The Muscular System


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  • Thick and thin filaments do not overlap completely
  • Transverse tubule infolding of the plasma memebrane
  • The Muscular System

    2. 2. Muscular System <ul><li>Main functions: </li></ul><ul><ul><li>Movement </li></ul></ul><ul><ul><li>Production of heat </li></ul></ul><ul><ul><li>Protection of other vital organs </li></ul></ul><ul><ul><li>Helps in posture </li></ul></ul>
    3. 3. Review of thermoregulation <ul><li>How do ectotherms balance heat loss and heat gain? </li></ul><ul><li>How do endotherms balance heat loss and heat gain? </li></ul><ul><li>Importance of daily torpor </li></ul><ul><li>Vasoconstriction and vasodilation </li></ul><ul><li>Countercurrent heat exchanger mechanism </li></ul>
    4. 4. Review of thermoregulation <ul><li>Huddling </li></ul><ul><li>Panting </li></ul><ul><li>Stress-induced proteins </li></ul><ul><ul><li>Heat-shock proteins </li></ul></ul>
    5. 5. Feedback mechanism in human thermoregulation
    6. 6. Balancing the internal environment <ul><li>Behavioral and physiological adaptations are used to budget heat loss and heat gain </li></ul><ul><ul><li>E.g. In a desert environment, evaporative cooling is not favored by the body rather conservation of water </li></ul></ul>
    7. 7. Characteristics of skeletal muscles <ul><li>Excitability- receive and respond to stimuli (the all-or-none response) </li></ul><ul><li>Contractility- shorten and thicken </li></ul><ul><li>Extensibility- stretch and extend (passively) </li></ul><ul><li>Elasticity- return to original shape after contraction or extension </li></ul>
    8. 8. Locomotion again.... <ul><li>Again, movement is done only if a muscle is working against some kind of skeleton </li></ul><ul><ul><li>Action is always to contract </li></ul></ul><ul><ul><li>Extension is passive </li></ul></ul><ul><ul><li>Muscles act in pairs (agonist and antagonist) </li></ul></ul><ul><ul><li>Also synergist (muscles that have the same action) </li></ul></ul>
    9. 9. Origin and insertion <ul><li>A muscle always originates from a specific part of a bone and is inserted also in a specific part of a bone (hahaha, kaya nga tinawag na skeletal muscle) </li></ul><ul><li>Origin- proximal </li></ul><ul><li>Insertion- distal </li></ul>
    10. 10. The muscle
    11. 11. Some terms (actually a lot of terms) <ul><li>Skeletal muscle- consists of bundle of muscle fiber </li></ul><ul><li>Fascicle- a bundle of muscle fiber surrounded by a connective tissue </li></ul><ul><li>Muscle fiber- a single muscle cell (syncytium) </li></ul><ul><li>Myofibril – makes up a muscle fiber </li></ul><ul><ul><li>Divided into two myofilaments (creates the striation) </li></ul></ul><ul><ul><ul><li>Thin filaments </li></ul></ul></ul><ul><ul><ul><li>Thick filaments </li></ul></ul></ul>
    12. 12. Still on terms...... <ul><li>Thin filaments- consists of two strands of actin and one strand of regulatory protein </li></ul><ul><li>Thick filament- consists of staggered array of myosin </li></ul><ul><li>Sarcomere- the basic contractile unit of a muscle </li></ul>
    13. 13. The sarcomere <ul><li>Z line- border of the sarcomere (striation visible in a light microscope) </li></ul><ul><ul><ul><li>Thin filaments- attached to the Z line </li></ul></ul></ul><ul><ul><ul><li>Thick filaments- center of the sarcomere </li></ul></ul></ul><ul><li>I band- area where there are only thin filaments </li></ul><ul><li>A band- shows the entire length of the thick filament </li></ul><ul><li>H zone- area that contains thick filament only </li></ul><ul><li>M line- the center of the thick filament </li></ul>
    14. 14. The sliding-filament model <ul><li>Contraction of a muscle is a result of the shortening of a sarcomere </li></ul><ul><li>The distance of two Z lines decreases </li></ul><ul><li>A bands do not change in length </li></ul><ul><li>I bands shortens and H zone disappears </li></ul>
    15. 15. The sliding-filament model <ul><li>Neither the thin nor the thick filaments shorten </li></ul><ul><li>The filaments slide past each other increasing the degree of overlap </li></ul><ul><li>Only the distance occupied specifically by both filaments disappears/shortens (I band, H zone) </li></ul>
    16. 16. The sliding-filament model <ul><li>Based on the interaction of the myosin head in the thick filament and actin molecules in the thin filament </li></ul><ul><li>Binding and hydrolysis of ATP is responsible for the change in shape of the myosin molecule </li></ul><ul><li>The myosin molecule goes into an energized stage </li></ul><ul><li>Formation of cross-bridge between the myosin and actin </li></ul><ul><li>Bond is broken when new ATP binds to myosin </li></ul>
    17. 17. Energy for contraction <ul><li>Stored ATP in muscle cell </li></ul><ul><li>Glycogen that can be degraded and form ATP </li></ul><ul><li>Phosphagens- supplies a phosphate to ADP to turn it into ATP </li></ul><ul><ul><ul><li>Vertebrate phosphagen- Creatine phosphate </li></ul></ul></ul>
    18. 18. Control of muscle contraction <ul><li>Contraction is due to stimulation from a motor neuron </li></ul><ul><li>At rest- tropomyosin blocks binding site in actin molecules </li></ul><ul><li>Troponin complex- controls position of tropomyosin </li></ul><ul><li>Calcium ions- binds to troponin </li></ul><ul><ul><li>Releases the active site of actin </li></ul></ul><ul><ul><li>High calcium in cytosol- contraction can occur </li></ul></ul><ul><ul><li>Low calcium in cytosol- contraction cannot occur </li></ul></ul>
    19. 19. The Sarcoplasmic Reticulum <ul><li>Specialized endoplasmic reticulum </li></ul><ul><li>Controls calcium ion concentration </li></ul><ul><li>Membrane- actively transports calcium ions </li></ul><ul><ul><li>Cytosol into the interior of the reticulum </li></ul></ul>
    20. 20. Action potential <ul><li>Stimulus of contraction- action potential from a motor neuron </li></ul><ul><li>Motor neuron- a type of neuron that is connected to a motor cell (as oppose to sensory) </li></ul><ul><li>Action potential is due to a change in the net charge of the surrounding muscle cell </li></ul>
    21. 21. Action potential <ul><li>Motor neuron- releases acetylcholine at neuromuscular junction </li></ul><ul><ul><li>ACh depolarizes the postsynaptic muscle cell creating the action potential </li></ul></ul><ul><ul><li>The action potential spreads to the T (transverse) tubule </li></ul></ul><ul><ul><li>T tubule is connected to the sarcoplasmic reticulum </li></ul></ul><ul><ul><li>Efflux of calcium ions is caused by a change in the permeability of sarcoplasmic reticulum due to the action potential </li></ul></ul>
    22. 22. Overview of muscle contraction <ul><li>Synaptic terminal of motor neuron releases Ach </li></ul><ul><li>ACh depolarizes postsynaptic muscle cell </li></ul><ul><li>Action potential is generated in the muscle cell </li></ul><ul><li>Action potential spreads through T tubules </li></ul><ul><li>Sarcoplasmic reticulum becomes permeable to calcium ions </li></ul><ul><li>Calcium ions bind to troponin </li></ul><ul><li>Troponin releases the active site of actin by moving the tropomyosin </li></ul>
    23. 23. Overview of muscle contraction <ul><li>Myosin head with attached ATP is under low energy state </li></ul><ul><li>ATP is hydrolyzed creating a high energy state myosin head </li></ul><ul><li>Cross-bridge is formed </li></ul><ul><li>Release of energy relaxes the myosin molecule </li></ul><ul><li>Another ATP molecule detaches the myosin from actin </li></ul><ul><li>Contraction occurs as the myosin head walks on the thin filament </li></ul>
    24. 24. End of contraction <ul><li>End of contraction occurs if calcium ion concentration in the cytosol falls leading to the blockage of the active site of actin </li></ul>
    25. 25. The all-or-none response <ul><li>Experience and experiments show that whole mucle contraction is graded </li></ul><ul><li>At the cellular level, contraction is an all-or-none response </li></ul><ul><li>Frequency of action potential creates the graded contraction </li></ul><ul><li>Summation of action potential occurs </li></ul><ul><li>Tetanus- a smooth and sustained contraction </li></ul>
    26. 26. Graded contraction of a muscle <ul><li>A muscle is innervated by one motor neuron </li></ul><ul><li>Motor neuron is branched- controls different muscle cell </li></ul><ul><li>Motor unit- a motor neuron and all muscle fiber it controls </li></ul><ul><li>Nervous system controls graded contraction by the number of motor unit in action </li></ul><ul><li>Recruitment of motor neurons- progressive inc in tension in a muscle </li></ul>
    27. 27. Muscle fatigue <ul><li>Caused by depletion of ATP, dissipation of ion gradient for a normal electrical signal, accumulation of lactate </li></ul><ul><li>Fatigue is prevented by alternating activation of motor units </li></ul>
    28. 28. Fast and slow muscle fibers <ul><li>Based on the duration of a muscle twitch </li></ul><ul><li>Fast (type II) muscle fibers- short, rapid, powerful contractions </li></ul><ul><li>Slow (type I) muscle fibers- maintain posture, long contractions, fatigue slowly </li></ul>
    29. 29. Type I (slow twitch) <ul><ul><li>Degrades ATP slowly </li></ul></ul><ul><ul><li>Less sarcoplasmic reticulum </li></ul></ul><ul><ul><li>Slower calcium pumps </li></ul></ul><ul><ul><li>High mitochondria, rich blood supply, myoglobin </li></ul></ul>
    30. 30. Type II (fast twitch) <ul><li>Divided in two types: </li></ul><ul><ul><li>Type IIa- fast-twitch oxidative </li></ul></ul><ul><ul><li>Type IIb- fast-twitch glycolytic </li></ul></ul><ul><li>Type IIa- sustained, strenous </li></ul><ul><ul><li>flight muscles of migratory birds </li></ul></ul><ul><li>Type IIb- contract very rapidly, fatigue quickly </li></ul><ul><ul><li>Few mitochondria </li></ul></ul><ul><ul><li>Anaerobic glycolysis </li></ul></ul><ul><ul><li>Breast muscle of fowls and non-migratory birds </li></ul></ul><ul><ul><li>Reptiles and amphibians use type IIb extensively </li></ul></ul>
    31. 31. Other type of muscle <ul><li>Cardiac muscle </li></ul><ul><ul><li>Intercalated discs- special gap junction </li></ul></ul><ul><ul><li>Provide direct electrical coupling </li></ul></ul><ul><ul><li>Longer contraction than skeletal (up to 20x) </li></ul></ul><ul><ul><li>Skeletal muscle- action potential triggers contraction, do not control duration </li></ul></ul><ul><ul><li>Cardiac muscle- duration of action potential controls duration of contraction </li></ul></ul>
    32. 32. <ul><li>Smooth muscle </li></ul><ul><ul><li>No striations </li></ul></ul><ul><ul><li>Filaments have a spiral aarangement </li></ul></ul><ul><ul><li>Less myosin than skeletal and not associated to actin </li></ul></ul><ul><ul><li>NO T tubule system and well-developed sarcoplasmic reticulum </li></ul></ul><ul><ul><li>Calcium ions enters via the plasma membrane through an action potential </li></ul></ul><ul><ul><li>Slow, long contraction </li></ul></ul><ul><ul><li>Found in tubular organs </li></ul></ul>
    33. 33. Invertebrate muscle <ul><li>Almost similar to vertebrate smooth and skeletal muscle </li></ul><ul><li>Arthropod skeletal muscle almost identical to vertebrate skeletal muscle </li></ul><ul><li>Flight muscle of insects- have independent rhythmic contraction </li></ul><ul><li>faster than action potential </li></ul><ul><li>Paramyosin- found in clams </li></ul><ul><ul><li>Regulatory protein that enables muscle fibers to sustain a fixed state of contraction with low energy rate </li></ul></ul>
    34. 34. Lastly <ul><li>Muscle is derived from mesoderm </li></ul><ul><li>First muscle is found in Planarians due t the presence of the mesoderm </li></ul>