The muscular system is essential in creating movement. There are three types of muscles in the human body: smooth muscle, cardiac muscle, and skeletal muscle. Smooth muscle tissue can be found in the digestive tract, aiding in moving food through the intestines to be removed as waste. Cardiac muscle, as the name implies, is only found in the heart and is responsible for pumping the heart to circulate blood around the body. Lastly, skeletal muscle is attached to bones to allow for body movements. By laying on top of the “frame” the skeleton provides, skeletal muscles are able to contract and shorten to allow motion (running, walking, etc.), creation of facial expressions, and interaction with your environment.
Basic Anatomy of the Muscular System
There are over 600 muscles in the body. Some of the major muscles are included in the diagram above.
Organization of Skeletal Muscle
Skeletal muscles attach to bones at either an insertion or origin. The insertion of a muscle is the point that is more movable and the origin is the point that is immovable. For example, the biceps brachii (bicep) muscle in your arm inserts just below the elbow and originates in the shoulder region.
At either of these points, the muscle is connected to the bone via a tendon – a strong, cord-like connective tissue that is able to withstand stress from being stretched. This dense bundle of connective tissue extends from the bone and expands to surround the entire muscle, forming the epimysium. The epimysium acts as an overcoat, wrapping around the fascicles (large bundles of muscle fibers) within the muscle. Each fascicle has its own wrapping called the perimysium. Within each fascicle’s perimysium are muscle fibers, which are further wrapped by an endomysium. The fibers within each endomysium are then divided into myofibrils, which are functional components of muscular contraction.
Method of Muscular Contraction
The functional unit of contraction is known as a sarcomere. There are four major components within the sarcomere: actin, myosin, troponin, and tropomyosin. As discussed above, myofibrils are small, rod-like structures in the muscle. Within each myofibril are smaller, thread-like structures called myofilaments. These filaments are either thick (myosin) or thin (actin) and slide past one another to create contractions. On each thin (actin) filament are cords or tropomyosin, that block the site where myosin attaches to pull the actin fibers.
Before a muscle can contract, calcium must attach to the troponin protein on actin filaments, causing the tropomyosin to change shape and move out of the way. Once this is complete, the thick (myosin) filaments are able to grab on to the thin (actin) filaments and contract the muscle. For the muscle to relax, calcium supply is shut off, tropomyosin recovers the myosin binding sites on actin filaments, and the filaments slide back to their original position. The process is visually explained in this short video.
Motor Units
In the nervous system there are various cells that are responsible for signalling other cells and tissues in the body called neurons. Specifically, motor neurons are responsible for aiding in muscular contraction. A neuron and the muscle fibers it stimulates is called a motor unit. Within a motor unit, the muscle fiber and neuron do not touch, creating a neuromuscular junction.
As mentioned before, a muscle requires calcium before it can contract. When chemical messengers diffuse across the space between the neuron and muscle fibers, they signal the release of calcium ions in the muscle, allowing for contraction. Motor units follow an all-or-none principle, meaning that all fibers connected to the signaling neuron are/are not activated. This is similar to flipping a light switch. Either all the lights the switch is connected to will turn on or they will not.
Muscle Fiber Types
There are three types of muscle fibers in the body: type 1, type 2A, and type 2B. Type 1 fibers are aerobic, having the ability to steadily produce low amounts of force during contraction. These would be seen in endurance athletes like marathon runners. Type 2 fibers are power fibers, being able to produce a high amount of power very quickly. The major difference between types 2A and 2B is the amount of power and duration of power production. Type 2B creates the most power and lasts the least amount of time. These fibers would be highly present in strength athletes like powerlifters.
Want to know more about the other body systems? Check out this article!
