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The Muscular System

The Muscular System. The Muscular System. Muscles are responsible for all types of body movement Three basic muscle types are found in the body Skeletal muscle Cardiac muscle Smooth muscle. Characteristics of Muscles. Muscle cells are elongated (muscle cell = muscle fiber)

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The Muscular System

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  1. The Muscular System

  2. The Muscular System Muscles are responsible for all types of body movement Three basic muscle types are found in the body Skeletal muscle Cardiac muscle Smooth muscle

  3. Characteristics of Muscles • Muscle cells are elongated (muscle cell = muscle fiber) • Contraction of muscles is due to the movement of microfilaments • All muscles share some terminology • Prefix myo refers to muscle • Prefix mys refers to muscle • Prefix sarco refers to flesh

  4. Smooth Muscle Characteristics • Has no striations • Spindle-shaped cells • Single nucleus • Involuntary – no conscious control • Found mainly in the walls of hollow organs

  5. Cardiac Muscle Characteristics • Has striations • Usually has a single nucleus • Joined to another muscle cell at an intercalated disc • Involuntary • Found only in the heart

  6. Voluntary Involuntary Viscera Limbs Heart Non-striated Striated Classification of muscle Skeletal Cardiac Smooth Note: Control, Location and Structure

  7. Type of muscle Nervouscontrol Type of control Example Controlled by CNS Voluntary Lifting a glass Regulated by ANS Involuntary Heart beating Controlled by ANS Involuntary Peristalsis Muscle Control Skeletal Skeletal Cardiac Smooth

  8. There are about 650 muscles in the human body. They enable us to move, maintain posture and generate heat. In this section we will only study a sample of the major muscles. Skeletal Muscle

  9. Function of Skeletal Muscles • Produce movement • Maintain posture • Stabilize joints • Generate heat

  10. Skeletal Muscle Characteristics • Most are attached by tendons to bones

  11. Skeletal Muscle Characteristics • Cells are multinucleate • Striated – have visible banding • Voluntary – subject to conscious control • Cells are surrounded and bundled by connective tissue

  12. Connective Tissue Wrappings of Skeletal Muscle • Endomysium – around single muscle fiber • Perimysium – around a fascicle (bundle) of fibers

  13. Connective Tissue Wrappings of Skeletal Muscle • Epimysium – covers the entire skeletal muscle • Fascia – on the outside of the epimysium

  14. Skeletal Muscle Attachments • Epimysium blends into a connective tissue attachment • Tendon – cord-like structure • Aponeuroses – sheet-like structure • Sites of muscle attachment • Bones • Cartilages • Connective tissue coverings

  15. Microscopic Anatomy of Skeletal Muscle • Cells are multinucleate • Nuclei are just beneath the sarcolemma

  16. Microscopic Anatomy of Skeletal Muscle • Sarcolemma – specialized plasma membrane • Sarcoplasmic reticulum – specialized smooth endoplasmic reticulum

  17. Microscopic Anatomy of Skeletal Muscle • Myofibril • Bundles of myofilaments • Myofibrils are aligned to give distinct bands • I band = light band • A band = dark band

  18. Microscopic Anatomy of Skeletal Muscle • Sarcomere • Contractile unit of a muscle fiber

  19. Microscopic Anatomy of Skeletal Muscle • Organization of the sarcomere • Thick filaments = myosin filaments • Composed of the protein myosin • Has ATPase enzymes

  20. Microscopic Anatomy of Skeletal Muscle • Organization of the sarcomere • Thin filaments = actin filaments • Composed of the protein actin

  21. Microscopic Anatomy of Skeletal Muscle • Myosin filaments have heads (extensions, or cross bridges) • Myosin and actin overlap somewhat

  22. Microscopic Anatomy of Skeletal Muscle • At rest, there is a bare zone that lacks actin filaments • Sarcoplasmic reticulum (SR) – for storage of calcium

  23. To Summarize . . . • Skeletal muscle tissue is made of cells called muscle fibers. • Muscle fibers contain small cylinders called myofibrils. • Myofibrils are made of sarcomereslinked end-to-end.

  24. Muscle Contraction

  25. Properties of Skeletal Muscle Activity • Irritability – ability to receive and respond to a stimulus • Contractility – ability to shorten when an adequate stimulus is received

  26. Nerve Stimulus to Muscles • Skeletal muscles must be stimulated by a nerve to contract • Motor unit • One neuron • Muscle cells stimulated by that neuron

  27. Nerve Stimulus to Muscles • Neuromuscular junctions – association site of nerve and muscle

  28. Nerve Stimulus to Muscles • Synaptic cleft – gap between nerve and muscle • Nerve and muscle do not make contact • Area between nerve and muscle is filled with interstitial fluid

  29. Transmission of Nerve Impulse to Muscle • Neurotransmitter – chemical released by nerve upon arrival of nerve impulse • The neurotransmitter for skeletal muscle is acetylcholine • Neurotransmitter attaches to receptors on the sarcolemma • Sarcolemma becomes permeable to sodium (Na+)

  30. Transmission of Nerve Impulse to Muscle • Sodium rushing into the cell generates an action potential • Once started, muscle contraction cannot be stopped

  31. The Steps in Muscle Contraction • When a muscle is relaxed, myosin and actin filaments are not attached.

  32. The Steps in Muscle Contraction • Myosin attaches to a binding site on an actin filament. Calcium is required to make a binding site available for myosin.

  33. The Sliding Filament Theory

  34. The Steps in Muscle Contraction • The myosin head rotates and causes the actin filament to slide along the myosin filament. The sliding causes the filaments to overlap more, and the sarcomere becomes shorter.

  35. The Steps in Muscle Contraction • During contraction, myosin attaches to binding sites on actin, forming cross-bridges. Using ATP, the cross-bridges pull the actin toward the center of the sarcomere.

  36. The Steps in Muscle Contraction • After the myosin head has rotated as far as it can, it must let go of the actin fiber. ATP is required for myosin to detach from actin. The myosin head snaps back into its original position, using the energy in the ATP. The ATP becomes ADP and releases a phosphate ion.

  37. The Steps in Muscle Contraction • Calcium exposes a new actin binding site and myosin reattaches to actin. Steps 1 through 3 happen again.

  38. The Steps in Muscle Contraction • The cross-bridges break, myosin binds to another site, and the cycle begins again until the muscle fiber is contracted.

  39. To summarize . . . • Myosin filaments bind to actin filaments, actin filaments move inward, and sarcomeres shorten to cause muscle contraction.

  40. Contraction of a Skeletal Muscle • Muscle fiber contraction is “all or none” • Within a skeletal muscle, not all fibers may be stimulated during the same interval • Different combinations of muscle fiber contractions may give differing responses • Graded responses – different degrees of skeletal muscle shortening

  41. Types of Graded Responses • Twitch • Single, brief contraction • Not a normal muscle function

  42. Types of Graded Responses • Tetanus (summing of contractions) • One contraction is immediately followed by another • The muscle does not completely return to a resting state • The effects are added

  43. Types of Graded Responses • Unfused (incomplete) tetanus • Some relaxation occurs between contractions • The results are summed

  44. Types of Graded Responses • Fused (complete) tetanus • No evidence of relaxation before the following contractions • The result is a sustained muscle contraction

  45. Muscle Response to Strong Stimuli • Muscle force depends upon the number of fibers stimulated • More fibers contracting results in greater muscle tension • Muscles can continue to contract unless they run out of energy

  46. Energy for Muscle Contraction • Initially, muscles used stored ATP for energy • Bonds of ATP are broken to release energy • Only 4-6 seconds worth of ATP is stored by muscles • After this initial time, other pathways must be utilized to produce ATP

  47. Energy for Muscle Contraction • Direct phosphorylation • Muscle cells contain creatine phosphate (CP) • CP is a high-energy molecule • After ATP is depleted, ADP is left • CP transfers energy to ADP, to regenerate ATP • CP supplies are exhausted in about 20 seconds

  48. Energy for Muscle Contraction • Aerobic Respiration • Series of metabolic pathways that occur in the mitochondria • Glucose is broken down to carbon dioxide and water, releasing energy • This is a slower reaction that requires continuous oxygen

  49. Energy for Muscle Contraction • Anaerobic glycolysis • Reaction that breaks down glucose without oxygen • Glucose is broken down to pyruvic acid to produce some ATP • Pyruvic acid is converted to lactic acid

  50. Energy for Muscle Contraction • Anaerobic glycolysis (continued) • This reaction is not as efficient, but is fast • Huge amounts of glucose are needed • Lactic acid produces muscle fatigue

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