Unit III Muscular System Structure and Physiology

1. Unit IIIMuscular System Structure and Physiology Chapter 10

2. Muscle functions • Producing body motions • Walking, running, nodding, grasping, etc. • Stabilizing body positions • Sustained contractions of neck hold head up • Storing & moving substances within body • GI tract, cardiac muscle • Generating heat (_______________) • Exercise, shivering

3. 4 Properties of muscle tissue • _________________ – respond to stimuli by producing electrical signals (AP) • ___________ – ability of muscle tissue to contract forcefully when stimulated by AP • _____________ – stretch w/out being damaged • ____________ – return to original length & shape after contraction or extension

5. Structure of skeletal muscle, fig 10.1 • Each _______________________________ • composed of 100-1000s cells • * muscle ______= muscle _____ = myofibers • Endomysium = CT surrounds each fiber • B.V. & nerves penetrate into muscle • Perimysium = CT surrounds the fascicle • _______= 10s-100s of cells (fibers) bundled together • Epimysium = CT surrounds many fascicles to bundle a whole muscle together as an organ

6. Structure of skeletal muscle (2) • All 3 layers of CT (endo, peri, epimysium) • protect and strengthen • extend from deep fascia - dense irregular CT bind muscles w/ similar function • ________ = fibrous membrane covering • Supporting & separating muscles

7. Submicroscopic skeletal muscle • _____________= muscle fiber’s plasma mem. • Skeletal muscle fibers can have >100 nuclei just beneath the sarcolemma • ____________ = invaginations in sarcolemma • tunnel into center of fiber, filled w/ ECF • assists in exciting entire muscle fiber during AP • _____________-cytoplasm within muscle fiber • contains ___________________  ATP • stuffed w/ myofibrils

8. Myofibrils – threadlike structures • contractile elements, extend entire length of muscle • Arranged in ___________- basic functional units • Striated appearance • Myofilaments - composed of contractile proteins • DO NOT extend whole length of fiber • 2 types: • _______________________ • _______________________

9. _____________- red, oxygen binding protein • only in muscle fibers • Contributes oxygen for ATP synthesis • Mitochondria- many in skeletal muscle • Myoglobin & sarcoplasm have ingredients for ATP production: • O2 • Glucose • _____________________ (SR)- fluid-filled system of membranous sacs, similar to sER • in relaxed muscle stores Ca 2+ • release of Ca 2+ causes muscle contraction

11. ____________- appearance due to light I bands and dark A bands • ____________- small mesodermal cells • Embryonic: skeletal muscle fibers arise from fusion of 100 or more • 100’s of nuclei • Satellite cells- myoblasts persisting in mature skeletal muscle • capacity to fuse w/ one another or w/ damaged muscle fibers • Regenerate functional muscle fibers

14. Motor neuron & its muscle fibers • Figure 10.11- neuromuscular junction (NMJ) • __________________- motor neuron & all the muscle fibers it stimulates • __________________- region of sarcolemma that includes Ach receptors • Near synaptic end bulbs • ___________________ (NMJ)- a synapse between axon terminals of a motor neuron & sarcolemma of a muscle fiber

16. Graded potential • ________________ from membrane potential that makes the membrane more or less polarized (Na+ & Ca 2+in, K+ out) • occur in dendrites & cell body of the motor neuron • if graded potential reaches the axon: • voltage-gated ion channels openAP

17. All-or-none principle • When ______________ voltage is reached voltage gated channels will open and an action potential occurs • Different neurons may have different thresholds BUT the point is: • ________________________________ • Push over first domino, the rest fall

18. __________________- molecules within axon terminals • Released into synaptic cleft in response to • Nerve impulse • Change in membrane potential • ___________________- NT released at NMJ • 1000s of molecules in each synaptic vesicle

20. Acetylcholine receptor (AChR)- at each motor end plate 30-40 million • Transmembrane protein binds ACh • Binding opens ligand-gated ion channels • Acetylcholinesterase (AChE)- enzyme breaks down ACh, attached to ECM in synaptic cleft • ACh binding lasts only briefly • Breaks down excess not bound to receptor

21. Nerve elicits a muscle action potential: • Release of ACh- diffuses across synaptic cleft. • Activation of AChR: binding opens gated ion channels allowing flow of small cations (most importantly Na+) • Production of muscle AP: inflow of Na+ inside fiber + charged, changing membrane potential, trigger AP • Termination of ACh activity: effect of ACh binding lasts briefly (ACh rapidly broken down by AChE) *NMJ usually located at midpoint of muscle fibers & propagate toward both ends

22. Excitation - depolarization • Increased Ca 2+ concentration in cytosol initiates muscle contraction • [Ca 2+]  due to depolarization of muscle cell membrane = sarcolemma • AP from neuron  ACh receptors opening Na+ channels on the sarcolemma which depolarizes the muscle- muscle AP travels along T tubules causing SR to release Ca 2+

23. Skeletal contraction & proteins • Myofibrils built from 3 types protein; • ________________ proteins- generate force • Actin and myosin • ________________ proteins- switch on and off • Troponin and tropomyosin • Both are part of the thin filament • ________________ proteins- proper alignment, elasticity, extensibility, link myofibrils to sarcolemma and ECM • 12: titan, myomesin, dystrophin

24. Sliding filament theory fig 10.7,8 • Muscle contraction: • ______ heads attach & “walk” along _____ • Walking towards both ends of sarcomere • Thin filaments  M line • Thick filaments  Z disc • Length of thick and thin NOT changing • Sarcomere shortening  whole muscle fiber shortens  shorten entire muscle

26. _______- motor protein (push and pull) • found in all 3 types muscle • 300 molecules/thick filament • ______________- convert chemical energy in ATP to mechanical energy of motion & force production • Shaped like 2 golf clubs twisted together • ______- molecules join to form filament in form of helix • On each actin molecule is a myosin binding site for myosin head to attach

28. The contraction cycle, fig 10.8 • Ca 2+ released from SR • Binds troponin-tropomyosin complex & move it • In relaxed muscle, myosin binding sites are blocked by: • ______________- Are held in place by: _______________ • Contraction cycle can begin: • ATP hydrolysis • Myosin attaches actin, form crossbridges • Power stroke • Detachment of myosin from actin

30. Contraction of skeletal muscle • Treadmill analogy • Myosin moving draws Z discs together • neighboring sarcomeres pulled together • Skeletal muscle shortens, pulls CT & tendons • Tension passes thru tendon, move bone • Fig. 10.11 to summarize contraction

33. Calcium’s role •  [Ca2+] in cytosol starts contraction • (decrease stops contraction) • Muscle fiber relaxed: [Ca2+] low, BUT huge amt of Ca2+ stored in SR. • AP propogates along sarcolemma T tubules, ________________________ in SR membrane open • Ca2+ flows out into cytosol, combines with troponin to change its shape • Myosin binding sites are now free

34. Sources of energy • Figure 10.12 • ______________: powering contraction cycle, pumping Ca2+ to SR for relaxation (& other metabolic rxns) • Relaxed state- modest amount used • Contracting- using at rapid pace • Amt present only enough for a few seconds of contraction • If strenous activity more ATP made…

36. 3 ways to produce ATP • _____________________________ • Unique to muscle fibers • While relaxed, muscle making more ATP than needed • Excess  creatine phosphate – an energy rich molecule • Enzyme: creatine kinase (CK) catalyzes transfer of one phosphate of ATP to creatine creatine phosphate & ADP • When contraction begins, ADP levels  so CK transfers phosphate group from creatine phosphate back to ADP creating ATP (enough energy to last 15 sec) • _______________________________ • _______________________________

37. Cellular respiration & muscle • Anaerobic- ATP-producing rxns, without O2 • Muscle activity but no creatine phosphate  glucose is catabolized to generate ATP • Glucose: blood  muscle fibers, & glycogen breakdown within muscle • Glycolysis: 1 glucose (10 rxns) 2 pyruvate yields 2 ATP • Pyruvic acid enters mitochondria & enters series of O2 requiring rxns to produce large amt of ATP • If no O2, pyruvic acid  lactic acid in cytosol • Liver cells take lactic acid  glucose

38. Aerobic cellular respiration- series of O2 requiring rxns in the mitochondria produce ATP • muscle activity longer than ½ minute • Pyruvic acid ATP, CO2, H2O, and heat • Slower than glycolysis BUT yields more: • 1 glucose  36-38 ATP molecules. • F.a. molecule  over 100 ATP molecules • Oxygen comes from: • Diffuses into muscle from blood • Released from myoglobin within muscle fibers

39. Motor unit recruitment • Process of ____________________________ • Typically different motor units in a whole muscle are NOT stimulated to contract in unison • Alternation delays muscle fatigue • Contraction of whole muscle can be sustained for long pds • One factor responsible for producing smooth movements rather than series of jerks • Recruitment causes small changes in muscle tension

40. Comparing isotonic & isometric • ___________ contraction- iso= same, tonic= tension • Contraction where tension remains same • Occurs when constant load is moved thru the range of motions possible at a joint • Lifting a book off a table • _______________ contraction- iso = same, metric = measure • Contraction in which tension of the muscle increases but there is only minimal shortening so that no visible movement is produced • Holding a book in an outstretched hand

41. Simple twitch figure 10.15 • __________________- brief contraction of all muscle fibers in motor unit due to a single AP in motor neuron • Myogram • Skeletal muscle twitch= 20-200msec • _____________ period- brief delay between application of stimulus & beginning of contraction (@2msec) • Ca2+ being released from SR & filaments exert tension, elastic components stretch, shortening begins

42. _______________ period= 10-100msec • _______________ period= 10-100msec • Active transport of Ca2+ back into SR • Duration of all periods depends on type of muscle fiber (see table 10.1) • Fast twitch (as in eye) – 10 msec for each contraction and relaxation • Slow twitch (as in legs) – 100 msec for each contraction and relaxation

43. Repolarization happens… • During the relaxation period Ca2+ is actively transported back to the SR • Refractory period= period of lost excitability • Characteristic of all muscle and nerve cells • Duration varies with muscle involved • Skeletal  5msec • Cardiac  300msec

45. ___________ = minimum stimulus – the least amount of voltage required for contraction • _________________ – the voltage at which maximal force is generated (increasing voltage will not increase force of contraction) • All motor units are stimulated • All muscle fibers are contracting • Graded response (graded potential) – small deviation in the membrane potential that makes the membrane more polarized or less polarized

46. Terms assoc. w/ frequency • _______________________________- • Maximal voltage applied to muscle in which all fibers in unit are stimulated • series of shocks at max voltage causes separate twitches • each twitch will stronger than the previous • Stimuli all at same intensity, cause muscle to contract more efficiently each time • May be warm-up effect, due to  intracellular Ca2+ needed for contraction

47. Wave summation- (summation of contraction)  strength of muscle contraction that results when muscle APs occur one after another in rapid succession •  frequency =  strength of contraction • Tetany- fused tetanus -hyperexcitability of neurons & muscle fibers • Sustained or fused contraction • Continuous tonic muscular contractions – individual twitches not discerned • May be due to hypoparathyroidism

49. Muscle fatigue • Inability to mantain force of contraction after prolonged activity • usually results from ∆ w/in muscle fiber • May feel tired, desire to cease activity • Central fatigue (CNS) • Mechanism unknown, possibly protective • Suspect contributing factors: • Inadequate release of Ca2+ from SR • Depletion of creatine phosphate (ATP levels not much change) • Insufficent oxygen, depletion of glycogen,build up of lactic acid and ADP, failure of AP to release Ach

50. Recovery oxygen uptake • Formerly called ___________________ – the added oxygen that is taken into body after exercise • Recovery: few minutes to several hours depending upon intensity of exercise • Depletions during exercise: • Convert lactic acid  glycogen in liver • Resynthesize creatine phosphate & ATP • Replace oxygen removed from myoglobin • Post exercise oxygen needs remain high: •  body temp  chemical rxns • Heart & muscles still working hard   ATP use • Tissue repair processes