Neuromuscular Junction

1. Neuromuscular Junction

2. Motor Unit • one motor neuron • all the skeletal muscles it stimulates

3. Fine Muscle Control • few muscle fibers stimulated by one motor neuron • single motor neuron may supply very few fibers (eye) • Result: - finer control of muscle fibers

4. Coarse Muscle Control • many muscle fibers stimulated by one motor neuron • single motor neuron may supply many fibers (large muscle) • Result: - less control of muscle fibers

5. Neuromuscular Junction • contact or junction between motor neuron and a skeletal muscle - thread-like extensions of neuron branch into many axonal terminals - each branch forms a junction with sarcolemma (one muscle fiber)

6. Nerve Endings • individual branches of axon near muscle fiber loose myelin sheath • divide into several bulb-shaped structures (synaptic end bulb) - bulbs contain neurotransmitter acetylcholine (ACh)

7. Nerve Endings • extremely close to muscle but never touch • space is called synaptic cleft - filled with interstitial fluid

8. Motor End Plate • portion of muscle fiber membrane adjacent to synaptic end bulb of motor neuron • contains receptors for acetylcholine

9. Polarization • muscle fiber relaxed (resting sarcolemma) • outside sarcolemma + charge (predominant extracellular ion is Na+) • inside sarcolemma - charge (predominant intracellular ion is K+) • sarcolemma is relatively impermeable to both ions

10. Depolarization(generation of the action potential) • stimulation of sarcolemma by motor nerve • patch of sarcolemma becomes permeable to sodium ions (sodium gates open) • + sodium ions rush into cell • inside sarcolemma + charge • outside sarcolemma - charge • this rush upsets electrical currents causing action potential

11. Propagation of the Action Potential • + charge inside sarcolemma changes permeability of adjacent patches on sarcolemma • depolarization is repeated - therefore action potential spreads along entire length of sarcolemma

12. Repolarization • events occur in reverse • sarcolemma permeability changes • Na+ gates close • K+gates open allowing diffusion of K+ ions out of cell • activation of sodium-potassium pump restores ionic resting state concentrations

13. Repolarization (cont.) • occurs in same direction as depolarization • must occur before muscle can be stimulated again

14. Sequence of Events of Muscle Stimulation

15. Muscle Stimulation • muscle fibers are stimulated by motor neurons • impulse arrives at axon terminal of motor neuron

16. Muscle Stimulation (cont.) • impulse depolarizes plasma membrane opening voltage-sensitive calcium channels (Ca+2) • calcium ions diffuse from extracellular fluid into the axon terminal - triggers release of acetylcholine from synaptic end bulb

17. Muscle Stimulation • ACh diffuses across synaptic cleft • ACh interacts with receptors in the motor end plate of the muscle fiber, thus altering its permeability to sodium ions (Na+) • sodium ions diffuse from extracelluar fluid into muscle fiber, producing local depolarization called end-plate potential

18. Muscle Stimulation(power stroke) • end-plate potential generates flow of ions or current to bring adjacent sarcolemma to threshold • current spreads in both directions triggering action potentials • action potentialinitiate wave of contraction by way of transverse tubules

19. Muscle Stimulation (power stroke) (cont.) • action potential triggers release of Ca+2 from sarcoplasmic reticulum • Ca+2 ions bind to troponin molecules on the thin filaments • tropomyosin moves, uncovering cross-bridge binding sites on actin • binding of actin and myosin causes ATP to split releasing energy for the power stroke

20. Muscle Stimulation (power stroke) (cont.) • rotational movement of a myosin cross-bridge • one power stroke of a cross-bridge results in a small movement of the thin filament • each cross-bridge produces many cycles of movement during a single twitch contraction

21. Muscle Stimulation (power stroke) (cont.) • acetylcholine is quickly decomposed by cholinesterase • its decomposition prevents generation of further end-plate potentials

22. Sliding Filament Mechanism • during muscle contraction, neither the thick nor the thin filaments decrease in length • the actin (thin) filaments slide like pistons inward among the myosin (thick) filaments

23. Sliding Filament Mechanism (cont.) • in the resting state, the ends of the actin barely overlap the myosin • during contraction, these ends overlap considerably while the two Z membranes approach the ends of the myosin filaments

24. Myosin has globular bridges Ca+2 ions help cross bridges react with actin Actin ADP molecules on surface act as sites for linkages with cross bridges

25. Sources of Energy • phosphate system • glycogen-lactic acid system • aerobic system

26. Phosphate System

27. Phosphate System • ATP and creatine phosphate • together they provide energy for muscles to contract maximally for approximately 15 seconds • this system is used for short bursts of energy

28. Energy Source for Muscle Contraction • immediate source is ATP(adenosine triphosphate) • supplied by mitochondria near myofibrils • enzyme ATPase splits a phosphate group from ATP, forming ADP (adenosine diphosphate) and P (phosphate group) • energy released when P is split from molecule of ATP activates myosin cross-bridges

29. Energy Source for Muscle Contraction • very little ATP present in muscle fibers • if exercise is to continue for more than a few seconds, additional ATP must be produced

30. Energy Source for Muscle Contraction • primary energy source available to regenerate ATP from ADP and phosphate is creatine phosphate • contains high-energy phosphate bonds • cannot directly supply energy to a cell • 3-5 times more abundant in muscle fibers than ATP

31. Creatine Phosphate • stores energy released from mitochondria • when sufficient ATP is present, creatine phosphokinase (enzyme) promotes synthesis of creatine phosphate • energy is stored in its phosphate bonds • when ATP is being decomposed, energy from CP is transferred to ADP and then quickly converted back to ATP

32. Glycogen-Lactic Acid System

33. Glycogen-Lactic Acid System • with continued activity, muscles require energy after the supply of creatine phosphate is depleted • glucose must be catabolized to generate ATP

34. Glycogen-Lactic Acid System • glucose passes into contracted muscles via blood (facilitated diffusion) • glucose is also produced by glycolysis (breakdown of glycogen in muscles)

35. Glycolysis • series of ten reactions • splits glucose into two molecules of pyruvic acid and two molecules of ATP • anerobic process (no oxygen)

36. Glycogen-Lactic Acid System • pyruvic acid formed by glycolysis enters mitochondria - its oxidation produces large quantities of ATP from ADP • some activities do not supply enough O2 to completely break down pyruvic acid • pyruvic acid is then converted to lactic acid

37. Glycogen-Lactic Acid System (cont.) • most lactic acid diffuses from skeletal muscles into the blood • heart muscle fibers, kidney cells and liver cells use lactic acid to produce ATP • liver cells can convert lactic acid back to glucose • some lactic acid is accumulated in blood and muscles

38. Glycogen-Lactic Acid System (cont.) • can provide energy for about 30-40 seconds of maximal muscle activity, e.g., a 50 meter swimming race

39. Aerobic System

40. Aerobic System • reactions that require oxygen carried by the blood • oxygen is bonded to molecules of hemoglobin

41. Cellular Respiration • when energy is exhausted, muscles become dependant upon cellular respiration of glucose as a source of energy for synthesis of ATP • muscle activity longer than 30 seconds requires an aerobic process

42. Aerobic System • conversion of pyruvic acid into CO2, H20, and ATP • yields 36 molecules of ATP from each glucose molecule • provides energy for muscular activity lasting longer than 30 seconds

43. Recovery Oxygen Consumption (oxygen debt) • elevated oxygen use after exercise • above resting oxygen consumption • elevated oxygen necessary to restore metabolic conditions to resting state

44. Recovery Oxygen Consumption • converts lactic acid back into pyruvic acid • reestablishes glycogen stores in muscle and liver cells • resynthesizes creatine phosphate and ATP • replaces O2 removed from myoglobin

45. Recovery Oxygen Consumption(cont.) • ATP production for metabolic reactions (increased rate due to increased body temperature) • ATP production for continued elevated activity of cardiac and skeletal muscles • ATP production needed for an increased rate of tissue repair

46. Muscle Fatigue(inability of a muscle to contract) • Condition may result from: - insufficient O2 delivered to muscle cells - depletion of glycogen stored in muscle cells - buildup of lactic acid in body fluids - insufficient acetylcholine