Chapter 12 Muscles

Chapter 12 Muscles - Chapter 12: Muscles Review muscle...

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Unformatted text preview: Chapter 12: Muscles Review muscle anatomy (esp. microanatomy of skeletal muscle) Terminology: q sarcolemma q t-tubules q sarcoplasmic reticulum q myofibers, myofibrils, myofilaments q sarcomere More Terminology: q Tension q Contraction q Load q Excitation-contraction coupling q Rigor q Relaxation Anatomy Fig 12-3 More Anatomy Fig 12-3 Myofibrils = Contractile Organelles of Myofiber Myofiber Contain 6 types of protein: q Actin q Myosin Contractile q Tropomyosin q Troponin q Titin q Nebulin Regulatory Accessory Fig 12-3 c-f Fig 12-3 Titin and Nebulin Titin and Nebulin q Titin: biggest protein known (25,000 aa); elastic! elastic! » Stabilizes position of contractile filaments » Return to relaxed location q Nebulin: inelastic inelastic giant protein giant » Alignment of A & M Fig 12-6 Sliding Filament Theory p 403 Sliding Filament Theory q q Sarcomere = unit of contraction Myosin “walks down” an actin fiber towards Zline » ? - band shortens » ? - band does not shorten q Myosin = motor protein: chemical energy → mechanical energy of motion Changes in a Sarcomere during Contraction Fig 12-8 The Molecular Basis of Contraction Rigor State Compare to Fig 12-9 myosin affinity changes due to ATP binding • Tight binding between G-actin and myosin • No nucleotide bound ATP ADP + Pi ATP binds ⇒ dissociation Released energy changes angle between head & long axis of myosin Myosin head acts as ATPase Relaxed muscle state Rotation and weak binding to new G-actin when sufficient ATP Power stroke begins as Pi released ADP released Tight binding to actin Myosin crossbridge movement pushes Regulation of Contraction by Troponin Regulation of Contraction by Troponin and Tropomyosin Tropomyosin blocks myosin binding site (weak binding possible but no powerstroke) Troponin controls position of tropomyosin and has Ca2+ binding site Ca2+ present: binding of A&M Ca2+ absent: relaxation Fig 12-10 Rigor mortis Rigor mortis Joint stiffness and muscular rigidity of dead body Begins 2 – 4 h post mortem. Can last up to 4 days depending on temperature and other conditions Caused by leakage of Ca2+ ions into cell and ATP depletion Maximum stiffness ∼ 12-24 h post mortem, then? Initiation of Contraction Initiation Excitation-Contraction Coupling explains how you get Excitation-Contraction from AP in axon to contraction in sarcomere from ACh released from somatic motor neuron at the Motor End ACh Plate Plate AP in sarcolemma and T-Tubules Ca2+ release from sarcoplasmic reticulum release Ca2+ binds to troponin binds Details of E/C Coupling Nicotinic cholinergic receptors on motor end plate = Na+ /K+ channels ⇒Net Na entry creates EPSP ⇒AP to T-tubules ⇒DHP (dihydropyridine) receptors in T+ tubules sense depolarization Fig 12-11 Nicotinic Cholinergic Receptors ExcitationExcitationContraction Contraction Coupling Coupling Fig 12-11 a DHP (dihydropyridine) receptors open Ca2+ channels in t-tubules Intracytosolic [Ca2+] Contraction Ca2+ re-uptake into SR Relaxation Fig 12-11 b Muscle Contraction Needs Steady Supply of ATP q Where / when is ATP needed? q Only enough ATP stored for 8 twitches » Phosphocreatine may substitute for ATP Twitch = single contraction relaxation cycle Where does all this ATP come from? Where q Phosphocreatine: backup energy backup source source C(P)K phosphocreatine + ADP creatine + ATP q CHO: aerobic and anaerobic resp. q Fatty acid breakdown always requires O2 – is too slow for heavy exercise » Some intracellular FA Oxidative only Muscle Fiber Classification Oxidative or glycolytic Muscle Adaptation to Exercise Muscle ( not in book) Endurance training: q More & bigger mitochondria q More enzymes for aerobic respiration Resistance training: q More actin & myosin proteins & more sarcomeres q q More myofibrils More myoglobin muscle hypertrophy no hypertrophy Muscle Tension is Function of Fiber Length q Sarcomere length reflects thick, thin filament overlap q Long Sarcomere: little overlap, few crossbridges ⇒ weak tension generation q Short Sarcomere: Too much overlap limited crossbridge formation ⇒ tension decreases rapidly Force of Contraction (all-or-none) q Increases With » muscle-twitch summation » recruitment of motor units recruitment motor Mechanics of body movement covered in lab only Fig 12-17 Smooth muscle q A few differences » » » » q Innervation by varicosities Smaller cells Longer myofilaments Myofilaments arranged in periphery Myofilaments of cell of Cardiac muscle contraction Cardiac covered later covered ...
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