Excitation - Contraction coupling

Nerve - Muscle Physiology

1. Mechanism of Skeletal Muscle Contraction (Excitation Contraction C oupling)

2. Contraction For contraction, skeletal muscle must: be stimulated by a nerve ending propagate an action potential, along its sarcolemma have a rise in intracellular Ca 2+ levels, the final stimulus for contraction Ca 2+ levels may rise from its resting level of less than 10 -7 M to greater than 10 -5 M

3. Theories of Muscle Contraction New elastic body theory (1840 – 1920 ) Fenn observed that total energy released by muscle (work +heat) increases as muscle work increases this is known as “FENN EFFECT” Continuous filament theory – According to this theory, during contraction actin & myosin combine to form 1 continuous filament which undergoes folding & shortening Electron microscope observations do not support this theory as after contraction length of thick & thin filament is not altered only their relative position changes

4. Theories of Muscle Contraction Sliding Filament Theory (1954): Huxley and Niedergerke Sliding filament theory was transformed into Cross Bridge cycle (1957): Huxley Thin filament slides past thick filament Molecular basis of sliding motion is by globular head of myosin forming cross bridges with actin monomers (Huxley’s Cross Bridge theory) or Ratchet theory of muscle contraction / Walk along Theory

5. Sliding Filament Theory Thin filament slides over the thick Width of A band constant Z-lines move closer – contraction move apart – relaxation A. Relaxed I band A band sarcomere M line Z disk B. Contracted

6. Sliding Filament Theory

7. Excitation –Contraction Coupling Sequence of events linking the transmission of an action potential along the sarcolemma to muscle contraction (the sliding of myofilaments )

8. Excitation Contraction(EC) coupling The entire process, extending from depolarization of the T-tubule membrane to the initiation of cross-bridge cycling, is termed Excitation Contraction coupling or EC coupling Action potential travels along T-Tubules leading to Ca ++ release from sarcoplasmic reticulum leading to contraction

9. Excitation Contraction(EC) coupling Electrical Event Action potential generated in muscle fiber memb . due to depolarization of motor end plate AP transmitted along muscle fiber – initiates contractile response Mechanical Event Contraction via contractile protein myosin and cytoskeletal protein actin ` Single AP causes a brief contraction followed by relaxation – Simple Muscle Twitch

10. Electrical and Mechanical Response Plotted on the same time scale Twitch starts about 2 ms after start of depolarization of membrane & before repolarization is complete Duration of twitch varies with type of muscle "Fast" muscle fibers- fine, rapid, precise movement - twitch durations as short as 7.5 ms "Slow" muscle fibers - strong, gross, sustained movements - twitch durations up to 100 ms

11. Action Potential RMP = -90mv AP lasts for 2-4 milliseconds (ms), conducted along muscle fiber at 5 m/sec with ARP = 1-3 msec Ends before contraction occurs Period between action potential initiation and the beginning of contraction is called the latent period Excitation-contraction coupling occurs within the latent period

12. Excitation Contraction(EC) coupling

13. STEPS IN CONTRACTION Neurotransmitter acetylcholine ( ACh ) binds to its receptors on the motor end plate Ligand gated ion channels in the receptors open and allow Na + and K + to move across the membrane depolarization

14. Action Potential Propagated along Sarcolemma Reaches the T Tubules Triads

15. T – system or Transverse Tubular System Contains L-type Ca 2+ channels clustered in groups of four called "tetrads“ Each is in fact a heteropentameric protein Each of the four Ca 2+ channels is also called a DHP receptor because inhibited by class of antihypertensive drugs known as dihydropyridines Function as voltage sensor in EC coupling Depolarization of T-tubule activates longitudinal sarcoplasmic reticulum via DHP receptors

16. Ca 2+ channel in Sarcoplasmic Reticulum a.k.a ryanodine receptor Because inhibited by class of drugs that include the plant alkaloids ryanodine and caffeine Homotetrameric structure Each of the four subunits of these channels has a large extension-also known as a "foot.“ Ligand gated Ca 2+ channel with calcium as its natural ligand Ca 2+ -induced Ca 2+ release (CICR)

17. Depolarization of T-tubule Each L-type Ca 2+ channel interacts with foot of one of the 4 subunits of the Ca 2+ -release channel Depolarization of T-tubule evokes conformational changes in each of the four L-type Ca 2+ channels and has two effects Conformational changes allow Ca 2+ to enter through the four channel pores Second, and much more important, the conformational changes in the four L-type Ca 2+ channels induce a conformational change in each of the four subunits of another channel-the Ca 2+ -release channel-that is located in the SR membrane

18. Ca ++ binds with Troponin C Troponin – Tropomyosin complex inhibits the interaction between actin and myosin When Ca ++ binds to Trop C, active sites of actin are uncovered & ATP is split to ADP releasing P- energy & contraction occurs Excitation Contraction(EC) coupling

19. Troponin Each troponin C molecule in skeletal muscle has 2 high-affinity Ca 2+ -binding sites 2 low-affinity Ca 2+ -binding sites Binding of Ca 2+ to low-affinity sites induces a conformational change in the troponin complex that has two effects troponin I moves away from the actin / tropomyosin filament, thereby permitting the tropomyosin molecule to move troponin T pushes tropomyosin away from the myosin-binding site on the actin and into the actin groove With the steric hindrance removed, the myosin head is able to interact with actin and engage in cross-bridge cycling

20. Cross Bridge Cycle In presence of calcium, myosin head binds to an actin filament Changes its orientation relative to myosin filament which causes filaments to slide relative to each other - Power Stroke During the Cross-Bridge Cycle, Contractile Proteins Convert the Energy of ATP Hydrolysis Into Mechanical Energy Each power stroke shortens sarcomere by 10nm Cross bridge cycling is asynchronous 500 myosin in one thick filament, each head cycling 5 times per second

21. Cross Bridge Cycle Occurs in 5 steps :- 1. Cross – Bridge formation – cocked myosin head (perpendicular or at a 90-degree angle to the thick and thin filaments) binds to actin filament Cocked head has the stored energy derived from the cleaved ATP

22. Cross Bridge Cycle 2. Release of Pi from the myosin Dissociation of Pi from the myosin head triggers power stroke Conformational change - myosin head bends approximately 45º about the hinge Pulls the actin filament about 11 nm toward the tail of the myosin molecule Generating force and motion

23. Cross Bridge Cycle 3. ADP release – Dissociation of ADP from myosin Myosin head remains in the same position (45º angle with respect to the thick and thin filaments) 4. ATP binding – ATP binding to the head of the myosin heavy chain (MHC) reduces the affinity of myosin for actin Myosin head releases actin filament

24. Cross Bridge Cycle 5. ATP hydrolysis – Breakdown of ATP to ADP and inorganic phosphate (Pi) occurs on myosin head Products of hydrolysis are retained on the myosin As a result of hydrolysis, the myosin head pivots around the hinge into a "cocked" position (perpendicular or at a 90º angle to the thick and thin filaments) Rotation causes the tip of the myosin to move about 11 nm along the actin filament so that it now lines up with a new actin monomer two monomers further along the actin filament

25. Cross Bridge Cycle Cycle repeats as long as Ca 2+ is elevated and sufficient ATP is there Muscle cells do not regulate cross-bridge cycling by modifying [ATP] i Instead, skeletal muscle and cardiac muscle control this cycle by preventing cross-bridge formation until the tropomyosin moves out of the way in response to an increase in [Ca 2+ ] i

26. Cross Bridge Cycle

27. Cross Bridge cycle

28. Cross Bridge cycle

29. Movement of 10 nm Force in pico N Cross Bridge cycle

30. Cross Bridge cycle

31. Cross Bridge cycle

32. Cross Bridge cycle

33. Cross Bridge cycle

34. Excitation Contraction(EC) coupling

35. Excitation Contraction(EC) coupling

36. Steps in Relaxation Cell may extrude Ca 2+ using either an Na-Ca exchanger (NCX) or a Ca 2+ pump(PMCA) However, would eventually deplete the cell of Ca 2+ and is thus a minor mechanism for Ca 2+ removal from the cytoplasm Instead, Ca 2+ re-uptake into the SR is the most important mechanism by which the cell returns [Ca 2+ ] i to resting levels Ca 2+ re-uptake by the SR is mediated by a SERCA ( s   arcoplasmic or e   ndoplasmic r   eticulum C  a 2+ A   TPase )-type Ca 2+ pump

37. Steps in Relaxation SR Ca 2+ -pump activity is inhibited by high [Ca 2+ ] within the SR lumen Inhibition of SR Ca 2+ -pump activity is delayed by Ca 2+ -binding proteins within the SR lumen Buffer the Ca 2+ increase in the SR during Ca 2+ re-uptake and thus markedly increase the Ca 2+ capacity of SR Proteins have a tremendous capacity to bind Ca 2+ with up to 50 binding sites per protein molecule Principal Ca 2+ binding protein in skeletal muscle, calsequestrin also present in cardiac and some smooth muscle Calreticulin - Ca 2+ -binding protein found in particularly high concentrations within the SR of smooth muscle

38. Steps in Relaxation When Ca ++ conc. outside has lowered, interaction of actin & myosin ceases & muscle relaxes ATP required for both contraction &relaxation Pump concentrates Ca ++ about 10,000fold Normal/Resting Ca ++ conc. (less than 10 -7 moles of Ca ++ ) rises to 10 -5 M Total duration of Ca ++ ions stay in fluid is 1/30th of sec

39. EC Coupling – Skeletal muscle

40. Contracture Ca 2+ movement inhibited Relaxation fails to occur Cross bridges don’t break Sustained contraction despite no action potential

41. Role of ATP Provides energy for power stroke of myosin head Brings about a dissociation of myosin head from actin filament Brings about muscle relaxation by pumping Ca 2+ back into sarcoplasmic reticulum

42. Rigor Mortis Muscles of body become very stiff and rigid shortly after death Due to loss of ATP in the muscle cell In absence of ATP, the myosin cross bridges with actin is not broken, so, no relaxation occurs 15-25 hrs later, muscle proteins deteriorate and rigor disappears

43. EC Coupling: Drugs Blocking release of Ca ++ from SR keeps muscle relaxed, even in the presence of action potential eg Ryanodine receptor blocker like Protamine sulphate Caffeine cause release of Ca ++ produces contraction without action potential Drugs which increase the release of Ca ++ from sarcoplasmic reticulum. eg . Digitalis increase the force of cardiac muscle contraction

44. Malignant Hyperthermia Channelopathy of calcium release channel in muscle ( Ryanodine receptors) constant leak of SR Ca 2+ through ryanodine receptor triggered by halogenated anesthetics ( isoflurane , halothane ) or severe exercise familial tendency - can be tested for by muscle biopsy Symptoms Normal muscle function under normal conditions increased body temperature -more heat produced skeletal muscle rigidity lactic acidosis ( hypermetabolism )

45. Motor Unit One motor neuron and all the muscle fibers it innervates Number of muscle fibers varies among different motor units Number of muscle fibers per motor unit and number of motor units per muscle vary widely

46. Motor Unit Muscles that produce precise, delicate movements contain fewer fibres per motor unit Muscles performing powerful, coarsely controlled movement have larger number of fibers per motor unit

47. Thank you References: Guyton- Textbook of Medical Physiology Ganong’s - Review of Medical Physiology Boron-Medical Physiology Kandel -Principles of Neural Science Silbernagl -Color atlas of Physiology Ira Fox- Medical Physiology