1. Physiology of the Muscular System Biology II Chapter 11

2. General Functions Body movement Heat production Homeostasis of body temperature Maintenance of posture Continued partial contraction Other functions Respiration Communication Constriction of organs and vessels Heart beat

3. Function of Skeletal Muscle Tissue Excitability (irritability) Ability to be stimulated and respond to that stimulus Respond to nerve signals Contractility Ability of a muscle to shorten with force Produces body movement Extensibility/Elasticity Ability to extend or stretch Ability of muscle to recoil to original resting length

4. Muscle Organ

5. Fascicle

6. Muscle Fiber

7. Function of Skeletal Muscle Tissue Sarcolemma Sarcoplasm Sarcoplasmic reticulum (SR) Triad T tubules Myofibrils Thick myofilament Thin myofilament Sarcomere Z lines M line A band I band H zone

8. Muscle Fiber

13. Function of Skeletal Muscle Tissue Myofilaments Each muscle fibers contains 1000+ myofibrils Each myofibril contains thousands of thick and thin myofilaments Four kinds of protein Myosin Actin Tropomyosin Troponin Thin filament Actin molecules strung together At rest, active sites covered by tropomyosin Tropomyosin molecules are held together by troponin Thick filament Myosin molecules Myosin heads are bridges between thick and thin filaments Muscle Contraction

14. Myofilaments

15. Sarcomere Shortening

19. Function of Skeletal Muscle Tissue The Mechanism of Contraction/Relaxation Excitation and Contraction Nerve impulse reaches the end of a motor neuron…release of acetylcholine ACH diffuses across the synaptic cleft of the synapse and binds to ACH receptors Stimulation of ACH receptors initiates an impulse that travels along sarcolemma, through the T tubules, to sacs of the SR Ca++ is released from SR into sarcoplasm, where it binds to troponin molecules Tropomyosin molecules in the thin myofilaments shift, exposing actin’s active site Energized myosin cross bridges bind to actin and use their energy to pull the thin myofilaments toward the center of each sarcomere. Repeats if ATP is available As the thin filaments slide past the thick filaments, the entire muscle fiber shortens Muscle Contraction

21. Function of Skeletal Muscle Tissue The Mechanism of Contraction/Relaxation Relaxation SR pumps Ca++ back into its sacs Tropomyosin returns to original position, blocking actin’s active sites Myosin cross bridges cannot bind to actin and contraction is not sustained Thick and thin myofilaments no longer connected, fibers return to resting length

22. Function of Skeletal Muscle Tissue Energy Sources for Muscle Contraction ATP Myosin cross bridges move into resting position when ATP binds to them Inorganic phosphate is released and energy is transferred to myosin cross bridge When myosin binds to actin, the stored energy is released and the myosin cross bridge returns to its original position Another ATP binds causing the release of actin and movement of myosin cross bridges into resting position (ready for next cycle)

23. Function of Skeletal Muscle Tissue Energy Sources for Muscle Contraction Glucose and Oxygen Potential energy in glucose is released during catabolism and transferred to ATP or CP Some muscles may store glucose as glycogen Excess oxygen in the sarcoplasm is bound to protein molecule called myoglobin Myoglobin supplies oxygen when the oxygen concentration inside a muscle fiber decreases rapidly Red fibers have large amounts of myoglobin; white fibers have small amounts

24. Function of Skeletal Muscle Tissue Energy Sources for Muscle Contraction Aerobic Respiration Catabolic process that produces the maximum amount of energy available from each glucose molecule Requires oxygen to be present Anaerobic Respiration Rapidly produces ATP in the absence of oxygen Results in the formation lactic acid Some lactic acid may later be converted back into glucose in the liver in an oxygen-consuming process Heat Production Some of the energy released is lost as heat Skeletal muscles can also be employed when the body’s temperature is too low

25. Function of Skeletal Muscle Tissue All-or-None Principle When sufficiently stimulated, muscle fibers contract with all the force possible under existing conditions, or they do not contract at all Threshold stimulus Minimal level of stimulation required to cause a fiber to contract

26. Motor Unit Somatic motor, branches, and muscle fibers May stimulate very few or many fibers Number of fibers contracted depends on the function of the muscle Function of Skeletal Muscle Organs

28. Skeletal Muscle Structure Myography A procedure in which the contraction of an isolated muscle is recorded as a line that rises and falls as the muscle contracts and relaxes Threshold stimulus produces a quick jerk of the muscles (twitch reaction)

29. Skeletal Muscle Structure Twitch Contraction Muscle does not begin to contract at the instant of stimulation (latent period) impulse travels through the sarcolemma and T tubules to the SR release of calcium ions into the sarcoplasm The muscle then increases its tension until a peak is reached (contraction phase) calcium binds to troponin and the sliding of the myofilaments begins Then it gradually returns to it resting state (relaxation phase) sliding of the myofilaments ceases

31. Skeletal Muscle Structure Treppe: The Staircase Phenomenon Gradual increase in the strength of contraction that can be observed in a series of twitch contractions muscle contracts more forcefully after it has contracted a few times may be related partly to the rise in temperature of active muscles and partly to their accumulation of metabolic products Muscle eventually stays partially contracted…state known as contracture

33. Skeletal Muscle Structure Tetanus (tetanic contractions) Smooth, sustained contractions A series of stimuli come in very rapidly, muscle does not have time to relax multiple wave summation Incomplete tetanus very short periods of relaxation occur between peaks of tension Complete tetanus single sustained peak, tension sustained completely Tetanus results from the coordinated contraction of different motor units within the muscle organ motor units fire in an overlapping time sequence to produce sustained contraction

36. Skeletal Muscle Structure Isotonic and Isometric Contractions Isotonic tone or tension within a muscle remains the same length of the muscle changes Isometric muscle length remains the same muscle tension increases Most body movements are a mixture of isotonic and isometric contractions

38. Skeletal Muscle Structure The Graded Strength Principle Skeletal muscles contract with varying degrees of strength only individual muscle cells contract according to the all-or- none principle Affected by various factors metabolic condition number of fibers contracting simultaneously depends on the number of motor units activated or recruited recruitment of motor neurons depends on intensity and frequency of stimulus strength of contraction reaches maximum even if strength of stimulus increases

40. Skeletal Muscle Structure The Graded Strength Principle Affected by various factors maximal strength that a muscle can develop is related to the initial length of its fibers max contraction occurs when muscle is at optimal length not too short, not too long

42. Skeletal Muscle Structure The Graded Strength Principle Affected by various factors amount of load imposed on the muscle heavier load = stronger contractions stretch reflex attempts to return muscle to set length if the load is too heavy, the reflex is abandoned and relaxation is triggered

44. Skeletal Muscle Structure Function of Cardiac and Smooth Muscle Tissue Cardiac Muscle (striated involuntary) Found only in the heart Contracts rhythmically and continuously Each cardiac muscle contains parallel myofibrils that comprise sarcomeres Muscle fibers form strong, electrically couple junctions (intercalated disks) Individual cells are often branched to form a syncytium T tubules are larger than in skeletal muscle and form diads with smaller SR Ca++ may enter sarcoplasm through T tubules rather than SR Sarcolemma sustains impulse longer…simultaneous, prolonged contractions Impulse cannot move fast enough to produce tetanus Does not exhibit tetanus or fatigue Self-exciting, but can be affected by nervous or hormonal input

45. Skeletal Muscle Structure Smooth Muscle Composed of small, tapered cells with single nuclei T tubules are absent Loosely organized SR Ca++ comes from outside the cell and binds to calmodulin (not troponin) Thin filaments criss-cross the cell and attach to plasma membrane contractions cause the muscles to “ball up” and contract the cell

46. Skeletal Muscle Structure Smooth Muscle Two types of smooth muscle tissue visceral: gap junctions join fibers into large, continuous sheets most common type of smooth muscle forms the muscular layer in the walls of hollow structures such as the digestive, urinary, and reproductive systems exhibits autorhythmicity that can push contents along its lumen = peristalsis multiunit: composed of many independent single-cell units each independent fiber responds to a nervous input most often found in bundles (arrector pili), but may be found as single fibers (surround blood vessels), or in thin sheets (walls of large blood vessels)

47. Muscle Tissue Types Skeletal Attached to bones Nuclei multiple and peripherally located Striated, Voluntary and involuntary (reflexes) Smooth Walls of hollow organs, blood vessels, eye, glands, skin Single nucleus centrally located Not striated, involuntary, gap junctions in visceral smooth Cardiac Heart Single nucleus centrally located Striations, involuntary, intercalated disks

48. Extra Slides and Images

49. Cross-bridge formation

65. Multiple Motor Unit Summation A whole muscle contracts with a small or large force depending on number of motor units stimulated to contract

66. Speed of contraction: