What is the structure of the muscular system? - Human Body Anatomy

The muscular system is composed of three types of muscle, Skeletal, Cardiac and Smooth. These muscle function to move the skeleton or move materials.
There are three types of muscle fibers cardiac, skeletal and smooth. Skeletal muscle is voluntary, multi-nucleated and striated. Cardiac muscle is involuntary, multi-nucleated and striated. While smooth muscle is involuntary, uni-nucleated and non-striated.

The typical gross anatomy of a muscle is the large belly and two tendons. The insertion that creates the actual movement and the origin which anchors the muscle in place.

Typically muscles move
by a process called a contraction which causes the muscle belly to shorten. Muscles work in opposition. The muscle that contracts is called the agonist, while the one that relaxes is called the antagonist. The muscle belly is composed of bundles of muscle fibers called fascicles. It is the muscle fibers which are composed of myofibrils that actually do the contracting because of specialized units called sarcomeres.

A sarcomere runs from z-lineto z-line. And is composed of a thick filament called myosin and a thin filament called actin. When energy is released in the form of ATP the thick filament called myosin spins very rapidly and pulls on the two proteins found on the thin actin filament. These two proteins are called tropomyosin and troponin.

When this occurs the z-lines are pulled closer together and the sarcomere contracts. This is better known as the sliding filament theory.

When the sarcomere contracts all of the sarcomeres contract and the myofibril contracts. Then the fiber contracts and the fascicle and finally, the belly contracts. When the belly contracts it pulls on the tendon which in turn pulls on the bone to make the skeleton move.