Muscle System If you enjoy action, then this is the system for you. No matter how still you try to be, there’s always a movement taking place in your muscles. Did you know there are over 650 muscles in the human body, and that muscles make up 1/3 of a grown human body? Muscles cover the entire skeleton, and bones can’t move without muscles. Flex your muscles and tour this exciting system! Three types of muscles are skeletal muscles, smooth muscles, and cardiac muscles. Skeletal muscle tissue contains many long, cylindrical cells. Usually, a number of skeletal muscle cells are bundled together, and then several bundles are enclosed in a tough connective tissue sheath to form “a muscle” such as biceps in the arms.
This muscle type functions in contraction for voluntary movements. Cardiac muscle tissue is made up of branching, striated cells that are fused together at their plasma membranes. This allows the cells to contract as a unit. This type of muscle is found in the walls of the heart and functions in the pumping of blood through circulatory system. Smooth muscle consists of long, spindle-shaped cells, each with a single nucleus, and connective tissue holds the cells together. Smooth muscle tissue can be found in the stomach and the walls of blood vessels.
This type of muscle functions in propulsion of substances along internal passageways. The fleshy part of all muscles is called the belly. Sarcomeres are the basic units of muscle contraction. They are made up of actin and myosin filaments which are a part of myofibrils. Myofibrils are threadlike structures within each muscle cell.
Actin is a contractile protein that is a thin filament, and myosin is another contractile protein but is a thick filament. According to the sliding-filament model, myosin filaments slide along and pull the actin filaments toward the center of a sarcomere during contraction. First, acetylcholine is produced in motor neurons. This is a transmitter substance that stimulates contraction in adjacent muscle cells. Cholinesterase is an enzyme present in all nerve tissue that breaks up acytylcholine after contraction and prevents build-up.
Once acetylcholine stimulates muscle cells, the head of a myosin molecule attaches to a binding site on actin, forming a cross-bridge. The myosin head is bent because an ATP molecule is associated with it. When energy is released, the head bends back to its normal position, bringing with it the actin filament. Another energy input from ATP causes the myosin head to detach, and then the process begins again until the muscle is fully contracted. Contraction of muscles is an all or nothing response because it is based on the action potential in motor neurons. In these neurons, a threshold must be reached before the action potential is stimulated and carried to the muscle cells.
Muscles, bones, and tendons are all related. Tendons are the tissues that connect muscles and bones. They are what ultimately allow movement. When muscles contract, the tendons move because of the contraction, causing the movement of the bones to which they are attached. Extensors are muscles that when contracted, extend or straighten out a limb or part of the body. Flexors, however, are muscles that bend the angle of a limb or other body part when contracted. Bones are connected to one another by ligaments. Muscle fatigue is a protective mechanism that protects muscles against deleterious energy depletion and irreversible impairment of muscle function.
As potassium channels are activated as a result of fatigue, potassium increases and action potential duration decreases. The shorter action potential and the increased potassium then contribute to the decrease in membrane excitability, allowing a muscle to preserve energy. With all its action, this system provides for an exciting experience. Enjoy your tour through the world of myology, as you will be able see first-hand the structure, function and diseases of muscles.