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

16. 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

17. 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

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

20. 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

21. Motor Units

22. Motor unit recruitment

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

24. 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

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