakdown of ATP (short term energy supply located in muscle tissue), to ADP and phosphate. Another molecule of ATP must replace the ADP on the myosin cross bridge head for the head to detach from the actin filament and recock. This allows the contraction or shortening process to continue, as long is calcium is present.

Measurable muscle contraction occurs when calcium ions are present, myosin and actin filaments couple, and the flexion, release and re-cocking of the myosin on the actin filiments occurs due to the breakdown of ATP.

This process continues as long as calcium is present, ATP is available, and the enzyme ATPase is present to help in the breakdown of ATP.

Relaxation occurs when the stimulation of the motor nerve stops. Calcium is pumped back into the sarcoplasmic reticulum (where it is stored). This prevents the myosin and actin from binding - and a relaxed muscle state occurs.