Muscular System

• Skeletal muscles are voluntary muscles attached to bones that allow for movement.
• Connective tissue layers surrounding Skeletal Muscle

  • Epimysium - Forms a connective tissue sheath that surrounds each muscle
  • Perimysium - Loose connective tissue; serves as a passageway for blood vessels and nerves that supply each fascicle
  • Endomysium (internal) - Separates the individual muscle fibers within each fascicle; serves as passageways for nerve fibers and blood vessels that supply each separate muscle fiber
• Structures involved in Muscle Contraction

  • Sarcolemma - Cell membrane of muscle fibers
  • Transverse Tubules - Carry impulses into the center of the muscle fiber
  • Sarcoplasmic Reticulum - Storage site in muscles for calcium release
  • Myofibrils - Extend the entire length of the muscle fiber; protein filaments in this structure interact to shorten the muscle fiber during contraction
  • Myofilaments: Actin (thin filaments) and Myosin (thick filaments)
  • Sarcomeres - Smallest portion of a muscle that can contract; forms myofibrils
• Muscle Contraction

  1. Electrical Component: Respond and transmit electrical signals

  2. Mechanical Component: General muscle property is due to main structures
• Components of Myofilaments


  • Actin (thin filaments): Tropomyosin - covers the attachment sites, Troponin - prevents tropomyosin from uncovering the attachment sites; binds Ca
  • Myosin (thick filaments): Composed of myosin molecules, Head - bind active site of actin, Head + Active Site = Cross Bridge
• Neuromuscular Junction Structure

  1. Point of contact of motor neuron axon branches with the muscle fiber

  2. Motor Neurons: Carry electrical signals → Generate action potential (cause contraction)
• General Properties of Muscle Tissue

  1. Contractility: Ability of the muscle to shorten forcefully or contract

  2. Excitability: Capacity of the muscle to respond to a stimulus

  3. Extensibility: Muscle can be stretched beyond its normal resting length and still be able to contract

  4. Elasticity: Ability of the muscle to recoil to its original resting length after it has been stretched
• Sliding Filament Model

  1. Parallel arrangement of myofilaments allows them to interact leading to muscle contraction

  2. Actin and Myosin slide past one another resulting in the shortening of Sarcomere
• Parts of Neuromuscular Junction

  • Pre-synaptic Terminal: Axon Terminal
  • Synaptic Cleft: Space between the pre-synaptic terminal and muscle fiber
  • Motor End-Plate/Post-Synaptic Membrane: Muscle cell membrane in the area of the junction
  • Synaptic Vesicles: Contain neurotransmitters
• Resting Membrane Potential


  The charge difference in an unstimulated cell resulting from the concentration differences of ions inside and outside the cell membrane
• Skeletal Muscle Fiber Physiology - Electrically excitable cells are polarized with more negative charge on the inside
• Function of Neuromuscular Junction - Involves Acetylcholine (Ach) as a neurotransmitter and Acetylcholinesterase (AchE) as an enzyme
• Neurotransmitters


  Communicators of neurons; promote/prevent action potential
• Neuromuscular Junction Structure

  1. Point of contact of motor neuron axon branches with the muscle fiber

  2. Motor Neurons carry electrical signals and generate action potential

  3. Action Potential must be within threshold
• Ion Channels

  • Na+ - Out
  • K+ - In
• Action Potential


  Occurs when excitable cells are stimulated leading to a reversal of the resting membrane potential
• Components of Sarcomere


  • At Rest: Same length
  • During Contraction: Shorten
  • During Relaxation: Lengthen
• Release of Acetylcholine (Ach) at Neuromuscular Junction

  Action potential in the pre-synaptic terminal leads to the opening of voltage-gated Ca+ channels, causing Ca+ diffusion into the axon terminal. Synaptic Vesicle containing Ach merges with the axon, releasing Ach through exocytosis. Ach binds to Na+ channels, leading to depolarization and action potential. AchE destroys free Ach into Acetic Acid & Choline, with Choline being reabsorbed
• Muscle Relaxation occurs when Acetylcholine (Ach) is no longer released at neuromuscular junctions, cross bridges cannot reform, and Concentration of Ca+ in SR is greater than in Sarcoplasm
• Synaptic Transmission

  Synaptic Vesicle (Neurotransmitter) releases Ach through exocytosis, Ach binds to Na+ channels causing depolarization and leading to action potential, AchE destroys free Ach into Acetic Acid & Choline, Choline is reabsorbed by axon terminal
• Muscle Twitch

  Response of a muscle fiber to a single action potential along with its motor neuron, Phases: Lag Phase, Contraction Phase, Relaxation Phase
• Muscle Contraction

  Depends on Summation & Recruitment, Concentration of Ca+ in Sarcomere and Sarcoplasm, Troponin binding Ca+, Uncovering active site of actin by Troponin + Ca+, Cross Bridge formation between Actin and Myosin Head leading to contraction
• Muscle Relaxation


  Na-K Pump actively transports Na+ out and K+ into the muscle fiber, ATP detaches myosin heads for recovery stroke, ATP is needed for active transport of Ca+ into the sarcoplasmic reticulum from the sarcoplasm