The muscular system is responsible for controlling the skeletal muscles and the muscles of the internal organs, including the heart, stomach and intestines. It connects with the nervous system and receives input from efferent neurons, allowing it to respond to external or internal stimuli.
The skeletal muscles attach to bones by tendons. When they receive the signal from the cerebellum to contract or extend, they pull the tendon, which in turn pulls the bone and moves the limb. For example, the muscles used in lifting an object with the arms include the biceps and forearm muscles. The biceps tendon connects to the forearm. When the biceps contracts, the tendon pulls the forearm. Skeletal muscles typically exist in pairs that complement one another, such as the biceps/triceps pair, the quadriceps/calf muscles and more.
By contrast, smooth muscle tissue and cardiac muscle are involuntary. Their functions are controlled by the medulla, which is responsible for vital functions such as heartbeat. One example of involuntary muscle action is expansion and contraction of the diaphragm. This allows the lungs to expand and take in air. Another example is peristalsis, which occurs in the esophagus and intestines. This is a wave of contraction that moves food along the digestive tract.