MUSCLE ENERGETICS AND FATIGUE

Summary of Mechanics 1 Muscles pull; they don’t push 2 Muscle lengthen by being yanked on by antagonists or gravity 3 Muscle force can be graded by recruitment of motor units 4 You activate small motor units first: the size principle 5 Muscle force can be graded by repetitive stimulation 6 Muscle force can be graded by changing length, but who cares 7 Muscle velocity is inversely related to muscle force: you can be strong or fast but not both at the same time 8 Muscle power peaks at 1/3 maximum force 9 Pinnate muscle fibers develop more force at lower velocity because of the angle 10 Muscles fatigue: they drop force on continued use 11 Muscles are heterogeneous based on contractile properties Slow twitch (S) Fast fatigue resistant (FR) Fast Intermediate (FI) Fast fatiguable (FF)

Summary of contractile mechanisms 1 Muscle cells are highly organized 2 Myofibrils consists of interdigitating hexagonal arrays of filaments 3 Thick filaments are mainly myosin: A bands 4 Thin filaments are actin + tropomyosin + TnI + TnC + TnT: I bands 5 Sliding filaments explains the length-tension curve 6 Cross-Bridge cycling couples ATP hydrolysis to force or shortening 7 Myosin isoforms have different turnover numbers 8 Costameres may transmit force from myofilaments to muscle exterior through the cytoskeleton

Summary of Excitation-Contraction Coupling 1 Contraction begins with neuromuscular transmission 2 Muscle action potential depolarizes the T-tubule 3 T-tubule depolarization tickles the DHPR 4 DHPR tickles RyR1 5 RyR1 release loads of Ca, but SR is not empty 6 Ca saturates TnC 7 TnC-Ca disinhibits Acto-myosin interaction 8 Acto-myosin cycles the cross bridge 9 SR re-uptake shuts off contraction 10 Series-elastic elements explain twitch time course 11 Prolonged Ca transient with series-elastic elements explains tetany

ATP is the energy currency of the cell; its free energy of hydrolysis drives muscle contraction and energy output

Acto-myosin cross bridge cycle: ATP hydrolysis is linked to force generation or shortening

Muscles are activated by brief trains of impulses followed by rest periods

Lactic acid levels increases with exercise intensity

Lactic acid is oxidized in mitochondria and carries in cytoplasmic NADH equivalents

Lactic acid shuttles: 1. to the mitochondria 2. to oxidative fibers 3 Lactic acid shuttles: 1. to the mitochondria 2. to oxidative fibers 3. to liver

Exercise increases GLUT4 population in muscle membranes and increases glucose uptake independent of insulin

Muscle glycogenoses

Signals for muscle hypertrophy

Nuclear domains in muscle

Fiber Type Switching