Human Anatomy & Physiology

1. Human Anatomy & Physiology Muscle Tissue Chapter 11 By Abdul Fellah, Ph.D.

2. Muscle Tissue • Types and characteristics of muscular tissue • Microscopic anatomy of skeletal muscle • Nerve-Muscle relationship • Behavior of skeletal muscle fibers • Behavior of whole muscles • Muscle metabolism • Cardiac and smooth muscle

3. Introduction to Muscle • Movement is a fundamental characteristic of all living things • Cells capable of shortening and converting the chemical energy of ATP into mechanical energy • Types of muscle • skeletal, cardiac and smooth • Physiology of skeletal muscle • basis of warm-up, strength, endurance and fatigue

4. Characteristics of Muscle • Responsiveness (excitability) • to chemical signals, stretch and electrical changes across the plasma membrane • Conductivity • local electrical change triggers a wave of excitation that travels along the muscle fiber • Contractility -- shortens when stimulated • Extensibility -- capable of being stretched • Elasticity -- returns to its original resting length after being stretched

5. Skeletal Muscle • Voluntary striated muscle attached to bones • Muscle fibers (myofibers) as long as 30 cm • Exhibits alternating light and dark transverse bands or striations • reflects overlapping arrangement of internal contractile proteins • Under conscious control (voluntary)

6. Connective Tissue Elements • Attachments between muscle and bone • endomysium, perimysium, epimysium, fascia, tendon • Collagen is extensible and elastic • stretches slightly under tension and recoils when released • protects muscle from injury • returns muscle to its resting length • Elastic components • parallel components parallel muscle cells • series components joined to ends of muscle

7. The Muscle Fiber

8. Muscle Fibers • Multiple flattened nuclei inside cell membrane • fusion of multiple myoblasts during development • unfused satellite cells nearby can multiply to produce a small number of new myofibers • Sarcolemma has tunnel-like infoldings or transverse (T) tubules that penetrate the cell • carry electric current to cell interior

9. Muscle Fibers 2 • Sarcoplasm is filled with • myofibrils (bundles of myofilaments) • glycogen for stored energy and myoglobin for binding oxygen • Sarcoplasmic reticulum = smooth ER • network around each myofibril • dilated end-sacs (terminal cisternea) store calcium • triad = T tubule and 2 terminal cisternea

10. Thick Filaments • Made of 200 to 500 myosin molecules • 2 entwined polypeptides (golf clubs) • Arranged in a bundle with heads directed outward in a spiral array around the bundled tails • central area is a bare zone with no heads

11. Thin Filaments • Two intertwined strands fibrous (F) actin • globular (G) actin with an active site • Groove holds tropomyosin molecules • each blocking 6 or 7 active sites of G actins • One small, calcium-binding troponin molecule on each tropomyosin molecule

12. Elastic Filaments • Springy proteins called titin • Anchor each thick filament to Z disc • Prevents overstretching of sarcomere

13. Regulatory and Contractile Proteins • Myosin and actin are contractile proteins • Tropomyosin and troponin = regulatory proteins • switch that starts and stops shortening of muscle cell • contraction activated by release of calcium into sarcoplasm and its binding to troponin, • troponin moves tropomyosin off the actin active sites

14. Overlap of Thick and Thin Filaments

15. Striations = Organization of Filaments • Dark A bands (regions) alternating with lighter I bands (regions) • anisotrophic (A) and isotropic (I) stand for the way these regions affect polarized light • A band is thick filament region • lighter, central H band area contains no thin filaments • I band is thin filament region • bisected by Z disc protein called connectin, anchoring elastic and thin filaments • from one Z disc (Z line) to the next is a sarcomere

16. Striations and Sarcomeres

17. Relaxed and Contracted Sarcomeres • Muscle cells shorten because their individual sarcomeres shorten • pulling Z discs closer together • pulls on sarcolemma • Notice neither thick nor thin filaments change length during shortening • Their overlap changes as sarcomeres shorten

18. Nerve-Muscle Relationships • Skeletal muscle must be stimulated by a nerve or it will not contract • Cell bodies of somatic motor neurons in brainstem or spinal cord • Axons of somatic motor neurons = somatic motor fibers • terminal branches supply one muscle fiber • Each motor neuron and all the muscle fibers it innervates = motor unit

19. Motor Units • A motor neuron and the muscle fibers it innervates • dispersed throughout the muscle • when contract together causes weak contraction over wide area • provides ability to sustain long-term contraction as motor units take turns resting (postural control) • Fine control • small motor units contain as few as 20 muscle fibers per nerve fiber • eye muscles • Strength control • gastrocnemius muscle has 1000 fibers per nerve fiber

20. Neuromuscular Junctions (Synapse) • Functional connection between nerve fiber and muscle cell • Neurotransmitter (acetylcholine/ACh) released from nerve fiber stimulates muscle cell • Components of synapse (NMJ) • synaptic knob is swollen end of nerve fiber (contains ACh) • junctional folds region of sarcolemma • increases surface area for ACh receptors • contains acetylcholinesterase that breaks down ACh and causes relaxation • synaptic cleft = tiny gap between nerve and muscle cells • Basal lamina = thin layer of collagen and glycoprotein over all of muscle fiber

21. The Neuromuscular Junction

22. Neuromuscular Toxins • Pesticides (cholinesterase inhibitors) • bind to acetylcholinesterase and prevent it from degrading ACh • spastic paralysis and possible suffocation • Tetanus or lockjaw is spastic paralysis caused by toxin of Clostridium bacteria • blocks glycine release in the spinal cord and causes overstimulation of the muscles • Flaccid paralysis (limp muscles) due to curare that competes with ACh • respiratory arrest

23. Electrically Excitable Cells • Plasma membrane is polarized or charged • resting membrane potential due to Na+ outside of cell and K+ and other anions inside of cell • difference in charge across the membrane = resting membrane potential (-90 mV cell) • Stimulation opens ion gates in membrane • ion gates open (Na+ rushes into cell and K+ rushes out of cell) • quick up-and-down voltage shift = action potential • spreads over cell surface as nerve signal

24. Muscle Contraction and Relaxation • Four actions involved in this process • excitation = nerve action potentials lead to action potentials in muscle fiber • excitation-contraction coupling = action potentials on the sarcolemma activate myofilaments • contraction = shortening of muscle fiber • relaxation = return to resting length • Images will be used to demonstrate the steps of each of these actions

25. Excitation of a Muscle Fiber

26. Excitation (steps 1 and 2) • Nerve signal opens voltage-gated calcium channels. Calcium stimulates exocytosis of synaptic vesicles containing ACh = ACh release into synaptic cleft.

27. Excitation (steps 3 and 4) Binding of ACh to receptor proteins opens Na+ and K+ channels resulting in jump in RMP from -90mV to +75mV forming an end-plate potential (EPP).

28. Excitation (step 5) Voltage change in end-plate region (EPP) opens nearby voltage-gated channels producing an action potential

29. Excitation-Contraction Coupling

30. Excitation-Contraction Coupling (steps 6 and 7) Action potential spreading over sarcolemma enters T tubules -- voltage-gated channels open in T tubules causing calcium gates to open in SR

31. Excitation-Contraction Coupling (steps 8 and 9) • Calcium released by SR binds to troponin • Troponin-tropomyosin complex changes shape and exposes active sites on actin

32. Contraction (steps 10 and 11) • Myosin ATPase in myosin head hydrolyzes an ATP molecule, activating the head and “cocking” it in an extended position • It binds to actin active site forming a cross-bridge

33. Contraction (steps 12 and 13) • Power stroke = myosin head releasesADP and phosphate as it flexes pulling the thin filament past the thick • With the binding of more ATP, the myosin head extends to attach to a new active site • half of the heads are bound to a thin filament at one time preventing slippage • thin and thick filaments do not become shorter, just slide past each other (sliding filament theory)

34. Relaxation (steps 14 and 15) Nerve stimulation ceases and acetylcholinesterase removes ACh from receptors. Stimulation of the muscle cell ceases.

35. Relaxation (step 16) • Active transport needed to pump calcium back into SR to bind to calsequestrin • ATP is needed for muscle relaxation as well as muscle contraction

36. Relaxation (steps 17 and 18) • Loss of calcium from sarcoplasm moves troponin-tropomyosin complex over active sites • stops the production or maintenance of tension • Muscle fiber returns to its resting length due to recoil of series-elastic components and contraction of antagonistic muscles

37. Rigor Mortis • Stiffening of the body beginning 3 to 4 hours after death • Deteriorating sarcoplasmic reticulum releases calcium • Calcium activates myosin-actin cross-bridging and muscle contracts, but can not relax. • Muscle relaxation requires ATP and ATP production is no longer produced after death • Fibers remain contracted until myofilaments decay

38. Length-Tension Relationship • Amount of tension generated depends on length of muscle before it was stimulated • length-tension relationship (see graph next slide) • Overly contracted (weak contraction results) • thick filaments too close to Z discs and can’t slide • Too stretched (weak contraction results) • little overlap of thin and thick does not allow for very many cross bridges too form • Optimum resting length produces greatest force when muscle contracts • central nervous system maintains optimal length producing muscle tone or partial contraction

39. Length-Tension Curve

40. Muscle Twitch in Frog • Threshold = voltage producing an action potential • a single brief stimulus at that voltage produces a quick cycle of contraction and relaxation called a twitch (lasting less than 1/10 second) • A single twitch contraction is not strong enough to do any useful work

41. Muscle Twitch in Frog 2 • Phases of a twitch contraction • latent period (2 msec delay) • only internal tension is generated • no visible contraction occurs since only elastic components are being stretched • contraction phase • external tension develops as muscle shortens • relaxation phase • loss of tension and return to resting length as calcium returns to SR

42. Contraction Strength of Twitches • Threshold stimuli produces twitches • Twitches unchanged despite increased voltage • “Muscle fiber obeys an all-or-none law” contracting to its maximum or not at all • not a true statement since twitches vary in strength • depending upon, Ca2+ concentration, previous stretch of the muscle, temperature, pH and hydration • Closer stimuli produce stronger twitches

43. Recruitment and Stimulus Intensity • Stimulating the whole nerve with higher and higher voltage produces stronger contractions • More motor units are being recruited • called multiple motor unit summation • lift a glass of milk versus a whole gallon of milk

44. Twitch and Treppe Contractions • Muscle stimulation at variable frequencies • low frequency (up to 10 stimuli/sec) • each stimulus produces an identical twitch response • moderate frequency (between 10-20 stimuli/sec) • each twitch has time to recover but develops more tension than the one before (treppe phenomenon) • calcium was not completely put back into SR • heat of tissue increases myosin ATPase efficiency

45. Incomplete and Complete Tetanus • Higher frequency stimulation (20-40 stimuli/second) generates gradually more strength of contraction • each stimuli arrives before last one recovers • temporal summation or wave summation • incomplete tetanus = sustained fluttering contractions • Maximum frequency stimulation (40-50 stimuli/second) • muscle has no time to relax at all • twitches fuse into smooth, prolonged contraction called complete tetanus • rarely occurs in the body

46. Isometric and Isotonic Contractions • Isometric muscle contraction • develops tension without changing length • important in postural muscle function and antagonistic muscle joint stabilization • Isotonic muscle contraction • tension while shortening = concentric • tension while lengthening = eccentric

47. Muscle Contraction Phases • Isometric and isotonic phases of lifting • tension builds though the box is not moving • muscle begins to shorten • tension maintained

48. ATP Sources • All muscle contraction depends on ATP • Pathways of ATP synthesis • anaerobic fermentation (ATP production limited) • without oxygen, produces toxic lactic acid • aerobic respiration (more ATP produced) • requires continuous oxygen supply, produces H2O and CO2

49. Immediate Energy Needs • Short, intense exercise (100 m dash) • oxygen need is supplied by myoglobin • Phosphagen system • myokinase transfers Pi groups from one ADP to another forming ATP • creatine kinase transfers Pi groups from creatine phosphate to make ATP • Result is power enough for 1 minute brisk walk or 6 seconds of sprinting

50. Short-Term Energy Needs • Glycogen-lactic acid system takes over • produces ATP for 30-40 seconds of maximum activity • playing basketball or running around baseball diamonds • muscles obtain glucose from blood and stored glycogen