Anaphy

• Skeletal muscle


  • Attached to bones
  • Nuclei multiple and peripherally located
  • Striated, Voluntary and involuntary (reflexes)

  
• Smooth muscle


  • Walls of hollow organs, blood vessels, eye, glands, skin
  • Single nucleus centrally located
  • Not striated, involuntary, gap junctions in visceral smooth

  
• Cardiac muscle


  • In the heart only
  • Single nucleus centrally located
  • Striations, involuntary, intercalated disks

  
• Muscular System Functions


  • Body movement
  • Maintenance of posture
  • Respiration
  • Production of body heat
  • Communication
  • Constriction of organs and vessels
  • Heart beat
• Properties of Muscle


  • Contractility - Ability of a muscle to shorten with force, it DOES NOT produce force by lengthening/pushing
  • Excitability - Capacity of muscle to respond to a stimulus
  • Extensibility - Muscle can be stretched to its normal resting length and beyond to a limited degree
  • Elasticity - Ability of muscle to recoil to original resting length after stretched
• Cardiac Muscle


  • Branching cells
  • One/two nuclei per cell
  • Striated
  • Involuntary
  • Medium speed contractions

  
• Smooth Muscle


  • Fusiform cells
  • One nucleus per cell
  • Nonstriated
  • Involuntary
  • Slow, wave-like contractions

  
• Skeletal Muscle


  • Long cylindrical cells
  • Many nuclei per cell
  • Striated
  • Voluntary
  • Rapid contractions

  
• A little less than half of the body's mass is composed of skeletal muscle, with most muscles linked to bones by tendons through which the forces and movements developed during contractions are transmitted to the skeleton.

  
• Muscle Proteins


  • Contractile Proteins (actin and myosin)
  • Regulatory Proteins (i.e. tropomyosin and troponin)
  • Structural Proteins (i.e. Titin)

  
• Myosin


  A hexamer: 2 myosin heavy chains, 4 myosin light chains

  
• Myosin head


  • Retains all of the motor functions of myosin, i.e. the ability to produce movement and force
• Working stroke produced by opening and closing of the nucleotide binding site, resulting in rotation of the regulatory domain (neck) about a fulcrum (converter domain). Sub-nanometer rearrangements at active site are geared up to give 5-10 nm displacement at the end of the lever arm.
• Skeletal Muscle Functions


  • Produce movement
  • Maintain posture & body position
  • Support Soft Tissues
  • Guard entrance / exits
  • Maintain body temperature
  • Store nutrient reserves
  • Makes up aprox. 40% of body weight

  
• Sarcoplasmic Reticulum (SR)


  Fluid filled sacks that encircle each myofibril, similar to the smooth endoplasmic reticulum in other cells
• Terminal cisterns (cistern-reservoir)


  Dilated ends of SR, butt against T tubule from both sides
• Triad


  T tubule and 2 terminal cisterns on either side of it
• Satellite cells


  Myoblasts that remain, they maintain (repair, etc.) skeletal muscle fibers

  
• Mature muscle fibers range from between 10 to 100 micrometers in diameter. The "typical" muscle fiber is around 4 inches (10cm). There are muscle fibers that are up to a foot (30cm) long.

  
• Sarcolemma


  The plasma membrane of a muscle cell
• Transverse tubules (T tubules)


  Invaginations of the sarcolemma that tunnel from the surface of the fiber to the center of the muscle fiber, open to the outside and filled with interstitial fluid
• Sarcoplasm


  Cytoplasm of the muscle fiber

  
• Myofibrils


  Contractile element of the skeletal muscle fibers, have prominent striations (stripes)

  
• Sarcomere


  The basic functional unit of the myofibril
• Z discs


  Narrow plate-shaped regions of dense material that separate sarcomeres from each other
• Thick and thin filaments overlap one another to varying degrees, dependent on whether the muscle is contracted, relaxed or stretched. The pattern of this overlap creates the striations that are characteristic of skeletal muscle.

  
• Sliding Filament Model


  Actin myofilaments sliding over myosin to shorten sarcomeres, actin and myosin DO NOT change length, shortening sarcomeres responsible for skeletal muscle contraction

  
• Elastic components


  Include titin molecules, connective tissue around muscle fibers (endomysium, perimysium and epimysium, as well as tendons

  
• Isometric contractions


  Where the myosin heads rotate and generate tension but thin filaments are unable to slide toward M line due to excessive opposing tension

  
• Isotonic contractions


  Result in the shortening of the muscle
• Motor unit


  Composed of one motor neuron and all the muscle fibers that it innervates
• The fewer the number of fibers per neuron, the finer the movement (more brain power)
• Slow motor units


  Contain slow fibers with myosin that has long cycle time and uses ATP at a slow rate, many mitochondria for efficient performance of repetitive slow isotonic contractions
• Fast motor units


  Contain fast fibers with myosin that has rapid cycling rates, for higher power or when isometric force produced by slow motor units is insufficient
• Muscle is plastic! Muscle "adapts" to meet the habitual level of demand placed on it, i.e. level of physical activity.
• Increase muscle use through endurance training or strength training, decrease muscle use through prolonged bed rest, limb casting, denervation, or space flight.
• Type 2A fibers


  • Fast and adapted for producing sustained power
• Type 2X fibers


  • Faster, but non-oxidative and fatigue rapidly
• 2X/2D not 2B
• Modified from Burke and Tsairis, Ann NY Acad Sci 228:145-159, 1974