determined mainly by the kind of joint it is associatedwith and the way the muscle is attached to the joint. Atone end of some muscles are long white tendons thatattach the muscles to bone. The point of fixedattachment of a muscle to bone is called the origin. Themore flexible attachments, especially attachments to amovable bone, are termed insertions.Muscles seldom act alone; they usually work ingroups held together by sheets of a white fibrous tissuecalled fascia. Muscles make up about one-half of thetotal body weight. Their main functions are threefold:Providing movement—including internalfunctions such as peristalsis (rhythmic waves ofmuscular contraction within the intestines).Maintaining body posture—through muscletone, as in the muscles of the head, neck and shoulders,which keep the head up.Providing heat—through chemical changes thattake place during muscle activity, such as exercise thatwarms the body.In addition, muscles are involved in such essentialbodily functions as respiration, blood circulation,digestion, and other functions such as speaking andseeing.MUSCLE CONTRACTIONMuscle tissue has a highly developed ability tocontract. Contractibility enables a muscle to becomeshorter or thicker, and this ability, along withinteraction with other muscles, produces movement ofinternal and external body parts. Muscle contraction ina tissue or organ produces motion and provides powerand speed for body activity. A contracting muscle isreferred to as a prime mover. A muscle that is relaxingwhile a prime mover is contracting is called theantagonist.STIMULUS FOR CONTRACTIONAll muscles respond to stimulus. This property iscalled excitability or irritability. The mechanicalmuscular action of shortening or thickening (alsocalled contraction) is activated by a stimulus sentthrough a motor nerve. All muscles are linked to nervefibers that carry messages from the central nervoussystem.CONTRACTION AND RECOVERYThe chemical action of muscle fibers consists oftwo stages, contraction and recovery. In thecontraction stage, two protein substances (actin andmyosin) react to provide energy through thebreakdown of glycogen into lactic acid. In the recoverystage, oxygen reacts with lactic acid to release carbondioxide and water.MUSCLE FATIGUEWhen a muscle contracts, it produces chemicalwaste products (carbon dioxide, lactic acid, and acidphosphate) which make the muscle more irritable. Ifcontraction is continued, the muscle will cramp andrefuse to move. This condition is known as fatigue. If itis carried too far, the muscle cells will not recover andpermanent damage will result. Muscles, therefore,need rest to allow the blood to carry away the wastematerials and bring in fresh glucose, oxygen, andprotein to restore the muscle protoplasm and theenergy that was used.TONICITYTonicity, or muscular tone, is a continual state ofpartial contraction that gives muscles a certainfirmness. Isometric muscle contraction occurs whenthe muscle is stimulated and shortens, but nomovement occurs, as when a person tenses his or hermuscles against an immovable object. Isotonic musclecontraction occurs when the muscle is stimulated. Themuscle shortens and movement occurs. An examplewould be lifting an object.EXTENSIBILITY AND ELASTICITYMuscles are also capable of stretching when forceis applied (extensibility) and regaining their originalform when that force is removed (elasticity).MAINTENANCE OF MUSCLE TISSUEDuring exercise, massage, or ordinary activities,the blood supply of muscles is increased. Thisadditional blood brings in fresh nutritional material,carries away waste products more rapidly, and enablesthe muscles to build up and restore their efficiency andtone.The importance of exercise for normal muscleactivity is clear, but excessive muscle strain isdamaging. For example, if a gasoline motor stands1-18
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