Unit 2.2 Muscle Physiology

• Common properties of muscle tissue


  • Excitability (& conductivity)
  • Contractility
• Functions of muscles


  • Movement
  • Stability
  • Joints
  • Maintain posture and position
• Thermoregulatory role of muscles


  Generating heat
• Skeletal muscle organization


  • Bundles with connective tissue wraps
  • Muscle cell
  • Sarcolemma = plasma membrane
• Components of skeletal muscle


  • Fibers
  • Blood vessels
  • Nerve fibers
  • Connective tissue
• Muscles and bones


  • Muscles attach to bones
  • Muscles can only pull, not push
• Muscle action with bones

  1. Immovable or less moveable bone

  2. Moveable bone

  3. Lever system
• Agonists and antagonists


  • Agonists (prime movers) responsible for a specific movement
  • Antagonists resist that movement
• Skeletal muscle cells


  • Very large
  • Contain glycogen
  • Myoglobin has higher oxygen affinity than hemoglobin
• Skeletal muscle fibers


  • Striated
  • Contain myofilaments in myofibrils
  • Contain many mitochondria
  • Sarcoplasmic reticulum is calcium storage organelle
• Sarcomeres


  • Functional contractile units of skeletal muscle
  • Made of myofilaments (thick filament = myosin, thin filament = actin, tropomyosin, troponin)
• Thick filament


  Made of myosin, has actin binding sites and ATP binding sites
• Thin filament


  Made of actin, has myosin attachment sites, also contains tropomyosin and troponin
• Elastin filament


  Made of titin, forms core of thick filament and tethers it to Z disc and M line
• Sarcoplasmic reticulum


  Specialized smooth ER that regulates intracellular calcium, stores and releases calcium
• Triads


  SR + T-Tubule, extensive in skeletal muscle
• 1. Tubules


  Deep protrusions of sarcolemma that conduct neuron impulses deep into muscle fiber
• Importance of T-Tubules


  Allow signal from motor neuron to reach throughout huge muscle fiber
• Muscle contractility


  Sliding filament model - thin filaments slide past thick ones, increasing overlap
• Motor units

  One motor neuron innervates many muscle fibers located throughout the muscle
• Neuromuscular junction


  Synapse where somatic motor neuron excites skeletal muscle
• Na+/K+ pump and Ca2+ pumps


  Establish ion concentration differences between intra- and extracellular fluid
• Membrane potential


  Potential difference established by selective permeability to K+
• Action potential generation and propagation


  1. AP travels down somatic motor neuron

  2. AP reaches axon terminal and transfers signal to muscle

  3. Excitation of muscle cell

  4. AP spreads along sarcolemma via T-tubules
• Neuromuscular signal transduction


  • AP opens Ca2+ channels, causing ACh release
  • ACh binds to nicotinic receptors, generating end-plate potential
  • End-plate potential initiates AP on muscle fiber
• Phases of muscle action potential


  • Rapid depolarization as Na+ channels open
  • Rapid repolarization as K+ channels open
• AP propagation along sarcolemma

  Na+ influx generates local depolarization, opening neighboring Na+ channels
• AP triggering Ca2+ release from SR


  DHP receptor in membrane changes shape, opening ryanodine receptor in SR
• Muscle contraction


  Ca2+ binds to troponin, shifting tropomyosin off myosin binding sites, allowing cross-bridge cycling
• Activation of voltage-gated K+ channels


  1. Voltage-gated Na+ channels open

  2. Voltage-gated K+ channels close

  3. Resting membrane potential
• Action potential generation


  1. Initially, the end plate potential depolarizes the membrane

  2. Rapid depolarization as voltage-gated Na+ channels open, Na+ influx

  3. Rapid repolarization as voltage-gated K+ channels open, K+ efflux
• State of voltage-gated channels

  • Open
  • Closed
  • Inactivated (voltage-gated Na+ only)
• Resting membrane potential


  Why negative?
• Action potential propagation


  1. Na+ entering through voltage-gated Na+ channels diffuses a short distance

  2. Generates a small local depolarization

  3. Depolarizes neighboring membrane and opens neighboring voltage-gated Na+ channels

  4. Repeats along the whole membrane
• 1. tubules


  What are they and why are they important?
• Excitation-contraction coupling


  1. AP passes along T-tubule

  2. Triggers Ca2+ release from SR via interaction between DHP receptor and ryanodine receptor

  3. Ca2+ spills out over sarcomeres and binds to troponin

  4. Shifts tropomyosin off myosin binding sites

  5. Allows energized myosin heads to bind, initiating cross-bridge cycling
• Myofilaments


  What is happening with thin filament (actin, troponin, tropomyosin) and thick filament (myosin) upon Ca2+ release?
• Cross-bridge cycle


  1. Ca2+ binds to troponin, changes shape and removes tropomyosin from myosin binding sites

  2. Myosin binds to actin, forming cross-bridge

  3. Power stroke: ADP and Pi are released, myosin head pivots and bends, pulling actin filament toward M line

  4. ATP binds to myosin and it detaches from actin (cross-bridge detachment)

  5. ATP is hydrolyzed and myosin head returns to high energy position
• Contraction


  Formation of cross-bridges
• Each myosin head attaches and detaches many times during a single contraction