Skeletal muscle is made up of long fibrous cylinders called myofibrils that run the length of the muscle cell. Muscle cells contract because myofibrils contract. Each myofibril contains a lot of thick and thin filaments, which run parallel to the myofibril axis. The thin filaments are called actin and the thick filaments are called myosin. These filaments create a light and dark banding pattern in the myofibrils that can be seen in the light microscope. (You really should look at a good picture that will clearly show these banding patterns in order to understand how the actin and myosin function in contraction since it involves the sliding of filaments past eachother.) The dark bands are called "A" bands and contain the thick myosin filaments. The light bands are called "I" bands and contain the thin actin filaments. The "A" and "I" bands are lined up next to eachother in an alternating pattern. There is a structure called a "Z-line" that runs perpendicular to the myofibril, through the "I" band, connecting neighboring myofibrils. The "A' band contains a lighter middle region, called the "H" zone. The "H" zone only contains myosin while the darker edges of the "A" band contain myosin and actin that overlaps from the "I" band. When a muscle contracts the "I" band and "H" zone appear to decrease in thickness. This is due to the actin and myosin filaments sliding past eachother. WHEW! It's hard to explain this stuff without showing you a picture!
Now, a sarcomere is the segement of a myofibril that runs between two adjacent "Z-lines" and represents the functional unit of striated muscle. Keep in mind what bands you see between the "Z-lines" and what happens to them when muscle contracts; hence the term "functional unit".
The actin and myosin filaments are able to slide past one another due to cross bridges, contained in the myosin filaments. (I'll try to keep this explanation as simple as possible). These cross bridges are detached from actin in relaxed muscle but are in a high energy state from prior binding and splitting of ATP. During contraction, the cross bridge attaches to actin, moving it relative to myosin; this releases the stored energy. In order for the myosin cross bridge to release from actin, ATP is needed. ATP binds to the myosin and is split, allowing it to release actin. The splitting of ATP reenergizes the cross bridge so a new cyle of contraction can occur. Ca++ is also required for the attachment phase of the cycle.....it is a trigger for contraction and it's removal triggers relaxation.....more on that when troponin and tropomyosin come into play!
Troponin and Tropomyosin are part of the actin containing thin filaments. Troponin is a molecule that is bound to tropomyosin. During relaxation the sites on actin where myosin cross bridges would normally "latch on", are covered by tropomyosin molecules. When Ca++ levels increase, troponin binds Ca++ and changes shape. This causes tropomyosin to move out of the way, exposing the sites where the myosin cross bridges can now "latch on". When Ca++ levels decrease, the troponin reverts back to its original shape and the attchment sites get blocked.
SO........when nerve impulses activate a muscle, the excitation is transmitted through the motor endplate( the ending of a motor nerve fiber in relation to a skeletal muscle) and a muscle action potential spreads over the surface of a muscle cell and the myofibrils contract. There is a system of tiny tubes, called T-tubules, that extend from the surface of the muscle cell deep inside it. These tubes encircle the perimiter of each myofibril at the level of the "Z-line". Surface action potentials are transmitted to the sarcomeres by way of T-tubules. Each sarcomere is surrounded by a sheath of something called a sarcoplasmic reticulum. The sarcoplasmic reticulum stores Ca++ and it is released due to the action potential/excitation, causing contraction to occur.
Sorry for being so verbose but it is a tricky subject.........it actually is a lot more complicated than that but I hope this is enough for now. I highly recommend the Physiology Coloring Book for this sort of stuff. Don't let the silly coloring part fool you, it has lots of great info that is explained in clear terms! Cheers!