Chapter 10: Muscle Tissue

1. Chapter 10:Muscle Tissue

2. Muscle Overview The three types of muscle tissue are skeletal, cardiac, and smooth These types differ in structure, location, function, and means of activation

3. Muscle Similarities Skeletal and smooth muscle cells are elongated and are called muscle fibers Muscle contraction depends on two kinds of myofilaments � actin and myosin Muscle terminology is similar Sarcolemma � muscle plasma membrane Sarcoplasm � cytoplasm of a muscle cell Prefixes � myo, mys, and sarco all refer to muscle

4. Functions of Skeletal Muscles Produce skeletal movement Maintain body position Support soft tissues Guard body openings Maintain body temperature

5. Organization of Connective Tissues Connective tissue wrappings epimysium-surrounds the �whole� muscle perimysium- surrounds the �fascicle� endomysium- surrounds each muscle fiber all of these sheaths are continuous w/ each other as well as tendons & aponeuroses that connect to bone transmits force of contraction to the bone to be moved

6. Skeletal Muscle: Nerve and Blood Supply Each muscle is served by one nerve, an artery, and one or more veins Each skeletal muscle fiber is supplied with a nerve ending that controls contraction Contracting fibers require continuous delivery of oxygen and nutrients via arteries Wastes must be removed via veins

7. Microscopic anatomy of skeletal muscle Muscle fiber = muscle cell skeletal muscle fibers are long & cylindrical & are multinucleate Fibers are 10 to 100 ?m in diameter, and up to hundreds of centimeters long Plasma membrane = sarcolemma lemma = husk Cytoplasm = sarcoplasm contain large amts of glycosomes (stored glycogen) & myoglobin (store O2 w/in mm cell)

8. Myofibrils The contractile elements of skeletal muscle cells account for 80% of cell volume 100�s to 1000�s are in a single muscle fiber Made up of bundles of protein filaments (myofilaments)

9. Striations Result from darker A Bands & lighter I Bands a sarcomere is the region of a myofibril between 2 successive Z-Discs the sarcomere is the smallest contractile unit of a muscle fiber (cell)

10. A band Dark contain thick filaments (myofilaments) has a central H zone visible only when the muscle is in a relaxed state (thin filaments do not overlap the thick ones in this region) has a slightly darker M line in middle of H zone because of protein strands (desmin) that hold adjacent thick filaments together Light interrupted in the middle by a darker line called the Z disc (AKA Z line) composed of proteins (connectins) anchors thin filaments & connects each myofibril to the next throughout the width of the muscle cell Titin - Strands of protein that reach from tips of thick filaments to the Z line to stabilize the filaments

11. Transverse Tubules (T tubules) Transmit action potential through cell Allow entire muscle fiber to contract simulataneously Have same properties as sarcolemma

12. Sarcoplasmic Reticulum A membranous structure surrounding each myofibril similar in structure to smooth e.r. Helps transmit action potential to myofibril Forms chambers (terminal cisternae) attached to T tubules Function to regulate intracellular levels of ionic calcium stores calcium & releases it on demand when the muscle fiber is stimulated to contract

13. Skeletal Muscle Contraction In order to contract, a skeletal muscle must: Be stimulated by a nerve ending Propagate an electrical current, or action potential, along its sarcolemma Have a rise in intracellular Ca2+ levels, the final trigger for contraction

14. Contraction vs Shortening Contraction = Caused by interactions of thick and thin filaments. Ends when cross bridges become inactive & the tension generated declines, inducing relaxation of the muscle fiber Shortening = forces generated by cross bridges on the thin filament is greater than forces opposing shortening

15. Types of Myofilaments Actin (thin filaments) AKA F actin subunit called G actin has sites to bind to myosin contains 2 types of proteins (troponin & tropomyosin) tropomyosin stiffens the actin & blocks active sites in a relaxed muscle so myosin cannot bind troponin helps to position tropomyosin on the actin & helps to bind calcium ions Myosin (thick filaments) each molecule has a tail & 2 heads (cross bridges) heads contain ATP binding sites & ATPase enzymes to split ATP to generate E for contraction ~200 myosin molecules per each thick filament within a sarcomere

16. 4 Thin Filament Proteins F actin: is 2 twisted rows of globular G actin the active sites on G actin strands bind to myosin Nebulin: holds F actin strands together Tropomyosin: is a double strand prevents actin�myosin interaction Troponin: binds tropomyosin to G actin controlled by Ca2+

17. The Mysosin Molecule Tail: binds to other myosin molecules Head: made of 2 globular protein subunits reaches the nearest thin filament During contraction, myosin heads interact with actin filaments, forming cross-bridges pivot, producing motion

18. Skeletal Muscle Contraction Sliding filament theory: Contractions that produce a shortening of the muscle cell thin filaments of sarcomere slide between thick filaments toward M line A bands move closer together but do not change in length Z lines move closer together I bands are shortened

19. Excitation�Contraction Coupling Action potential reaches a triad: releasing Ca2+ and triggering contraction Requires myosin heads to be in �cocked� position: loaded by ATP energy

20. Exposure of binding sites Cross bridge attachment to actin requires calcium ions nerve impulse leading to contraction causes an increase in intracellular calcium [] low levels of intracellular calcium causes muscle relaxation & tropomyosin blocks binding sites on actin w/available Ca, the Ca binds to sites on troponin causing it to change shape & the tropomyosin moves away from the myosin binding sites

21. 5 Steps of the Contraction Cycle Exposure of active sites Formation of cross-bridges Pivoting of myosin heads Detachment of cross-bridges Reactivation �cocking� of myosin

22. The Contraction Cycle

23. A single working stroke of all the cross bridges in a muscle results in a shortening of only about 1 % routinely muscles contract between 30-35% of their total resting length probably only one-half of the myosin heads are actively exerting a pulling force at the same time the other half are actively seeking their next binding site

24. The Process of Contraction A skeletal muscle fiber must be stimulated by a nerve ending and must propagate an electrical current (action potential) along its sarcolemma in order to contract. causes excitation�contraction coupling Cisternae of SR release Ca2+ which triggers interaction of thick and thin filaments consuming ATP and producing tension

25. Voluntary nervous system Motor neurons of the somatic (voluntary) nervous system stimulate skeletal muscle to contract cell bodies reside in the brain/spinal cord axons (efferent) travel to the muscle cells they serve axons divide many times to form neuromuscular junctions w/ individual muscle fibers each muscle fiber has only one neuromuscular junction located at the approximate middle of the fiber

26. Neuromuscular Junction The neuromuscular junction is formed from: Axonal endings, which have small membranous sacs (synaptic vesicles) that contain the neurotransmitter acetylcholine (ACh) The motor end plate of a muscle, which is a specific part of the sarcolemma that contains ACh receptors and helps form the neuromuscular junction Though exceedingly close, axonal ends and muscle fibers are always separated by a space called the synaptic cleft

27. The Neurotransmitter Acetylcholine or ACh: travels across the synaptic cleft & binds to receptors on sarcolemma (motor end plate) Causes Na+ to rush into sarcoplasm causing interior of cell to become less negative (more positive) This event is called depolarization Is quickly broken down by enzyme (acetylcholinesterase or AChE) This destruction prevents continued muscle fiber contraction in the absence of additional stimuli

28. Action Potential Generated by increase in sodium ions in sarcolemma Travels along the T tubules Leads to excitation�contraction coupling

29. Skeletal Muscle Innervation

30. A Review of Muscle Contraction

31. A Review of Muscle Contraction

32. Rigor Mortis A fixed muscular contraction after death Caused when: ion pumps cease to function calcium builds up in the sarcoplasm

33. Tension Production The all�or�none principal: as a whole, a muscle fiber is either contracted or relaxed

34. Tension of a Single Muscle Fiber Depends on: The number of pivoting cross-bridges The fiber�s resting length at the time of stimulation Normal resting sarcomere length is 75% to 130% of optimal length The frequency of stimulation

35. Length�Tension Relationship Number of pivoting cross-bridges depends on: amount of overlap between thick and thin fibers Optimum overlap produces greatest amount of tension: too much or too little reduces efficiency

36. Frequency of Stimulation A single neural stimulation produces: a single contraction or twitch which lasts about 7�100 msec Length of twitch depends on type of muscle Sustained muscular contractions: require many repeated stimuli

37. 3 Phases of Twitch Latent period before contraction: the action potential moves through sarcolemma causing Ca2+ release Contraction phase: calcium ions bind tension builds to peak Relaxation phase: Ca2+ levels fall active sites are covered tension falls to resting levels

38. Treppe Repeated stimulations immediately after relaxation phase: stimulus frequency < 50/second Causes a series of contractions with increasing tension Basis for warm-up in athletes

39. Wave (Temporal) Summation Repeated stimulations before the end of relaxation phase: stimulus frequency > 50/second Causes increasing tension or summation of twitches Incomplete tetanus - twitches reach maximum tension

40. Complete Tetanus If stimulation frequency is high enough, muscle never begins to relax, and is in continuous contraction

41. Motor Units in a Skeletal Muscle Contain hundreds of muscle fibers that contract at the same time Controlled by a single motor neuron Recruitment � multiple motor unit stimulation

42. Muscle Tone The normal tension and firmness of a muscle at rest Muscle units actively maintain body position, without motion Increasing muscle tone increases metabolic energy used, even at rest

43. 2 Types of Skeletal Muscle Tension Isotonic contraction - skeletal muscle changes length resulting in motion If muscle tension > resistance: muscle shortens (concentric contraction) If muscle tension < resistance: muscle lengthens (eccentric contraction) Isometric contraction - skeletal muscle develops tension, but is prevented from changing length

44. Isotonic Contraction

45. Resistance and Speed of Contraction Are inversely related The heavier the resistance on a muscle: the longer it takes for shortening to begin and the less the muscle will shorten

46. Muscle Relaxation After contraction, a muscle fiber returns to resting length by: elastic forces: the pull of elastic elements (tendons and ligaments) Expands the sarcomeres to resting length opposing muscle contractions : antagonists gravity

47. Muscle metabolism ATP is the only E source for contraction mm only store ~2-4 seconds worth of ATP 3 main pathways for the regeneration of ATP 1. Creatine phosphate (CP) 2. Anaerobic glycolysis - breaks down glucose from glycogen stored in skeletal muscles 3. Aerobic respiration - of fatty acids in the mitochondria

48. ATP and CP Reserves Adenosine triphosphate (ATP): the active energy molecule Creatine phosphate (CP): The storage molecule for excess ATP energy in resting muscle Energy recharges ADP to ATP using the enzyme creatine phosphokinase (CPK)

49. Anaerobic glycolysis & lactic acid formation Remember�w/ initial phase of glycolysis 1 glucose molecule is divided into 2 pyruvic acid molecules and 2 ATP molecules are yielded Remember�glycolysis occurs w/ or w/o oxygen (pyruvic acid or lactic acid) With an oxygen deficit lactic acid is the main byproduct of glycolysis rather than carbon dioxide & water

50. Anaerobic glycolysis & lactic acid formation, cont. Anaerobic pathway yields only ~5% as much ATP as aerobic pathway but is occurs 2 � times faster Glycolysis can provide enough ATP to sustain strenuous activity for 30-40 seconds CP & Glycolysis together can provide enough E to sustain exercise for nearly a minute

51. Muscle Metabolism: Anaerobic Glycolysis When muscle contractile activity reaches 70% of maximum: Bulging muscles compress blood vessels Oxygen delivery is impaired Pyruvic acid is converted into lactic acid

52. Aerobic respiration Provides up to 95% of the ATP used for mm activity w/ prolonged exercise�these reactions are collectively known as oxidative phosphorylation Remember�it occurs in the mitochondria and yields 36 ATP per molecule of glucose Can continue �indefinitely� in the presence of oxygen� When exercise demands (oxygen) exceed the ability of mm to carry out the reactions, glycolysis kicks back in�lactic acid build-up� muscle fatigue Aerobic endurance- length of time a muscle can continue to contract using aerobic pathways Anaerobic threshold- the point at which muscle metabolism converts to anaerobic glycolysis Is ATP demands are kept below the anaerobic threshold moderate activity can continue for several hours in a trained individual

53. Muscle fatigue Definition- the physiological inability to contract Different than psychological fatigue Absence of ATP leads to contractures i.e. writer�s cramp No ATP available detach the cross-bridges Intense exercise produces rapid muscle fatigue (with rapid recovery) Na+-K+ pumps cannot restore ionic balances quickly enough Low pH (lactic acid) SR is damaged and Ca2+ regulation is disrupted

54. Oxygen debt Definition- the extra amount of oxygen to be taken in by the body to restore reserves of glycogen, ATP, and CP Example To run the 100 yard dash in 12 seconds your body would need ~ 6 L of oxygen for totally aerobic respiration. VO2 max (amt of oxygen delivered to & used by your mm) is ~ 1.2 L during that interval. The oxygen debt is then 4.8L which gets repaid by heavy breathing after exercise triggered by increased lactic acid in the blood.

55. The Cori Cycle The removal and recycling of lactic acid by the liver Liver converts lactic acid to pyruvic acid Glucose is released to recharge muscle glycogen reserves

56. Heat Production and Loss Only 30-40% of the energy released in muscle activity is useful as work The remaining 60-70% is given off as heat Dangerous heat levels are prevented by radiation of heat from the skin and sweating

57. Hormones and Muscle Metabolism Growth hormone Testosterone Thyroid hormones Epinephrine

58. Muscle Performance Power: the maximum amount of tension produced Endurance: the amount of time an activity can be sustained Power and endurance depend on: the types of muscle fibers physical conditioning

59. 3 Types of Skeletal Muscle Fibers Fast fibers (white)- Have large diameter, large glycogen reserves, few mitochondria Have strong contractions, fatigue quickly Slow fibers (red)- Have small diameter, more mitochondria, contain myoglobin (red pigment, binds oxygen) Have high oxygen supply Intermediate fibers (pink)- Have more capillaries than fast fiber, slower to fatigue Have low myoglobin

60. Fast versus Slow Fibers

61. Comparing Skeletal Muscle Fibers

62. Muscle Hypertrophy Muscle growth from heavy training: increases diameter of muscle fibers increases number of myofibrils increases mitochondria, glycogen reserves

63. Structure of Cardiac Tissue Automaticity: contraction without neural stimulation controlled by pacemaker cells Variable contraction tension: controlled by nervous system Extended contraction time Prevention of wave summation and tetanic contractions by cell membranes

64. Characteristics of Cardiocytes Unlike skeletal muscle, cardiac muscle cells: are small, branched & uninucleate have short, wide T tubules have SR with no terminal cisternae are aerobic (high in myoglobin, mitochondria) have intercalated discs Are specialized contact points between cardiocytes Join cell membranes of adjacent cardiocytes (gap junctions, desmosomes) Because intercalated discs link heart cells mechanically, chemically, and electrically, the heart functions like a single, fused mass of cells

65. Smooth Muscle in Body Systems Found lining hollow organs In blood vessels: regulates blood pressure and flow In reproductive and glandular systems: produces movements In digestive and urinary systems: forms sphincters produces contractions In integumentary system: arrector pili muscles cause goose bumps

66. Characteristics of Smooth Muscle Cells Long, slender, and spindle shaped Have a single, central nucleus Have no T tubules, myofibrils, or sarcomeres Have no tendons or aponeuroses Have scattered myosin fibers Myosin fibers have more heads per thick filament Have thin filaments attached to dense bodies Dense bodies transmit contractions from cell to cell

67. Excitation�Contraction Coupling Actin & myosin are scattered in sarcoplasm Free Ca2+ in cytoplasm triggers contraction Ca2+ binds with calmodulin in the sarcoplasm activates myosin light chain kinase

68. Regulation of contraction 30x longer to contract than skeletal mm Can maintain contraction at 1% the E cost of skeletal muscle ATP efficiency is very important to homeostasis i.e. maintaining smooth muscle tone in arteries Smooth muscle tone - Maintains normal levels of activity Modified by neural, hormonal, or chemical factors

69. Regulation of contraction, cont. Neural regulation The effect of a neurotransmitter on a smooth mm cell depends on the types of receptors on the sarcolemma ( + or - ) Ach in sk mm is always excitatory Examples of neurotransmitters include Ach, epi/norepi

70. Regulation of contraction, cont. Local regulation Some smooth mm cells have no nerve supply at all Some respond to both neural & chemical stimuli Examples include�hormones, lack of oxygen, rise in carbon dioxide, low pH All either enhance or inhibit calcium ion entry into the sarcoplasm

71. Response to Stretch Smooth muscle exhibits a phenomenon called stress-relaxation response in which: Smooth muscle responds to stretch only briefly, and then adapts to its new length The new length, however, retains its ability to contract This enables organs such as the stomach and bladder to temporarily store contents Skeletal & cardiac muscle respond to stretch with a more forceful contraction

72. Length & tension changes Smooth mm can function normally from ~30% shorter to 30% longer than its resting length It can also contract from twice its normal length to half of its normal (resting) length This is a change of 150%!

73. Control of Contractions Subdivisions: multiunit smooth muscle cells: connected to motor neurons visceral (single unit) smooth muscle cells: not connected to motor neurons rhythmic cycles of activity controlled by pacesetter cells

74. Characteristics of Skeletal, Cardiac, and Smooth Muscle