The Muscular System By Bryce Tappan

Functions of the Muscular System One of the most important roles muscles play is to give the body the ability to move. Muscles contract and expand to move bones and complete actions, and they are directed by the central nervous system. Actions are divided into voluntary and involuntary muscular movements, and there are different types of muscle that perform different functions.

Skeletal Muscle Skeletal muscles are joined to the bones by tendons. Their purpose is to sustain posture and stability, as well as control coordination. When skeletal muscles contract or expand, they move the bones that they are attached to. Skeletal muscles are voluntary muscles, meaning that we can consciously control them. Skeletal muscles are also striated, which means that they are divided into parallel fibers.

Cardiac Muscle Cardiac muscles are the striated muscles within the heart. They must contract and expand to sustain blood flow, and so they must not become fatigued. Cardiac cells have a high number of mitochondria which allows for continuous aerobic respiration to occur and the prevention of fatigue. Cardiac muscle is involuntary muscle; organisms do not consciously control its functioning.

Smooth Muscle Smooth muscle is involuntary and is not striated, and it is found in the walls of blood vessels and within tracts of the digestive and reproductive systems. In many smooth muscles, called single-unit muscles, the entire muscle contracts or expands at once. Smooth muscles have the ability to stretch farther than striated muscles, which makes them important in the digestion process in the intestines. Smooth muscles are in charge of regulating blood flow within the veins and thus blood pressure.

Sliding Filament Model The sliding filament model is a theory that depicts how muscles contract. Within muscle cells are sarcomeres. Sarcomeres are the basic unit of muscles and are composed of actin and myosin, two different types of filaments. The movement of actin past myosin creates tension within the muscle. ATP powers the movement of fibers within the sarcmere. Actin is anchored to both sides of the sarcomere, so when it slides, the sarcomere shortens with the tension. As a result of sarcomere shortening, muscles shorten as well. This is the sliding filament model

Acetylcholine Acetlycholine is a neurotransmitter found in both the PNS and CNS. Its function in the PNS is to activate muscles. It binds to acetlycholine receptors in skeletal muscle fibers which then opens sodium ion channels in the cell membrane of the muscle cells. Sodium ions flood the cell which induces the contraction of the muscle. In cardiac muscle, acetlycholine actually stops the contraction of muscle.

CA and Crossbridges A crossbridge is the head of a myosin molecule that is projected from a myosin filament. In the sliding filament model, calcium ions are released and they allow a bond to be formed between myosin and actin filaments. While attached, the myosin and actin pull on each other, which contracts muscles.

Actin & Myosin Actin is a microfilament that gives support to cells and gives myosin (another filament) a binding place in order for muscle contraction to occur. It is a protein that has many different functions within different types of cells. Myosin is known as a “motor protein” as it is responsible for motility in conjunction with actin in cells. It is also an important protein in muscular contraction. (See Sliding Filament Model)

Disorders: Muscular Dystrophy A)A disorder in which muscles strength and deteriorate in time. B)Loss of coordination, stiffness, immobility, mental retardation C) The most common type of muscular dystrophy, Myotonic dystrophy, occurs for 1 out of every 20,000 people. D)Physical therapy, surgery, mexiltine (medication)

Disorders: Rhabdomyolysis A)Breaking down of muscles, often as a result of taking drugs such as cocaine and PCP. B)Break down of muscles into myoglobin, which then goes to the blood and kidneys, causing kidney damage. Symptoms such as red urine, muscle weakness and pain may occur C)Rhabdomyolysis isn’t common, but is most prevalent among drug users. D)Hydration, diuretics, and bicarbonates can alleviate the symptoms of Rhabdomyolysis