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Skeletal Muscle Mechanics Muscle fibers into whole muscle Whole muscle tension: • Number of muscle fibers contracting • Tension developed by each fiber Motor units: Motor unit recruitment Figure 8.15 Page 269 Also influenced by fiber type! 1 Whole muscle tension depends on: 1. Frequency of stimulation Tetanus Contractile activity Single twitch Stimulation ceases or fatigue begins Twitch summation Action potentials Figure 8.17 Page 271 Whole muscle tension depends on: 2. Length of fiber at onset of contraction Figure 8.18 Page 272 Whole muscle tension depends on: 3. Extent of fatigue 4. Thickness of fiber Duration of activity Amount of motor unit recruitment Fiber type 2 Types of Contractions Isotonic 1. • Muscle tension remains constant as muscle changes length 9 Concentric & eccentric contractions Isometric 2. • Tension develops at constant length Isokinetic 3. • Fixed movement Skeletal Muscle Metabolism Major requirement of contraction-relaxation coupling is ATP… 1. Splitting of ATP by myosin ATPase 2. Binding of ATP to myosin 3. Active transport of Ca2+ back into sarcoplasmic reticulum Must be in constant supply (readily available) 3 Intramuscular ATP supplied by 3 means 1. Creatine Phosphate 2. Glycolysis 3. Oxidative Phosphorylation Creatine Phosphate: ¾ Immediate energy source Creatine + Phosphate ¾ Creatine Kinase ADP Creatine + ATP Concentrations will drive reaction • Resting muscle: ~ 5x the amount of CP than ATP ¾ Only 1 enzyme (rapid reaction) ¾ Limited supply • Short bursts, high-intensity exercise Glycolysis: ¾ No O2 requirement (like CP) ~ anaerobic ¾ Continuous high-intensity exercise ¾ Breakdown of glucose or glycogen: Pyruvic acid Lactic acid Pyruvic acid Oxidative phosphorylation 2 ATP Mitochondria 4 Oxidative Phosphorylation: ¾ If energy requirement continues ¾ Multiple steps • Time • Pathway fueled primarily by glucose & fatty acids ¾ Occurs within the mitochondria (O2!!) • O2 comes from hemoglobin & myoglobin • Electron-transport chain 9Yields 36 ATP (glucose), ~ 128 ATP (Fat) Muscle fiber Blood Figure 8.23 Page 278 Fatigue: Muscular 1. • • • Neural (Central & Peripheral) 2. • 3. Increased concentration of Pi (inorganic phosphate) Accumulation of lactic acid (lactate) Glycogen or glucose depletion Psychological O2 debt & nutrient depletion 5 Skeletal Muscle Fibers Slow-oxidative (Type I) 1. • Mitochondrial density 9 More resistant to fatigue Fast-oxidative (Type IIa) 2. • Higher myosin ATPase activity Fast-glycolytic (Type IIb) 3. • Higher myosin ATPase activity Changes in fibers: Endurance training Weight lifting Drugs Motor Control 6 Input to Motor Neurons Input from afferent neurons 1. • Intervening interneurons (spinal reflexes) 2. Input from primary motor cortex 3. Input from brainstem Premotor and supplementary motor areas Cortical level Subcortical level Sensory areas of cortex Basal nuclei Primary motor cortex Thalamus Brain stem level Cerebellum = Pathways conveying afferent input Brain stem nuclei (including reticular formation and vestibular nuclei) Spinal cord level Afferent neuron terminals = Corticospinal motor system Motor neurons Muscle fibers Peripheral receptors Periphery Movement Other peripheral events, such as visual input Sensory consequences of movement = Multineuronal motor system Figure 8.24 Page 285 Muscle Receptors 7 Coordinated movement: Learned muscle behaviors • • CNS & muscle input Proprioception 2 types of muscle receptors: 1. Muscle spindles • Stretch reflex (knee-jerk) 2. Golgi tendon organs • Respond to tension changes Figure 8.25 Page 286 Capsule Alpha motor neuron axon Intrafusal (spindle) muscle fibers Gamma motor neuron axon Contractile end portions of intrafusal fiber Secondary (flower-spray) endings of afferent fibers Extrafusal (“ordinary”) muscle fibers Noncontractile central portion of intrafusal fiber Primary (annulospiral) endings of afferent fibers Extrafusal skeletal muscle fiber Spinal cord Intrafusal muscle spindle fiber Afferent input from sensory endings of muscle spindle fiber Alpha motor neuron output to regular skeletal-muscle fiber Stretch reflex pathway Gamma motor-neuron output to contractile end portions of spindle fiber Descending pathways coactivating alpha and gamma motor neurons Figure 8.26 (1) Page 287 8 Figure 8.26 (2) Page 287 Relaxed muscle; spindle fiber sensitive to stretch of muscle Contracted muscle in hypothetical situation of no spindle coactivation; slackened spindle fiber not sensitive to stretch of muscle Extensor muscle of knee (quadriceps femoris) Contracted muscle in normal situation of spindle coactivation; contracted spindle fiber sensitive to stretch of muscle Muscle spindle Patellar tendon Alpha motor neuron Figure 8.27 Page 288 9