Week 4

• Cardiac Muscle


  • Endomysium covers muscle fibers
  • Gap junctions let ions through from cell to cell
  • Desmosomes are adhesions to keep cells together
  • Intercalated disk coordinates impulses
  • Sarcolemma is the membrane of a muscle cell
• Skeletal Muscle


  • Skeletal movement
  • Posture maintenance
  • Heat generation
• Structure of a Muscle

  1. Fascicle: bundle of muscle fibers (cells)

  2. Connective tissue holds fascicles together

  3. Three layers: Endomysium, Perimysium, Epimysium (deep fascia)

  4. Tendon: band of dense CT that attaches muscle to bone

  
• Individual muscle cells, or muscle fibers, are bundled into fascicles, which in turn are bundled into muscles
• How muscle contraction works

  1. Nerve impulses (from the brain & spinal cord) stimulate skeletal muscle fibers contraction

  2. Nerve impulses are motor impulses carried by motor neurons

  3. The axon (neuron fiber) carries the impulses to the muscles

  4. Each nerve cell fiber may stimulate 100s of individual muscle cells

  5. Motor unit = the neuron & all stimulated muscle fibers

  6. The Neuromuscular Junction (NMJ) is the point at which a nerve fiber contacts a muscle cell

  7. A neurotransmitter (Acetylcholine) is released from the neuron to stimulate the muscle fiber

  8. Acetylcholine crosses the synaptic cleft and attaches to a receptor on the motor end plate (receiving muscle membrane)

  9. Action potential is initiated by the Acetylcholine/receptor
• Motor unit


  • A single neuron and all the muscle fibers it stimulates
  • Stronger or weaker contractions use more or fewer motor units respectively
  • Fine movements use small motor units (fewer motor units), more controlled and precise
  • Broad movements use larger motor units (more motor units), maintaining posture, walking, golfing
• Synapse


  Point of communication between a neuron and another cell
• Synaptic cleft


  Tiny space between cells, across which the neurotransmitter must travel
• Neurotransmitter


  Chemical released from a neuron to stimulate the muscle fiber
• Receptors


  Proteins embedded in the muscle cell membrane, to which neurotransmitters attach once they cross the cleft
• Motor end plate


  Receiving membrane of the muscle cell
• Properties of Muscle Tissue

  • Excitability: ability to transmit an electric current along the cell membrane
  • Contractility: capacity of a muscle fiber to undergo shortening, becoming thicker
• Muscle Contraction


  1. The sarcomere is the functional unit of contraction in the skeletal muscle fiber

  2. Sarcomeres contract via the sliding filament mechanism

  3. Myosin heads bind to actin, forming cross-bridges

  4. Using stored energy (ATP), myosin heads pull actin filaments together within the sarcomeres, and the cell shortens

  5. New ATP is used to detach myosin heads and move them back into position for another "power stroke"
• The actin and myosin filaments do not change in length as contraction proceeds
• Calcium's Role in Muscle Contraction

  1. Calcium is released from the sarcoplasmic reticulum into the cytoplasm

  2. Calcium binds to troponin causing both troponin and tropomyosin to shift off of actin and expose the binding sites

  3. This allows cross-bridges to form between actin and myosin so that the sliding action can begin

  4. Muscle relaxes when stimulation ends and calcium is pumped back into the sarcoplasmic reticulum
• Summary of Events in a Muscle Contraction

  1. ACh is released from neuron ending into synaptic cleft at the NMJ

  2. ACh binds to motor end plate and produces an action potential

  3. The action potential travels to the sarcoplasmic reticulum

  4. Sarcoplasmic reticulum releases calcium into cytoplasm

  5. Calcium shifts troponin and tropomyosin off of thin filaments so that binding sites on actin are exposed

  6. Myosin heads bind to actin, forming cross-bridges

  7. Using stored energy, ATP, myosin heads pull actin filaments together within sarcomeres, and cell shortens

  8. New ATP is used to detach myosin heads and move them back to position for another "power stroke"

  9. Muscle relaxes when stimulation ends and calcium is pumped back into sarcoplasmic reticulum
• Energy Sources for Muscle Contraction

  • Myoglobin (stores oxygen)
  • Glycogen (storage form of glucose)
  • Creatine phosphate (interim energy source)
  • Fatty acids (stored as triglycerides)
• Aerobic metabolism


  Efficient but has limitations: slow to start generating ATP, requires a lot of oxygen
• Anaerobic ATP metabolism


  Breakdown of creatine phosphate generates ATP rapidly, anaerobic glycolysis produces lactic acid
• Excess postexercise oxygen consumption (EPOC)


  Represents the energy required to reestablish the resting state, from rapid breathing
• Effects of Exercise


  • Improved balance, joint flexibility
  • Increase in muscle size (hypertrophy)
  • Improvements in muscle tissue
  • Vasodilation
  • Strengthens heart muscle
  • Improves breathing and respiratory efficiency
  • Weight control
  • Stronger bones
• Three types of Muscle Contractions

  • Muscle tone or tonus: partially contracted state for posture maintenance
  • Isotonic contraction: no change in tension, muscle length changes, movement
  • Isometric contraction: great increase in tension, muscle length unchanged, no movement
• Mechanics of Muscle Movement

  1. Tendons attach muscles to bones

  2. Origin: attached to more fixed part of skeleton

  3. Insertion: attached to the more moveable part of skeleton

  4. Aponeurosis: broad sheet CT that attaches muscles to bones or to other muscles
• How Muscles Work Together

  • Prime movers (contract): muscle that performs the movement
  • Antagonists (relax): muscle that produces the opposite movement to that of the prime mover
  • Synergists: muscles that assist the prime movers to accomplish a movement, stabilize the body during movement
  • Agonists: synergists and prime movers
• Levers and Body Mechanics

  Musculoskeletal system as a lever system: bone is the lever, joint is the fulcrum, muscle is the effort (force)
• Types of Muscle Contractions

  • Muscle Contractions
• Tendons


  • Attach muscles to bones
• Origin


  Attached to more fixed part of skeleton
• Insertion


  Attached to the more moveable part of skeleton
• Aponeurosis


  A broad sheet CT that attaches muscles to bones or to other muscles
• How Muscles Work Together

  1. Prime movers (contract)

  2. Antagonists (relax)

  3. Synergists
• Prime movers


  Muscle that performs the movement
• Antagonists


  Muscle that produces the opposite movement to that of the prime mover
• Synergists


  • Muscles that assist the prime movers to accomplish a movement
  • Muscles that stabilize the body during movement
• Synergists and prime movers are collectively known as Agonists
• As muscles work together, body movement is coordinated
• Levers and Body Mechanics


  • Musculoskeletal system as a lever system
  • Lever => bone
  • Fulcrum (fixed pivot point) => joint
  • Effort (force) => applied by muscle
  • Resistance (load/weight) => body part
• Three classes of levers

  • First class: fulcrum is between the resistance & effort
  • Second class: load (R) is between the fulcrum & effort
  • Third class —> most body systems; effort is between load & fulcrum
• Remembering the names of muscles can be made easier if you remember the following characteristics
• Characteristics for Naming Muscles

  • Location
  • Size
  • Shape
  • Direction
  • Number of heads
  • Action