Sliding filament theory in its simplest form states that muscle fibres shorten when actin filaments slide inward on myosin filaments - pulling the z-lines closer together.
If that's all Greek to you then have a quick look at the article on muscle anatomy which outlines the different components of a muscle.
Have a look at the diagram below:
When actin filaments (the light bands in the diagram above) slide over myosin filaments (the dark bands) the H-zone and I-band decrease.
What causes actin filaments to move?
Myosin filaments contain tiny globular heads, called cross bridges at regular intervals. These cross bridges attach to the actin filaments pulling on them to create movement.
See figure 2 below:
Each flexion of a cross bridge produces only a very small movement in the actin filament so many cross bridges throughout the muscle must flex repeatedly and rapidly for any measurable movement to occur.
Surrounding each myofibril (remember a myofibril is the portion of the muscle fibre that houses actin and myosin) is a system of tubules called the sarcoplasmic reticulum. The sarcoplasmic reticulum stores calcium and it is the regluation of calcium release that causes muscular contraction.
During rest most of the calcium resides in the sarcoplasmic reticulum and and very few myosin cross bridges are attached to actin filaments - nor can they flex.
When the brain sends a nerve impulse (called an action potential)
How does that cause a muscle fibre to contract?
The action potential arrives at the nerve terminal and causes the release of a chemical called acetylcholine. Acetylcholine travels across the neuromusclualr junction and stimulates the sarcoplasmic reticulum to release its stored calcium ions throughout the muscle.
As calcium is relased it binds with a protein called troponin that is situated along the actin filaments. Sliding filament theory states that this binding causes a shift to occur in another chemical called tropomyosin. Because these chemicals have a high affinity for calcium ions they cause the myosin cross bridges to attach to actin and flex rapidly.
For contraction to contiune the myosin cross bridges must detach, "recock" and reattach. Significant muscle shortening depends on the continuous sequence of the following events:
And that's sliding filament theory in a nutshell!