It All Starts in the Brain
Some muscles, like the muscles in your heart and digestive system, are involuntary. They work by themselves without you consciously telling them to do anything. Other muscles are voluntary, meaning you order them to move. Both types of muscles are commanded by the brain. The brain sends an electric signal down a series of nerve cells ordering muscles to contract. This is done automatically by your brain with involuntary muscles and manually by your brain with voluntary muscles.
Muscles are made up of thick and thin muscle filaments. Thick muscle filaments are composed of myosin protein molecules stacked in a cylinder shape. Thin muscle filaments are made of actin protein resembling a pair of twisting pearl necklaces. Both thick and thin muscle filaments are required for muscle contraction.
The Muscle Trigger
There are grooves in the thin muscle filaments that allow them to move alongside the thick muscle filaments. In these grooves are a pair of proteins called tropomyosin and troponin. They are molecular switches essential to muscle contraction. Calcium ions are released to trigger the movement and the proteins help the thick myosin filaments and thin actin filaments work together.
When a muscle contracts, the thick myosin filaments seize the thin actin filaments creating a "crossbridge" pattern. The thick myosin filaments pull the thin actin filaments by them like trains passing each other on parallel tracks. These filaments are encased in tube like structures called sarcomeres. The sarcomeres get shorter as the thick and thin muscle filaments slide by each other.
The only thing an individual muscle can do is contract in a single direction. In a joint, there are opposing muscles that contract in opposing directions so you can move in a range of motion. For example, your biceps bend your arm at the elbow and your triceps straighten your arm. One group of muscles contracts while the opposing muscle group relaxes.
A human being has hundreds of muscles in every region of the body. All of these muscles must coordinate with each other to move a person effectively. A simple task like smiling, throwing a ball or standing up from a chair involves dozens of muscles contracting and relaxing with different amounts of force simultaneously. This is a learned process that the human body figures out with experience.
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