muscular system n.
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Muscular System
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  1. Muscular System

  2. essential function of muscle is contraction or shortening • muscles are responsible for essentially all body movement • "machines" of the body

  3. 3 types (skeletal, cardiac, smooth) differ in cell structure, body location, & how they are stimulated to contract • all muscle cells are elongated and called muscle fibers • the ability of muscle to shorten or contract depends on two types of myofilaments • (the muscle cell equivalents of the microfilaments of the cytoskeleton) • terminology is different • (“myo” and “mys” mean muscle / “sarco” means flesh)

  4. Skeletal Muscle • organs that attach to the body's skeleton • help form the smoother contours of the body • fibers are cigar-shaped, multinucleate cells, and the largest of the muscle fiber types • known as striated muscle because its fibers appear to be striped • called voluntary muscle because it is the only muscle type subject to conscious control • often activated by reflexes contract rapidly and with great force • tires easily and must rest after short periods of activity • soft and surprisingly fragile • can exert tremendous power

  5. thousands of their fibers are bundled together by connective tissue • (provides strength and support to the muscle as a whole) • each muscle fiber is enclosed in a delicate connective tissue sheath called an endomysium • several sheathed muscle fibers are then wrapped by a coarser fibrous membrane called a perimysium • (forms a bundle of fibers called a fascicle) • many fascicles are bound together by an even tougher layer of connective tissue called an epimysium • (covers the entire muscle) • epimysia blend into the strong, cordlike tendons • (tendons anchor muscles, provide durability, and conserve space around rough bony projections)

  6. Smooth Muscle • no striations, involuntary (not under conscious control), slow and sustained contractions • are spindle-shaped with a single nucleus arranged in sheets or layers • (most often there are two Iayers, one running circularly and the other longitudinally) • found in the walls of hollow visceral organs such as the stomach, urinary bladder, respiratory passages • propels substances along a definite tract (pathway) within the body

  7. Cardiac Muscle • striated but involuntary heart muscle • cardiac fibers are cushioned by small amounts of soft connective tissue • arranged in spiral or figure 8-shaped bundles • when the heart contracts, internal chambers become smaller, forcing blood into large arteries leaving the heart • cardiac muscle fibers are branching cells joined by special junctions called intercalated disks • structure of muscle fibers allow heart activity to be closely coordinated.

  8. Muscle Function • four important roles in the body (produces movement, maintains posture, stabilizes joints, generates heat)

  9. Muscle Functions • Producing Movement • skeletal muscles are responsible for all locomotion and manipulation • respond quickly to changes in the external environment • express our emotions • Maintaining Posture • skeletal muscles that maintain body posture • makes one tiny adjustment after another almost continuously

  10. Producing Muscle Functions • Stabilizing Joints • muscle tendons are extremely important in reinforcing and stabilizing joints • Generating Heat • by-product of muscle activity • nearly ¾ of energy used to power muscle contraction escapes as heat • lost energy is important in maintaining normal body temperature

  11. Show Nerve Stimulation Video: •

  12. The Nerve Stimulus and the Action Potential Part 1 • skeletal muscle cells must be stimulated by nerve impulses to contract • one motor neuron (nerve cell) may stimulate hundreds of muscle cells • (depending on the particular muscle and the work it does) • one neuron and all the skeletal muscle cells it stimulates are a motor unit • nerve fiber (axon) forms neuromuscular junctions with the sarcolemma of a different muscle cell • they never touch and the synaptic cleft is filled with tissue (interstitial) fluid

  13. Part 2 • when nerve impulse reaches the axonal terminals a neurotransmitter is released (acetylcholine - ACh) • AChdiffuses across the synaptic cleft and attaches to receptors in the sarcolemma • if enough acetylcholine is released the sarcolemma becomes temporarily permeable to sodium ions (Na+) • Na+ rushes into the muscle cell • cell interior has an excess of positive ions which changes the electrical conditions of the sarcolemma

  14. Part 3 • this generates an electrical current called an action potential (unstoppable once it begins) • action potential travels over the entire surface of the sarcolemma • an electrical impulse is conducted from one end of the cell to the other • result is contraction of the muscle cell • the cell returns to its resting state by • (1) diffusion of potassium ions (K+) out of the cell • (2) activation of the sodium-potassium pump (moves Na+ and K+ back to their initial positions)

  15. Contraction of a Skeletal Muscle as a Whole • skeletal muscles are organs with thousands of muscle cells • muscles react to stimuli with graded responses (different degrees of shortening) • graded muscle contractions can be produced two ways • (1) changing the speed of muscle stimulation • (2) by changing the number of muscle cells being stimulated

  16. muscle twitches • single, brief, jerky contractions • sometimes occur as a result of certain nervous system problems • nerve impulses are delivered to muscle at a very rapid rate • cells do not relax completely between stimuli • contractions are "summed" and get stronger and smoother • when muscle is stimulated rapidly with no evidence of relaxation, contractions are smooth/sustained, the muscle is said to be in tetanus • primary role of tetanus is to produce smooth and prolonged muscle contractions • depends mostly on how many muscle cells are stimulated

  17. Providing Energy for Muscle Contraction • as a muscle contracts, ATP bonds are hydrolyzed to release energy • muscles only store 4 to 6 seconds' worth ATP • contracting muscles different pathways for ATP regeneration:

  18. aerobic (cellular) respiration in the mitochondria • anaerobic glycolysis and lactic acid formation • during intense muscle activity oxygen/glucose delivery is temporarily inadequate • pyruvicacid generated during glycolysis is converted to lactic acid • produces about 5% as much ATP as aerobic respiration • works 2 ½ times faster and provides enough ATP for 30-40 seconds of activity • uses huge amounts of glucose for a small ATP harvest • accumulated lactic acid promotes muscle fatigue and muscle soreness

  19. Muscle Fatigue and Oxygen Debt • a muscle is fatigued when it is unable to contract, even if it is still being stimulated • when muscle begins to tire, it contracts more weakly until it stops reacting /contracting • believed to result from oxygen debt/lactic acid accumulation occurring during prolonged muscle activity • muscle's ATP supply starts to run low • increasing acidity / decreasing ATP cause muscle to contract less effectively and finally stop • individual breathes rapidly/deeply and muscles receive sufficient oxygen • lactic acid broken down and ATP/creatine phosphate made

  20. Types of Muscle Contractions • Isotonic • tension develops as actin/myosin myofilaments attempt to slide past each other in muscle fibers • contractions occur when myofilaments slide, the muscle shortens, and movement occurs • Isometric • contractions in which the muscles do not shorten because they are working against an immovable object • tension in the muscle keeps increasing without movement

  21. Muscle Tone • even a voluntarily relaxed muscle has some contracting fibers • contraction is not visible but keeps muscle firm, healthy, ready

  22. Effect of Exercise on Muscles • inactivity (loss of nerve supply, immobilization) leads to muscle weakness and wasting • regular exercise increases muscle size, strength, endurance

  23. aerobic/endurance exercise (jogging, biking, or aerobics class) • stronger, more flexible muscles with greater resistance to fatigue • cells form more mitochondria and store more oxygen • overall body metabolism more efficient • improves digestion/elimination • enhances neuromuscular coordination • makes the skeleton stronger • heart enlarges (hypertrophies) • more blood is pumped with each beat, fat deposits are cleared, gas exchanges more efficient

  24. resistances or isometric exercise • bulging muscles result mainly from muscles being worked against some immovable object • require very little time and little or no special equipment

  25. 5 Golden Rules of Skeletal Muscle Activity • all muscles cross at least one joint • bulk of the muscle lies proximal to the joint crossed • all muscles have at least two attachments: the origin and the insertion • muscles puII not push • during contraction the muscle insertion moves toward the origin

  26. Naming Skeletal Muscles • muscles come in many shapes/sizes and named with several criteria • direction of the muscle fibers - rectus (straight), oblique (slanted) • relative size of the muscle - maximus (largest), minimus (smallest), longus (long) • location of the muscle - named for the bone with which they are associated • number of origins - biceps, triceps, or quadriceps have two, three, or four origins

  27. location of the muscle's origin and insertion - named for their attachment sites • sternocleidomastoidmuscle – origin on the sternum (sterno)/clavicle (cleido) and inserts on the mastoid process • shape of the muscle - distinctive shape that helps to identify them (deltoid means "triangular") • action of the muscle - flexor, extensor, abductor appear in names