1. Suzanne D'Anna1 Neuromuscular Junction

2. Suzanne D'Anna2 Motor Unit l one motor neuron l all the skeletal muscles it stimulates

3. Suzanne D'Anna3 Fine Muscle Control l few muscle fibers stimulated by one motor neuron l single motor neuron may supply very few fibers (eye) l Result: - finer control of muscle fibers

4. Suzanne D'Anna4 Coarse Muscle Control l many muscle fibers stimulated by one motor neuron l single motor neuron may supply many fibers (large muscle) l Result: - less control of muscle fibers

5. Suzanne D'Anna5 Neuromuscular Junction l 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. Suzanne D'Anna6 Nerve Endings l individual branches of axon near muscle fiber loose myelin sheath l divide into several bulb-shaped structures (synaptic end bulb) - bulbs contain neurotransmitter acetylcholine (ACh)

7. Suzanne D'Anna7 Nerve Endings l extremely close to muscle but never touch l space is called synaptic cleft - filled with interstitial fluid

8. Suzanne D'Anna8 Motor End Plate l portion of muscle fiber membrane adjacent to synaptic end bulb of motor neuron l contains receptors for acetylcholine

9. Suzanne D'Anna9 Polarization l muscle fiber relaxed (resting sarcolemma) l outside sarcolemma + charge (predominant extracellular ion is Na + ) l inside sarcolemma - charge (predominant intracellular ion is K + ) l sarcolemma is relatively impermeable to both ions

10. Suzanne D'Anna10 Depolarization (generation of the action potential) l stimulation of sarcolemma by motor nerve l patch of sarcolemma becomes permeable to sodium ions (sodium gates open) l + sodium ions rush into cell l inside sarcolemma + charge l outside sarcolemma - charge l this rush upsets electrical currents causing action potential

11. Suzanne D'Anna11 Propagation of the Action Potential l + charge inside sarcolemma changes permeability of adjacent patches on sarcolemma l depolarization is repeated - therefore action potential spreads along entire length of sarcolemma

12. Suzanne D'Anna12 Repolarization l events occur in reverse l sarcolemma permeability changes l Na + gates close l K + gates open allowing diffusion of K + ions out of cell l activation of sodium-potassium pump restores ionic resting state concentrations

13. Suzanne D'Anna13 Repolarization (cont.) l occurs in same direction as depolarization l must occur before muscle can be stimulated again

14. Suzanne D'Anna14 Sequence of Events of Muscle Stimulation

15. Suzanne D'Anna15 Muscle Stimulation l muscle fibers are stimulated by motor neurons l impulse arrives at axon terminal of motor neuron

16. Suzanne D'Anna16 Muscle Stimulation (cont.) l impulse depolarizes plasma membrane opening voltage-sensitive calcium channels (Ca +2 ) l calcium ions diffuse from extracellular fluid into the axon terminal - triggers release of acetylcholine from synaptic end bulb

17. Suzanne D'Anna17 Muscle Stimulation l ACh diffuses across synaptic cleft l ACh interacts with receptors in the motor end plate of the muscle fiber, thus altering its permeability to sodium ions (Na+) l sodium ions diffuse from extracelluar fluid into muscle fiber, producing local depolarization called end-plate potential

18. Suzanne D'Anna18 Muscle Stimulation (power stroke) l end-plate potential generates flow of ions or current to bring adjacent sarcolemma to threshold l current spreads in both directions triggering action potentials l action potential initiate wave of contraction by way of transverse tubules

19. Suzanne D'Anna19 Muscle Stimulation (power stroke) (cont.) l action potential triggers release of Ca +2 from sarcoplasmic reticulum l Ca +2 ions bind to troponin molecules on the thin filaments l tropomyosin moves, uncovering cross- bridge binding sites on actin l binding of actin and myosin causes ATP to split releasing energy for the power stroke

20. Suzanne D'Anna20 Muscle Stimulation (power stroke) (cont.) l rotational movement of a myosin cross- bridge l one power stroke of a cross-bridge results in a small movement of the thin filament l each cross-bridge produces many cycles of movement during a single twitch contraction

21. Suzanne D'Anna21 Muscle Stimulation (power stroke) (cont.) l acetylcholine is quickly decomposed by cholinesterase l its decomposition prevents generation of further end-plate potentials

22. Suzanne D'Anna22 Sliding Filament Mechanism l during muscle contraction, neither the thick nor the thin filaments decrease in length l the actin (thin) filaments slide like pistons inward among the myosin (thick) filaments

23. Suzanne D'Anna23 Sliding Filament Mechanism (cont.) l in the resting state, the ends of the actin barely overlap the myosin l during contraction, these ends overlap considerably while the two Z membranes approach the ends of the myosin filaments

24. Suzanne D'Anna24 Myosin l has globular bridges l Ca +2 ions help cross bridges react with actin Actin l ADP molecules on surface act as sites for linkages with cross bridges

25. Suzanne D'Anna25 Sources of Energy l phosphate system l glycogen-lactic acid system l aerobic system

26. Suzanne D'Anna26 Phosphate System

27. Suzanne D'Anna27 Phosphate System l ATP and creatine phosphate l together they provide energy for muscles to contract maximally for approximately 15 seconds l this system is used for short bursts of energy

28. Suzanne D'Anna28 Energy Source for Muscle Contraction l immediate source is ATP (adenosine triphosphate) l supplied by mitochondria near myofibrils l enzyme ATPase splits a phosphate group from ATP, forming ADP (adenosine diphosphate) and P (phosphate group) l energy released when P is split from molecule of ATP activates myosin cross- bridges

29. Suzanne D'Anna29 Energy Source for Muscle Contraction l very little ATP present in muscle fibers l if exercise is to continue for more than a few seconds, additional ATP must be produced

30. Suzanne D'Anna30 l primary energy source available to regenerate ATP from ADP and phosphate is creatine phosphate l contains high-energy phosphate bonds l cannot directly supply energy to a cell l 3-5 times more abundant in muscle fibers than ATP Energy Source for Muscle Contraction

31. Suzanne D'Anna31 Creatine Phosphate l stores energy released from mitochondria l when sufficient ATP is present, creatine phosphokinase (enzyme) promotes synthesis of creatine phosphate l energy is stored in its phosphate bonds l when ATP is being decomposed, energy from CP is transferred to ADP and then quickly converted back to ATP

32. Suzanne D'Anna32 Glycogen-Lactic Acid System

33. Suzanne D'Anna33 Glycogen-Lactic Acid System l with continued activity, muscles require energy after the supply of creatine phosphate is depleted l glucose must be catabolized to generate ATP

34. Suzanne D'Anna34 Glycogen-Lactic Acid System l glucose passes into contracted muscles via blood (facilitated diffusion) l glucose is also produced by glycolysis (breakdown of glycogen in muscles)

35. Suzanne D'Anna35 Glycolysis l series of ten reactions l splits glucose into two molecules of pyruvic acid and two molecules of ATP l anerobic process (no oxygen)

36. Suzanne D'Anna36 Glycogen-Lactic Acid System l pyruvic acid formed by glycolysis enters mitochondria - its oxidation produces large quantities of ATP from ADP l some activities do not supply enough O 2 to completely break down pyruvic acid l pyruvic acid is then converted to lactic acid

37. Suzanne D'Anna37 Glycogen-Lactic Acid System (cont.) l most lactic acid diffuses from skeletal muscles into the blood l heart muscle fibers, kidney cells and liver cells use lactic acid to produce ATP l liver cells can convert lactic acid back to glucose l some lactic acid is accumulated in blood and muscles

38. Suzanne D'Anna38 Glycogen-Lactic Acid System (cont.) l can provide energy for about 30-40 seconds of maximal muscle activity, e.g., a 50 meter swimming race

39. Suzanne D'Anna39 Aerobic System

40. Suzanne D'Anna40 Aerobic System l reactions that require oxygen carried by the blood l oxygen is bonded to molecules of hemoglobin

41. Suzanne D'Anna41 Cellular Respiration l when energy is exhausted, muscles become dependant upon cellular respiration of glucose as a source of energy for synthesis of ATP l muscle activity longer than 30 seconds requires an aerobic process

42. Suzanne D'Anna42 Aerobic System l conversion of pyruvic acid into CO 2, H 2 0, and ATP l yields 36 molecules of ATP from each glucose molecule l provides energy for muscular activity lasting longer than 30 seconds

43. Suzanne D'Anna43 Recovery Oxygen Consumption (oxygen debt) l elevated oxygen use after exercise l above resting oxygen consumption l elevated oxygen necessary to restore metabolic conditions to resting state

44. Suzanne D'Anna44 Recovery Oxygen Consumption l converts lactic acid back into pyruvic acid l reestablishes glycogen stores in muscle and liver cells l resynthesizes creatine phosphate and ATP l replaces O 2 removed from myoglobin

45. Suzanne D'Anna45 Recovery Oxygen Consumption (cont.) l ATP production for metabolic reactions (increased rate due to increased body temperature) l ATP production for continued elevated activity of cardiac and skeletal muscles l ATP production needed for an increased rate of tissue repair

46. Suzanne D'Anna46 Muscle Fatigue (inability of a muscle to contract) l Condition may result from: - insufficient O 2 delivered to muscle cells - depletion of glycogen stored in muscle cells - buildup of lactic acid in body fluids - insufficient acetylcholine