Chapter 6 The Muscular System

1. Chapter 6 The Muscular System Biology 112 Tri-County Technical College Pendleton, SC

2. Functions of Muscles • Essential function of muscle is CONTRACTION (shortening) • Separates it from other body tissue • Responsible for essentially all body movement • Can be viewed as “machines” of body • Makes up nearly half of the body’s mass

3. Types of Muscle • Skeletal, Cardiac, and Smooth • Share certain characteristics • Muscle cells are elongated and are called MUSCLE FIBERS • Ability to shorten depends on TWO types of MYOFILAMENTS • Muscle cells =“microfilaments of cytoskeleton” • “Myo,” “Mys,” and “Sarco” refer to muscle • Sarcoplasm-cytoplasm of muscle cells

4. Types of Muscles, visual

6. Skeletal Muscle • Skeletal muscle fibers packed into organs called skeletal muscles that attach to skeleton • Known as striated muscle = fibers appear striped • Only muscle subject to CONSCIOUS control • Each fiber enclosed in CT sheath called ENDOMYSIUM • Several sheathed muscle fibers wrapped by membrane called PERIMYSIUM to form bundle of fibers called a FASCICLE

7. Skeletal Muscle, cont. • Many fascicles bound by EPIMYSIUM which covers entire muscle • EPIMYSIS blend into strong, cordlike TENDONS (sheetlike aponeuroses) which attach muscles indirectly to bone, cartilages, or connective tissue covering of each other • Spend some time on 6.1; page 164

8. Skeletal Muscle, visual

9. Sarcomere & Myofibrils • Plasma membrane of muscle cell called SARCOLEMMA • Long ribbonlike organelles called MYOFIBRILS nearly fill cytoplasm • Alternating light (I) and dark (A) bands along length of myofibril = striped appearance • Light I band has midline interruption; a darker area called the Z DISC (line) • Dark A band has lighter central area called the H zone

10. Sarcomere, cont. • Myofibrils are chains of tiny contractile units called SARCOMERES which are aligned end to end like boxcars in train • IT is arrangement of smaller MYOFILAMENTS within sarcomere that actually produces banding pattern • Spend some quality time on Figure 6.3; page 167

12. Myofilament Arrangement • Each sarcomere contains two types of myofilaments • Large thick filaments (MYOSIN) extend entire length of dark A band • Midparts of thick filaments are smooth but ends are studded with small projections (myosin heads or cross bridges) • Thin filaments (ACTIN) anchored to Z line which is actually disclike membrane • Light I band is area that includes parts of two adjacent sarcomeres and contains ONLY thin filaments

13. Myofilament Arrangement, cont. • Thin filaments overlap ends of thick filaments but DO NOT extend into middle of relaxed sarcomere • Thus central region (H zone) looks bit lighter • Contraction occurs, actin containing filaments slide toward each other into center of sarcomere and light zones disappear • Actin and myosin filaments completely overlap

14. Sarcomere Contraction • Fibers activated by NS, cross bridges on myosin attach to myosin binding sites of actin filaments • Attaches and detaches several times during contraction and PULLS thin filaments toward center of sarcomere • Event occurs simultaneously in sarcomeres throughout cell, muscle cell shortens • Z lines move closer together • H zone disappears • A bands move closer together but do NOT change in length • Millions of sarcomeres in millions of fibers = contraction of entire skeletal muscle

15. Contraction Visual

16. Connective Tissue Wrappings • Skeletal muscle wrapped by connective tissue • ENDOMYSIUM wraps each individual muscle fiber (cell) • PERIMYSIUM wraps bundles of fibers into a FASCICLE • EPIMYSIUM covers the entire muscle • Epimysium is continuous with tendons or aponeuroses

17. Smooth Muscle • Has NO striations and is involuntary • Walls of hollow visceral organs like stomach, urinary bladder, digestive tract, bronchi, uterus, and blood vessels • Visceral-nonstriated-involuntary • Spindled-shaped fibers with single nucleus and arranged in sheets/layers • Contractions SLOW and SUSTAINED-does Not tire easily • Movement of food through digestive tract, emptying bowels and bladder, & maintenance of blood pressure

18. Smooth Muscle, Visual

19. In a heartbeat…so to speak • Cardiac muscle best described as CARDIAC, STRIATED, AND INVOUNTARY • Branching cells joined by special junctions called intercalated disks • Cardiac muscle arranged in spiral shape • Allows contractions to be closely coordianted

20. Nerve Aspects • Each muscle fiber must be stimulated separately by nerve impulses to contract • MOTOR UNIT-one motor neuron (nerve cell) and all the skeletal muscle cells it stimulates • Threadlike extensions of neuron (axon/nerve fiber) branch into number of axonal terminals at muscle • Each axonal terminal forms junctions with sarcolema of different muscle cell • Junctions called neuromuscular junctions

21. Motor Unit, Visual

22. Neuromuscular Junction, Visual

23. Nerve Aspects, cont. • Nerve endings and muscle cells’ membranes NEVER TOUCH • Gap between them called synaptic cleft and is filled with interstitial fluid • Nerve impulse reaches axonal terminals, a neurotransmitter is released • Specific neurotransmitter that simulates muscle cells is ACETYLCHOLINE (Ach) • Acetylcholine diffuses across synaptic cleft

24. Nerve Aspects, cont. • Acetylcholine attaches to receptors on sarcolemma • If enough released, sarcolemma becomes temporary permeable to sodium ions (Na+) which rush into muscle cell • Generates electrical current called action potential • AP travels over entire surface of sarcolemma conducting impulse from one end of cell to the other • Result is CONTRACTION of the cell

25. Action Potential, Visual

26. Sliding Filament Theory • Nerve impulseneuromuscular junction  acetylcholine releasedAP in sarcolemma • AP in sarcolemma causes sarcoplasmic reticulum to releases stored calcium ions into sarcoplasm • Calcium ions cause cross-bridges to from • Thin myofilaments (actin) pulled over thick (myosin) myofilaments • Energy provided by ATP

27. SF Theory, cont. • Sacromere contracts • AP ends, calcium ions reabsorbed • Cross-bridges turn loose & sarcomere relaxes • Neurotransmitter acetylcholine degraded by enzymes in synaptic cleft • Prevents continuous stimulation of muscle fiber • Acetylcholinesterase (care for some Raid, anyone?)

28. Contraction Mechanism, Visual

29. To twitch or not to twitch • Muscle fiber contracts in all-or-none fashion • Whole muscles do NOT contract that way • Skeletal muscles are organs composed of 1000s of muscle cells which react to stimuli with GRADED RESPONSES (different degrees of shortening) • Graded muscle contractions produced in 2 ways • Changing speed of muscle stimulation • Changing number of muscle cells being stimulated

30. Twitching time, cont. • Muscle twitch is single, brief, jerky contraction that occurs as result of certain nervous system problems • NOT the way muscle normally operates • Single stimulus-contraction-relaxation sequence in muscle fiber • DOES NOT accomplish anything useful in skeletal muscle

31. Twitch and more, cont. • Incomplete tetanus results when nerve impulses delivered to muscle at very high rate • Delivered so rapidly cells do not get chance to relax completely between stimuli • Stimulation continues and muscle never allowed to relax completely will cause tension to peak • Muscle producing peak tension during rapid cycles of contraction/relaxation said to be in incomplete tetanus

32. Enough on twitching already • Complete tetanus results when muscle is stimulated so rapidly that NO evidence of relaxation is seen • Contractions are completely smooth and sustained • Complete tetanus major role = smooth and prolonged muscle contractions • Force of muscle contraction depends on how many of its cells are stimulated • Few cells = contraction as whole is slight • All cells = muscle contraction as strong as it can be

33. Fatigue and Debt • Muscle subject to continual contraction for long time = muscle fatigue occurs • Muscle is fatigued when unable to contract even though still be stimulated • Without rest, active/working muscle begins to tire and contracts more weakly until finally ceases reacting and stops contracting • MF believed to result from oxygen debt that occurs during prolonged muscle activity

34. Fatigue & Debt, cont. • Work muscle can do and how long it can work w/o becoming fatigued depend on how good its blood supply is • If muscle runs out of O2, it must depend on glycolysis for ATP and converts pyruvic acid to lactic acid • Lack of adequate ATP and >acidity cause muscle to contract less effectively and finally to stop contracting all together

35. Regeneration of ATP, Visual

36. Isotonic Contractions • Isotonic (same tone/tension) contractions most familiar • Myofilaments are successful in sliding movements, muscle shortens, and movement occurs • Bending knee, rotating arms, and smiling are examples of isotonic contractions

37. Isometric Contractions • Isometric (same measurement/length) contractions are contractions in which muscles do NOT shorten • Mysoin myofilaments skidding their “wheels” and tension in muscle keeps increasing • Trying to slide but muscle pitted against some more/less immovable object(s) • Trying to lift 400 lb dresser along or pushing against immovable wall

38. Muscle tone • When muscle voluntarily relaxed, some of its fibers are contracting (one group then another) • As result, muscle remains firm, healthy, and ready for action • This state of continuous partial contractions is called MUSCLE TONE • Is the result of different motor units scattered through muscle being stimulated by nervous system in systematic way

39. Muscle tone, cont. • If nerve supply to muscle is destroyed, muscle NO longer stimulated in this manner • It loses TONE, and becomes paralyzed • Soon after, becomes flaccid (soft/flabby) and begins to atrophy (waste away)