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Skeletal Muscle Physiology

Skeletal Muscle Physiology. How do contractions occur? Remember that muscles are excitable. Electrochemical gradient. Both neurons and muscle cells maintain electrochemical gradients across their plasma membranes Intracellular fluid is negatively charged. 2 Electrochemical gradients.

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Skeletal Muscle Physiology

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  1. Skeletal Muscle Physiology How do contractions occur? Remember that muscles are excitable

  2. Electrochemical gradient • Both neurons and muscle cells maintain electrochemical gradients across their plasma membranes • Intracellular fluid is negatively charged

  3. 2 Electrochemical gradients

  4. Remember that skeletal muscle: • Is under voluntary control…so we need a stimulus to begin the process of contraction • Where does the stimulus come from? • Motor neurons! • One motor neuron may innervate many muscle cells

  5. What do you need to produce a contraction? • Must transfer message (action potential) from the neuron throughout the muscle cell (via transverse tubules) • Thick and thin filaments must interact • What ions play a role? Na+, Ca2+ • Where does the Energy to contract come from? • ATP

  6. What is an Action Potential (AP)? • A propagated change in the transmembrane potential of excitable cells • This is the message telling the cell to contract! • initiated by a change in the membrane’s permeability to Na+

  7. Cell @ rest; Gated channels closed • Stimulus arrives! Na+ channels open & Na+ rushes IN; Depolarization • Slow moving K+ channels open & K+ rushes OUT; Repolarization

  8. What connection? How is a signal transferred from neuron to muscle cell?

  9. Signal transduction • AP arrives @ presynaptic terminal; causes Ca2+ channels to open • Ca2+ ions enter & stimulate neurotransmitter release (ACh) from synaptic vessicles into synaptic cleft

  10. Signal transduction • ACh diffuses across synaptic cleft & binds to ACh receptors on Na+ channel proteins in sarcolemma of muscle cell

  11. Signal transduction • Influx of Na+ ions results in depolarization of postsynaptic membrane; when “threshold” is reached, postsynaptic cell (muscle cell) fires an AP

  12. Ca2+ ions released • Ca2+ binds to troponinof thin filaments • Allows interaction of thick and thin filaments • Causing a contraction

  13. Exposure of attachment sight: Ca2+ binds to troponin; allows tropomyosin to move, exposing myosin attachment sight Cross-bridge formation: Myosin heads attach to actin subunits. P released

  14. Power Stroke: Stored E in myosin heads used to pull actin filament toward M line. ADP released from myosin head ATP regenerated & attached to myosin head: Could be new ATP or phosphorylated ADP from previous step Cross-bridge release: ATP broken down to ADP + P. Myosin head releases Recovery Stroke: Myosin heads return to resting position. E still stored in myosin head

  15. Recruitment & Summation • We know single muscle cells contract when AP arrives • One single AP stimulus produces a single Twitch • Twitches produce muscle tension • How long does one twitch take? • How do twitches achieve whole muscle contraction? • By building tension • Multiple motor units are stimulated (recruited) • AP’s arrive more frequently

  16. Twitch Contraction • Three phases • Latent: AP reaches sarcolemma; SR releases Ca2+; 2ms • Contraction: Cross-bridge formation; Ca2+, troponin; 15ms • Relaxation: Ca2+ uptake; tropomyosin covers actin; 25ms

  17. What happens when AP’s arrive? relaxation phase Complete relaxation phase Incomplete relaxation phase Eliminated

  18. Motor units control tension • 1 motor unit = all the muscle fibers controlled by a single motor neuron • Can the size (of motor units) vary? • Yes! Why would it vary? • Level of control required • Muscles of the eye - precise control; 4-6 fibers • Muscles of the leg - gross control; 1-2k fibers

  19. Motor Units

  20. Motor unit recruitment

  21. What ultimately controls muscle tension? • Presence of Ca2+ ions • More Ca2+ ions present = more to potentially bind to troponin • Stronger contraction (more tension produced)

  22. Cardiac muscle • Heart muscle • Cells directly connected via intercalated discs (pores through which ions pass) • Allows all connected cells to contract as one • Cardiac muscle is autorhythmic (spontaneous generation of AP) • Involuntary (influenced by hormones) • Metabolism is always aerobic

  23. Smooth muscle • Less actin & myosin, no sarcomeres • Contracts slowly • No O2 debt • Autorhythmic • Involuntary control

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