Muscle

• Fascicles


  Columns that a whole muscle is divided into
• Muscle fibers


  Muscle cells that make up each fascicle
• Sarcolemma


  Cell membrane that surrounds muscle fibers
• Myofibrils


  Composed of protein filaments called myofilaments that make up muscle fibers
• Sarcomere


  Functional unit of muscle fiber from Z disk to Z disk
• Sarcomere


  • Highly ordered repeating units of myofilaments
  • Z disk serves as attachment for actin myofilaments
  • I bands from Z disks to ends of thick filaments
  • A bands length of thick filaments
  • H zone region in A band where actin and myosin do not overlap
  • M line middle of H zone; delicate filaments holding myosin in place
• Actin (thin) myofilaments


  • Two strands of fibrous (F) actin attached at either end at sarcomere
  • Each F actin is composed of G actin molecules
  • Tropomyosin elongated protein winds along the groove of the F actin double helix
  • Troponin binds to actin, to tropomyosin, and to Ca2+
• Myosin (thick) myofilament


  • Many elongated myosin molecules shaped like golf clubs
  • Myosin heads can bind to active sites on the actin molecules to form cross-bridges
  • Have ATPase that breaks down ATP, releasing energy
• Titin


  Protein that acts like a spring and contributes to the elastic recoil of muscles that helps them to return to their resting lengths
• Motor unit


  Each somatic motor neuron and all the muscle fibers it innervates
• Neuromuscular junction (NMJ)


  Where the nervous and muscle systems meet up
• Function of neuromuscular junction

  1. Action potential arrives at NMJ causing voltage-gated Ca2+ channels to open

  2. Ca2+ stimulates Ach release from vesicles, and into synaptic cleft by exocytosis

  3. Ach binds to ligand-gated Na+ channels on postsynaptic membrane, causing channels to open

  4. Ach unbinds from the channel, which then closes

  5. Acetylcholinesterase removes Ach from synaptic cleft by breaking it down into acetic acid and choline

  6. Choline is brought back to presynaptic terminal to be recycled to make Ach
• Excitation-contraction coupling


  Mechanism where an action potential causes muscle fiber contraction
• Action potentials and muscle contraction

  1. Action potential from NMJ travels along sarcolemma of skeletal muscle

  2. Depolarization also travels down the T tubules and opens voltage-gated Ca2+ channels

  3. Depolarization of T tubules causes Ca2+ release channels in SR to open

  4. Ca2+ diffuses from SR in sarcoplasm

  5. Ca2+ binds to troponin, allowing tropomyosin to move and expose active sites of G actin

  6. Myosin heads bind to active sites forming cross-bridges
• Sliding filament model


  Sarcomere shortening - in relaxed muscle actin and myosin myofilaments overlap slightly, in contracted muscle myosin and actin filaments do not shorten in length but Z disks are brought closer together
• Muscle relaxation


  1. Ca2+ moves back into sarcoplasmic reticulum by active transport via Ca2+-ATPase pumps or out of cell by Ca2+-Na+ cotransport

  2. Ca2+ moves away from troponin-tropomyosin complex which re-establishes its position and blocks binding sites
• Muscle twitch


  Muscle contraction in response to a stimulus that causes action potential in one or more muscle fibers
• Muscle twitch phases


  • Lag or latent
  • Contraction
  • Relaxation
• Treppe


  Increase in the strength of successive contractions in a muscle rested for a prolonged period
• Summation


  Two or more twitches which build to greater tension, where the muscle does not completely relax before the next stimulus
• Incomplete tetanus


  Muscle fibers partially relax between contractions
• Complete tetanus


  No relaxation between contractions
• All-or-none response


  Contraction of equal force in response to each action potential
• Recruitment


  Larger and larger motor units are activated when contractions of greater strength are required
• Tonus


  Maintenance of a partially contracted state
• Isometric contraction


  No change in length but tension increases
• Isotonic contraction


  Change in length but tension constant
• Concentric contraction


  Overcomes opposing resistance and muscle shortens
• Eccentric contraction


  Tension maintained but muscle lengthens
• Length-tension relationship of the sarcomere


  The length of the sarcomere determines the force it can supply, a function of the cross bridge/binding site relationship
• Energy sources for muscle contraction

  • ATP
  • Aerobic respiration
  • Creatine phosphate
  • Anaerobic respiration
• Slow-twitch fibers (Type I)

  Highly aerobic (red fibers), more in lower than upper limbs, more fatigue-resistant than fast-twitch
• Fast-twitch fibers (Type IIa and IIx)


  Respond rapidly to nervous stimulation, fewer and smaller mitochondria than slow-twitch, more in upper limbs
• Muscle spindle apparatus


  Monitors muscle length
• Extrafusal fibers


  Fibers outside of muscle spindle, innervated by alpha motor neurons
• Intrafusal fibers


  Fibers inside the muscle spindle, innervated by gamma motor neurons
• Annulospiral sensory endings


  Send information to CNS
• Monosynaptic reflex


  Sensory neuron directly synapses with motor neuron in CNS, e.g. knee-jerk reflex
• Disynaptic reflex


  Two synapses are crossed in the CNS, e.g. Golgi tendon organ inhibitory reflex regulates muscle contraction force
• Reciprocal innervation


  Dual stimulatory and inhibitory activity, e.g. muscle spindle activation causes agonist to contract and antagonist to relax