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Lecture 2 muscle tissue cont


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Lecture 2 muscle tissue cont

  1. 1. Chapter 10 Muscle Tissue Lecture Outline
  2. 2. Overview: How Does Contraction Begin? <ul><li>1. Nerve impulse reaches an axon terminal </li></ul><ul><li>2.synaptic vesicles release acetylcholine (ACh) </li></ul><ul><li>3. ACh diffuses to receptors on the sarcolemma </li></ul><ul><li>4. stimulus provided by binding of ACh to the </li></ul><ul><li>sarcolemma; </li></ul><ul><li>5. Na+ channels open and Na+ rushes into the cell </li></ul><ul><li>6. resulting action potential travels along sarcolemma and into T tubules, triggering release of calcium ions from SR; </li></ul><ul><li>7. calcium ions bind to troponin; resulting shape change causes myosin binding site to be exposed; </li></ul><ul><li>8. myosin heads bind to actin, and swivel (power stroke), pulling Z discs closer together, shortening myofiber.-the contraction cycle begins </li></ul>
  3. 3. Contraction Cycle <ul><li>Repeating sequence of events that cause the thick & thin filaments to move past each other. </li></ul><ul><li>4 steps to contraction cycle </li></ul><ul><ul><li>ATP hydrolysis </li></ul></ul><ul><ul><li>attachment of myosin to actin to form crossbridges </li></ul></ul><ul><ul><li>power stroke </li></ul></ul><ul><ul><li>detachment of myosin from actin </li></ul></ul><ul><li>Cycle keeps repeating as long as there is ATP available & there is a high Ca+2 level near the filaments. </li></ul>
  4. 4. Steps in the Contraction Cycle <ul><li>Notice how the myosin head attaches and pulls on the thin filament with the energy released from ATP </li></ul>
  5. 5. ATP and Myosin <ul><li>Myosin heads are activated by ATP </li></ul><ul><li>Activated heads attach to actin & pull (power stroke) </li></ul><ul><li>ADP is released. (ATP released P & ADP & energy) </li></ul><ul><li>Thin filaments slide past the thick filaments </li></ul><ul><li>ATP binds to myosin head & detaches it from actin </li></ul><ul><li>All of these steps repeat over and over </li></ul><ul><ul><li>if ATP is available & </li></ul></ul><ul><ul><li>Ca+ level near the troponin-tropomyosin complex is high </li></ul></ul>
  6. 6. Excitation - Contraction Coupling <ul><li>All the steps that occur from the muscle action potential reaching the T tubule to contraction of the muscle fiber. </li></ul>
  7. 7. Relaxation <ul><li>Acetylcholinesterase (AChE) breaks down ACh within the synaptic cleft </li></ul><ul><li>Muscle action potential ceases </li></ul><ul><li>Ca+2 release channels close </li></ul><ul><li>Active transport pumps Ca2+ back into storage in the sarcoplasmic reticulum </li></ul><ul><li>Calcium-binding protein (calsequestrin) helps hold Ca+2 in SR (Ca+2 concentration 10,000 times higher than in cytosol) </li></ul><ul><li>Tropomyosin-troponin complex recovers binding site on the actin </li></ul>
  8. 8. Overview: From Start to Finish <ul><li>Nerve ending </li></ul><ul><li>Neurotransmittor </li></ul><ul><li>Muscle membrane </li></ul><ul><li>Stored Ca+2 </li></ul><ul><li>ATP </li></ul><ul><li>Muscle proteins </li></ul>
  9. 9. Neuromuscular Junction (NMJ) or Synapse <ul><li>NMJ = myoneural junction </li></ul><ul><ul><li>end of axon nears the surface of a muscle fiber at its motor end plate region (remain separated by synaptic cleft or gap) </li></ul></ul>
  10. 10. Anatomy of Cardiac Muscle <ul><li>Striated , short, quadrangular-shaped, branching fibers </li></ul><ul><li>Single centrally located nucleus </li></ul><ul><li>Cells connected by intercalated discs with gap junctions </li></ul><ul><li>Same arrangement of thick & thin filaments as skeletal </li></ul>
  11. 11. CARDIAC MUSCLE TISSUE - Overview <ul><li>Cardiac muscle tissue is found only in the heart wall Its fibers are arranged similarly to skeletal muscle fibers. </li></ul><ul><ul><li>Cardiac muscle fibers connect to adjacent fibers by intercalated discs which contain desmosomes and gap junctions </li></ul></ul><ul><ul><li>Cardiac muscle contractions last longer than the skeletal muscle twitch due to the prolonged delivery of calcium ions from the sarcoplasmic reticulum and the extracellular fluid. </li></ul></ul><ul><ul><li>Cardiac muscle fibers contract when stimulated by their own autorhythmic fibers. </li></ul></ul><ul><li>This continuous, rhythmic activity is a major physiological difference between cardiac and skeletal muscle tissue. </li></ul>
  12. 12. Appearance of Cardiac Muscle <ul><li>Striated muscle containing thick & thin filaments </li></ul><ul><li>T tubules located at Z discs & less SR </li></ul>
  13. 13. Physiology of Cardiac Muscle <ul><li>Autorhythmic cells </li></ul><ul><ul><li>contract without stimulation </li></ul></ul><ul><li>Contracts 75 times per min & needs lots of O2 </li></ul><ul><li>Larger mitochondria generate ATP aerobically </li></ul><ul><li>Extended contraction is possible due to slow Ca+2 delivery </li></ul><ul><ul><li>Ca+2 channels to the extracellular fluid stay open </li></ul></ul>
  14. 14. SMOOTH MUSCLE <ul><li>Smooth muscle tissue is nonstriated and involuntary and is classified into two types: visceral (single unit) smooth muscle and multiunit smooth muscle </li></ul><ul><ul><li>Visceral (single unit) smooth muscle is found in the walls of hollow viscera and small blood vessels; the fibers are arranged in a network and function as a “single unit.” </li></ul></ul><ul><ul><li>Multiunit smooth muscle is found in large blood vessels, large airways, arrector pili muscles, and the iris of the eye. The fibers operate singly rather than as a unit. </li></ul></ul>
  15. 15. Two Types of Smooth Muscle <ul><li>Visceral (single-unit) </li></ul><ul><ul><li>in the walls of hollow viscera & small BV </li></ul></ul><ul><ul><li>autorhythmic </li></ul></ul><ul><li>Multiunit </li></ul><ul><ul><li>individual fibers with own motor neuron ending </li></ul></ul><ul><ul><li>found in large arteries, large airways, arrector pili muscles,iris & ciliary body </li></ul></ul>
  16. 16. Microscopic Anatomy of Smooth Muscle <ul><li>Sarcoplasm of smooth muscle fibers contains both thick and thin filaments which are not organized into sarcomeres. </li></ul><ul><li>Smooth muscle fibers contain intermediate filaments which are attached to dense bodies. </li></ul><ul><li>Small, involuntary muscle cell -- tapering at ends </li></ul><ul><li>Single, oval, centrally located nucleus </li></ul><ul><li>Lack T tubules & have little SR for Ca+2 storage </li></ul>
  17. 17. Microscopic Anatomy of Smooth Muscle <ul><li>Thick & thin myofilaments not orderly arranged so lacks sarcomeres </li></ul><ul><li>Sliding of thick & thin filaments generates tension </li></ul><ul><li>Transferred to intermediate filaments & dense bodies attached to sarcolemma </li></ul><ul><li>Muscle fiber contracts and twists into a helix as it shortens -- relaxes by untwisting </li></ul>
  18. 18. Physiology of Smooth Muscle <ul><li>Contraction starts slowly & lasts longer </li></ul><ul><ul><li>no transverse tubules & very little SR </li></ul></ul><ul><ul><li>Ca+2 must flows in from outside </li></ul></ul><ul><li>In smooth muscle, the regulator protein that binds calcium ions in the cytosol is calmodulin (in place of the role of troponin in striated muscle); </li></ul><ul><ul><li>calmodulin activates the enzyme myosin light chain kinase , which facilitates myosin-actin binding and allows contraction to occur at a relatively slow rate. </li></ul></ul>