brasheranatomymuscle

The primary function of the muscular system is to produce movement of the body. It is also responsible for maintaining the posture of your body in a sitting or standing position; as well as stabilizing your joints. Another important function of the muscular system is to generate heat for the body. As ATP is split to cause muscle contraction, heat is produced which helps to maintain body temperature.

TYPES OF MUSCLE CELLS 1) Smooth muscle 2) Cardiac muscle 3) Skeletal muscle
 * Not striated
 * Single nucleus
 * Spindle-shaped
 * Involuntary
 * Found in the walls of hollow organs such as the stomach, intestines, and blood vessels
 * Striated
 * Single, branched cells with one nucleus
 * Cells are connected by an intercalated disk
 * Involuntary
 * Found in the heart
 * Long, straight, multinucleate cells
 * Striations due to protein structures
 * Voluntary
 * Found in all skeletal muscles; known as the muscular system
 * Be able to define: sarcolemma, myofibril, sarcoplasmic reticulum, sarcomere, actin, myosin

THE NEUROMUSCULAR JUNCTION For a muscle to contract, it must receive a message from the nervous system. The connection between the nervous system and a muscle is called the //neuromuscular// //junction//. The nerve cell, or //neuron,// does not actually touch the muscle cell. The small space between the neuron and muscle is called the //synaptic cleft.// A neuron may send messages to more than one muscle cell. The neuron and all of the muscle cells that it stimulates is called a //motor unit.// When a neuron is stimulated, it releases a chemical called a //neurotransmitter.// This neurotransmitter diffuses across the synaptic cleft and onto the surface of the muscle cell. The neurotransmitter in a neuromuscular junction is __always__ //acetylcholine.// When the acetylcholine reaches the cell membrane, or //sarcolemma,// of the muscle cell, the membrane becomes permeable to sodium ions. As the sodium ions move into the muscle cell, potassium ions move out of the muscle cell. At the same time, the muscle contracts. An active transport system called the //sodium-potassium pump// is activated to pump sodium out of the muscle and potassium back into the muscle. The muscle cell has returned to its resting state.

MUSCLE CONTRACTION Muscle contraction is dependent on the presence of ATP, oxygen, and calcium ions. ATP can be replaced by one of three methods:
 * Creatine phosphate can be used to change ADP to ATP. Creatine phosphate supplies are used quickly.
 * Aerobic respiration breaks glucose down to carbon dioxide and water, creating ATP. This is the most efficient route, but is slow and requires a continuous oxygen supply.
 * Anaerobic glycolysis, which occurs in the absence of oxygen, results in the formation of lactic acid. ATP is produced, but the process is not very efficient. The buildup of lactic acid causes muscle soreness.

MUSCLE DISORDERS Very few disorders affect the muscular system. Muscular dystrophy is a group of inherited disorders which causes muscles to get larger because of fat and connective tissue deposits. The muscle cells themselves atrophy and degenerate. Duchenne muscular dystrophy is almost always found in males and is usually diagnosed between the ages of 2 and 6. Death generally occurs by young adulthood. Myasthenia gravis is a rare disease that occurs in adults. Symptoms are drooping eyelids, difficulty in swallowing and talking, and general weakness and fatigue. This seems to be an autoimmune disorder in which acetylcholine receptors are destroyed. Death occurs by respiratory failure.

ADDITIONAL MUSCLE TERMS AND FACTS:
 * A muscle fiber either contracts or doen't contract. This is called the "all-or-none" law.
 * Because varying numbers of muscle cells are stimulated, an entire muscle reacts by a //graded response,// or varying degrees of contraction
 * Muscle twitches are single, brief, jerky contractions which usually are a result of nervous system problems.
 * Tetanus is a smooth, sustained muscle contraction. The disease tetanus, or lockjaw, is caused by a toxin produced by a bacteria which results in uncontrolled spasms and respiratory arrest.
 * If enough oxygen is not available to replenish the ATP supply, we have an //oxygen debt.// Lactic acid builds up in the muscle cells and causes soreness.
 * Isotonic contractions result in movement.
 * Isometric contractions occur when the muscle does not shorten. If you tried to bench press 500 pounds, your muscles would contract isometrically.
 * Muscle tone is a state of continuous partial contractions.