MUSCULAR

• Functions of the Muscular System

  • Movement
  • Maintain posture
  • Respiration
  • Production of body heat
  • Communication
  • Heart beat
  • Contraction of organs and vessels
• Types of Muscles


  • Skeletal
  • Cardiac
  • Smooth
• General Properties of Muscle

  • Excitability: respond to stimulus
  • Contractility: ability to shorten
  • Extensibility: can stretch
  • Elasticity: recoil
• Clinical Significance of Muscle

  • Muscles are sites for the introduction of drugs
  • Muscles are efficiently used in lifting and moving clients (body mechanics)
• Skeletal Muscle Characteristics


  • Makes up 40% of body weight
  • Named because attached to bones (skeleton)
  • Many nuclei per cell (near periphery)
  • Striated
  • Longest of muscle types

  
• Connective Tissue Coverings


  • Epimysium
  • Musle Fasciculus
  • Perimysium
  • Muscle fiber
  • Endomysium
• Muscle Fiber Structure


  • Sarcolemma: Cell membrane, contains T-tubules
  • Sarcoplasm: cytoplasm of muscle fiber (cell)
  • Myofibril: thread-like proteins that make up muscle fibers
  • Myofilament: Proteins that make up myofibrils, e.g. actin and myosin
  • Sarcoplasmic reticulum: Stores and releases Ca2+
  • Sarcomere: Basis for muscle contraction theory known as sliding filament theory

  
• Actin


  Thin myofilament, resemble 2 strands of pearls
• Myosin


  Thick myofilament, resemble golf clubs
• Troponin


  Attachment site on actin for Ca2+
• Tropomyosin


  Filament on grooves of actin, Attachment site on actin for myosin
• Sarcomere Components


  • Z disk: separate one sarcomere from the next
  • H zone: contains only myosin myofilaments
  • I band: contains only actin
  • A band: where actin and myosin overlap
  • M line: where myosin are anchored
• Excitability of Muscle Fibers

  Muscle fibers are highly specialized, electrically excitable cells
• Action Potential


  Electricity, stimulus that causes rapid depolarization and repolarization, causes muscle to contract
• Depolarization


  Change in charges, inside becomes more + and outside more -, Na+ channels open, Na+ diffuses down its concentration gradient
• Repolarization


  Na+ channels close, change back to resting potential, K+ movement out of the cell increases, making the inside of the cell membrane negatively charged compared to the outside
• Sodium-Potassium Pump


  Pumps Na+ out of cell and transports K+ into cell, restores balance
• Nerve Supply and Muscle Fiber Stimulation

  • Motor neuron
  • Neuromuscular junction (synapse)
  • Presynaptic terminal
  • Synaptic cleft
  • Postsynaptic membrane
  • Synaptic vesicle
  • Neurotransmitter

  
• Steps in a Muscle Contraction (Sliding Filament Theory)

  1. Action potential travels down motor neuron to presynaptic terminal causing Ca2+ channels to open

  2. Ca2+ causes synaptic vesicles to release acetylcholine into synaptic cleft

  3. Acetylcholine binds to receptor sites on Na+ channels, Na+ channels open, and Na+ rushes into postsynaptic terminal (depolarization)

  4. Na+ causes sarcolemma and t-tubules to increase the permeability of sarcoplasmic reticulum which releases stored calcium

  5. Ca2+ binds to troponin which is attached to actin

  6. Ca2+ binding to troponin causes tropomyosin to move exposing attachment sites for myosin

  7. Myosin heads bind to actin

  8. ATP is released from myosin heads and heads bend toward center of sarcomere

  9. Bending forces actin to slide over myosin

  10. Acetylcholinesterase (enzyme breaks down acetylcholine) is released, Na+ channels close, and muscle contraction stops
• ATP and Muscle contractions


  Exposure of attachment sites, Cross-bridge formation, Power stroke, ATP binds to myosin heads, Cross-bridge release, Recovery stroke
• ATP is made in mitochondria from aerobic or anaerobic respiration
• During a muscle contraction, H zone and I band shorten but A band stays the same
• Striations of skeletal and cardiac muscle are due to sarcomeres (actin and myosin)
• Rigor mortis: person dies and no ATP is available to release cross-bridges
• Threshold


  Weakest stimulus needed to produce a response
• All or None Law

  Muscle contracts or doesn't (no in between)
• Twitch


  Rapid contraction and relaxation of a muscle
• Tetanus


  Muscle remains contracted
• Muscle contraction cycle


  1. Myosin heads return to resting position (green arrows)

  2. Energy stored in myosin heads

  3. If Ca2+ still attached to troponin, cross-bridge formation and movement repeated

  4. Cycle occurs many times during muscle contraction

  5. Not all cross-bridges form and release simultaneously
• Types of muscle contractions

  • Isotonic
  • Isometric
• Isotonic


  Amount of repetitions increases
• Concentric


  Movement is against gravity
• Eccentric


  Movement is with gravity
• Isometric


  Amount of tension increases (weight)
• Slow twitch fibers


  • Contract slowly
  • Fatigue slowly
  • Used by long distance runners
  • Use aerobic respiration
  • Energy from fat
  • Dark meat
  • Red or dark because of myoglobin
  • Myoglobin helps O2 bind in muscle
• Fast twitch fibers


  • Contract quickly
  • Fatigue quickly
  • Used by sprinters
  • Use anaerobic respiration
  • Energy from glycogen
  • White meat
• Humans have both types of twitch fibers
• Distribution of twitch fibers is genetically determined
• Neither type of twitch fiber can be converted but capacity can be increased through intense exercise
• Origin


  Nonmovable end of a skeletal muscle