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Unit XIII

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  1. Biology 220 Anatomy & Physiology I Unit XIII MUSCULAR SYSTEM Chapter 9, pp. 276-321 Chapter 10, pp. 322-330 E. Gorski/ E. Lathrop-Davis/S. Kabrhel

  2. Sliding Filament Theory (con’t) 2)activated (“cocked”) myosin head (cross bridge) binds MBS on actin a) in relaxed muscle, ATP bound to myosin head as ADP and Pi (inorganic phosphate) - head is in high energy configuration b) when MBSs on actin become available, high energy head of myosin binds to actin  head changes position  pulls on actin = POWER STROKE of contraction

  3. Return to Resting Length • recoil of elastic series elements • as muscle contracts --> pulls on titin (elastic protein) within sarcomere, connective tissue coverings and tendons [all of these must stretch and become tense before contraction actually moves body part] • after muscle relaxes, recoil of stretched elastic elements pulls on muscle to lengthen it • contraction of antagonist muscles • gravity

  4. Internal vs External Tension • Internal tension - within muscle itself due to contraction of sarcomeres (within myofibrils) • results in stretch of series elastic elements • External tension - within series elastic elements • noncontractile structures (connective tissue coverings and tendons) • recoil when contraction ends

  5. Skeletal Muscle Tension • each motor unit/muscle fiber exhibits all-or-none contraction • whole muscle exhibits graded contraction due to recruitment of motor units (more units give greater contraction) • Depends on: • frequency of stimulation (no. APs reaching fiber/time) • length of fiber (stretch) before contraction • number of fibers (motor unit) contracting ADAM Interactive Physiology--Muscular System--Contraction of Whole Muscle, pp. 3-5

  6. Number of Fibers Contracting vs. Tension • Muscle Tone - alternating contractions in motor units stimulated by spinal reflexes • gives firmness to relaxed muscle without movement • keeps muscle healthy, ready to respond • important to posture

  7. Alternating Motor Units to Maintain Muscle Tone Motor unit 1 includes all “green” fibers; Motor unit 2 includes all “orange” fibers; Motor unit 3 includes all “blue” fibers. Alternating motor units helps slow onset of fatigue in muscle

  8. Types of Skeletal Muscle Contractions • Isometric contraction (iso = equal; metric = length) • increase in muscle tension but no significant change in overall muscle length (individual fibers shorten) • e.g., trying to lift overly heavy object Fig. 9.15, p. 297

  9. Types of Skeletal Muscle Contractions • Isotonic contraction (tonic = tension) • change in length while tension remains the same • concentric - muscle shortens to do work • eccentric - muscle contracts as it is stretched e.g., squats - quadriceps are stretched as you squat --> contract to maintain balance and slow down-ward movement Fig. 9.15, p. 297

  10. Disorders of Skeletal Muscles • spasticity - muscle has excessive tone • spasm - sudden, strong, usually painful, involuntary contraction • flaccidity - relaxed muscle appears soft or loose, contractions are weak • atrophy - loss of muscle mass (deterioration or wasting) due to disuse (couch potato), immobility (e.g., in cast or bed-ridden), abnormal innervation (e.g., damage to spinal cord or spinal nerves)

  11. Muscular Disorders (con’t) • Myasthenia gravis - autoimmune disease in which ACh receptors on the MEP are attacked • onset between 20-30 years of age, females • treated with neostigmine • anticholinesterase • allows ACh to build up at neuromuscular junction  keeps channels open longer  allows muscle cell to depolarize sufficiently to reach threshold  AP in fiber • does not cross BBB

  12. Muscular Disorders (con’t) • Muscular dystrophy - inherited diseases resulting in progressive muscle weakness and deterioration • Duchenne’s muscular dystrophy (DMD) • x-linked recessive gene, more often in males • onset usually 3-7 years old • skeletal muscle fibers structurally different • treated with injections of normal myoblasts fuse with abnormal fibers to provide normal dystrophin genes

  13. Muscle Fatigue • physiological inability to maintain contraction • results from: • relative deficit of ATP and creatine phosphate • accumulation of lactate (decreases pH) and ionic imbalances • insufficient oxygen • depletion of glycogen, lipids, amino acid reserves • physical damage to SR (after prolonged exercise) --> interferes with Ca2+ dynamics *ADAM Interactive Physiology--Muscular System

  14. Muscle Recovery • lactic acid removal • converted back into pyruvate and burned in muscle fiber • Cori cycle – some lactic acid diffuses into blood --> carried to liver --> converted back into pyruvate and either burned for ATP or converted back into glucose (requires ATP) --> glucose released, or stored as glycogen Fig. 9.17 p. 299

  15. Muscle Recovery (con’t) • oxygen debt - need for O2 to support aerobic ATP synthesis for: • restoring ATP, creatine phosphate and glycogen reserves in muscle • converting lactic acid to pyruvate --> to glucose in liver • heat dissipation - return to normal, resting body temperature • at rest, ~ 58% of energy released during aerobic formation of ATP becomes heat • at peak exertion, ~ 70% becomes heat

  16. Types of Skeletal Muscle Fibers • based on speed of contraction (slow or fast) and pathway for forming ATP (mainly aerobic = oxidative; or mainly anaerobic = glycolytic) • whole muscles composed of different types of fibers • major type varies depending on use of muscle • aerobic exercise • increases in blood vessels serving muscle (brings in more O2) • increases in number of mitochondria and amount of myoglobin • does not increase number of muscle fibers

  17. Types of Skeletal Muscle Fibers 1. Slow-Twitch Red = slow oxidative fibers • slow myosin ATPase --> slow contraction • high level of myoglobin (stored O2), many mitochondria; little stored glycogen --> aerobic ATP synthesis • highly vascularized (lots of capillaries --> O2) • fatigue resistance --> high endurance • don’t generate much power • important to posture • marathon runners (80% of total fibers)

  18. Types of Skeletal Muscle Fibers 2. Fast-Twitch White = fast glycolytic fibers • large diameter, pale cells • fast myosin ATPase --> rapid contraction • little myoglobin, fewer mitochondria; large amount of glycogen --> anaerobic ATP synthesis • fatigable --> low endurance • use up glycogen reserves • formation of lactic acid • powerful, short-term movements • sprinters (60% of total fibers)

  19. Types of Skeletal Muscle Fibers 3. Fast-Twitch Intermediate (red or pink) = fast oxidative fibers • fast myosin ATPase (as found in fast white fibers) --> fast contraction • intermediate amount of myoglobin, many mitochondria; intermediate amount of glycogen --> aerobic ATP synthesis • moderately fatigue resistant • moderate power

  20. Hormones and Skeletal Muscles • GH, testosterone - stimulate production of myosin and actin (hypertrophy = increase muscle mass) • thyroid hormones (T3, T4) - increase metabolic rate (increased activity including heat production) • epinephrine - stimulates breakdown of glycogen and glucose catabolism --> improves strength and endurance (does NOT stimulate contraction) • insulin - stimulates uptake and storage of glucose • glucagon - stimulates breakdown of glycogen

  21. Skeletal Muscle Movement • Origin - held in position • Insertion - attachment on part that’s moved • Belly - between origin and insertion Belly Fig. 10.14, p. 353

  22. Skeletal Muscle Group Actions • agonist = prime mover - contracts for action • synergist - aids prime mover (same action) • antagonist - opposite action; normally, relaxes while agonist is contracting • fixator - stabilizes origin of agonist • especially important at scapula (held to axial skeleton only by muscle)

  23. Smooth Muscle: Features • spindle-shaped fibers, single central nucleus • fibers covered by endomysium (no perimysium or epimysium) • sarcoplasmic reticulum not well developed • myofilaments not organized as sarcomeres • actin/myosin ratio ~ 13:1 (2:1 in skeletal muscle) • calmodulin on myosin regulates contraction (not troponin on actin) Fig. 9.23, p. 306

  24. Smooth Muscle: Features • involuntary, autonomic nervous system • varicosities = swellings along axons of ANS • self-excitatory pacemakercells – stomach, small intestines Fig. 9.24, p. 307

  25. Smooth Muscle: Features dense bodies provide anchor points for: • intermediate filament - non-contractile proteins that help resist tension • myosin/actin complexes • proteins that attach to sarcolemma and bind adjacent smooth muscle cells Fig. 9.25, p. 308

  26. Smooth Muscle: Excitation-Contraction Coupling autonomic nervous system stimulation • > entrance of Ca2+ into sarcoplasm initially from SR, also from extracellular fluid (outside cell) • > Ca2+ binds to and activates calmodulin • > calmodulin activates myosin light chain kinase • > kinase splits ATP on myosin (activates it) • > activated myosin pulls on actin • > fiber shortens (contraction) • relaxation due to return of Ca2+ to SR

  27. Smooth Muscle:Excitation-Contraction Coupling • slow, sustained contraction, fatigue-resistant, fewer mitochondria --> anaerobic ATP synthesis • smooth muscle tone - sustained partial contraction • hyperplasia - smooth muscle fibers can divide mitotically to produce new fibers • important during puberty, and during pregnancy in females

  28. Types of Smooth Muscle • Single-Unit - visceral • large network of fibers that contract together • communicate through gap junctions --> action potential spreads through rapidly • e.g., small arteries, veins, stomach, intestines • Multiunit • individual fibers with motor neuron contract • few gap junctions • e.g., large arteries, airways, arrector pili muscle

  29. Control of Smooth Muscle • can be excited or inhibited by neurotransmitters (depends on type of receptors present in sarcolemma) • autonomic NS: • ACh or norepinephrine used as neurotransmitters • varicosities (swellings) - release NT • hormones (e.g., epinephrine, gastrin) • chemical or physical factors (e.g., pH, O2, CO2, temperature) • stretching - stimulates contraction

  30. Smooth Muscle: Special Features • Stress-relaxation response • stretching stimulates --> initial contraction, increased tension -- only last a short time before tension decreases by itself and muscle returns to resting tone • important to movement of substances, especially in gut (allows time for adequate digestion and absorption) • also important to storage of fluid in gall bladder and urinary bladder • Length vs tension - total length change 150%