Muscular System

Cards (56)

  • Three basic muscle types
    • Skeletal muscle
    • Cardiac muscle
    • Smooth muscle
  • Muscle cells
    • They are elongated (muscle cell = muscle fiber)
  • Contraction of muscles
    Movement of myofilaments – the muscle cell equivalent of the microfilaments of cytoskeletons
  • All muscles
    • Share some terminology
    • Prefix myo refers to muscle
    • Prefix mys refers to muscle
    • Prefix sarco refers to flesh
  • Skeletal Muscle
    • Most are attached by tendons to bones
    • Cells are multinucleate & cigar-shaped
    • Striated – have visible banding
    • Voluntary – subject to conscious control
    • Cells are surrounded and bundled by connective tissue
  • Connective Tissue Wrappings of Skeletal Muscle
    • Endomysium – connective tissue around single muscle fiber
    • Perimysium – around a fascicle (bundle) of fibers
    • Epimysium – covers the entire skeletal muscle
    • Fascia – on the outside of the epimysium
  • Skeletal Muscle Attachments
    • Epimysium blends into a connective tissue attachment
    • Tendon – cord-like structure
    • Aponeuroses – sheet-like structure
    • Sites of muscle attachment: Bones, Cartilages, Connective tissue coverings
  • Smooth Muscle
    • Has no striations
    • Spindle-shaped cells
    • Single nucleus
    • Involuntary – no conscious control
    • Found mainly in the walls of hollow organsvisceral
    • Arranged in two sheets or layers
  • Cardiac Muscle

    • Has striations
    • Usually has a single nucleus
    • Joined to another muscle cell at an intercalated disc
    • Involuntary
    • Found only in the heart
  • Function of Muscles
    1. Produce movement
    2. Maintain posture
    3. Stabilize joints
    4. Generate heat
  • Microscopic Anatomy of Skeletal Muscle
    • Cells are multinucleate
    • Nuclei are just beneath the sarcolemma – plasma membrane
    • Sarcolemma – specialized plasma membrane
    • Sarcoplasmic reticulum – specialized smooth endoplasmic reticulum
    • Myofibril
    • Bundles of myofilaments
    • Myofibrils are aligned to give distinct bands
    • I band = light band
    • A band = dark band
    • Sarcomere
    • Contractile unit of a muscle fiber
    • Thick filaments = myosin filaments composed of the protein myosin with ATPase enzymes
    • Thin filaments = actin filaments composed of the protein actin and regulatory proteins
  • Components of a sarcomere
    • Thin filaments = actin filaments
    • Myosin filaments with heads
    • Bare zone that lacks actin filaments - the H zone
    • Sarcoplasmic reticulum (SR) for storage of calcium
  • Thin filaments
    • Composed of the protein actin and regulatory proteins
  • Myosin filaments
    • Have heads (extensions, or cross bridges)
    • Overlap somewhat with actin
  • Sarcoplasmic reticulum (SR)

    • For storage of calcium
  • Properties of Skeletal Muscle
    • Irritability - ability to receive and respond to a stimulus
    • Contractility - ability to shorten when an adequate stimulus is received
  • Nerve Stimulus to Muscles
    1. Skeletal muscles must be stimulated by a nerve to contract
    2. Neuromuscular junctions - association site of nerve and muscle
    3. Synaptic cleft - gap between nerve and muscle
    4. Transmission of Nerve Impulse to Muscle - Neurotransmitter attaches to receptors on the sarcolemma, Sarcolemma becomes temporarily permeable to sodium (Na+) that rushes into the cell giving it a positive charge, Sodium rushing into the cell generates an action potential, Once started, muscle contraction cannot be stopped, To return to resting state, Potassium ions (K+) diffuse out of the cell, Sodium-potassium pump pumps sodium and potassium back to their original positions
  • The Sliding Filament Theory of Muscle Contraction
    Activation by nerve causes myosin heads (cross bridges) to attach to binding sites on the thin filament, Myosin heads then bind to the next site of the thin filament when ATP is present, This continued action causes a sliding of the myosin along the actin, The result is that the muscle is shortened (contracted), Calcium ions are required for the attachment of myosin cross bridges to actin
  • Muscle fiber contraction is “all or none”
  • Types of Graded Responses
    • Twitch - Single, brief jerky contraction
    • Tetanus (summing of contractions) - One contraction is immediately followed by another
  • Types of Graded Responses: Tetanus (summing of contractions)
  • Types of Graded Responses: Unfused (incomplete) tetanus
  • Types of Graded Responses: Fused (complete) tetanus
  • Muscle force depends upon the number of fibers stimulated

    More fibers contracting results in greater muscle tension
  • Muscles can continue to contract unless they run out of energy
  • Energy for Muscle Contraction: Initially, muscles used stored ATP for energy
    Bonds of ATP are broken to release energy
  • Only 4-6 seconds worth of ATP is stored by muscles
  • Energy for Muscle Contraction: Direct phosphorylation

    1. Only muscle cells contain creatine phosphate (CP)
    2. CP is a high-energy molecule
  • Energy for Muscle Contraction: Aerobic Respiration

    1. Series of metabolic pathways that occur in the mitochondria
    2. Glucose is broken down to carbon dioxide and water, releasing energy
    3. This is a slower reaction that requires continuous oxygen
  • Energy for Muscle Contraction: Anaerobic glycolysis
    1. Reaction that breaks down glucose without oxygen
    2. Glucose is broken down to pyruvic acid to produce some ATP
    3. Pyruvic acid is converted to lactic acid
  • Lactic acid produces muscle fatigue
  • When a muscle is fatigued, it is unable to contract even when stimulated
  • The common reason for muscle fatigue is oxygen debt
  • Oxygen must be “repaid” to tissue to remove oxygen debt
  • Oxygen is required to get rid of accumulated lactic acid
  • Increasing acidity (from lactic acid) and lack of ATP causes the muscle to contract less
  • Types of Muscle Contractions: Isotonic contractions“same tone” or tension
  • Types of Muscle Contractions: Isometric contractions – “same measurement” or length
  • Muscle Tone: Some fibers are contracted even in a relaxed muscle
  • The process of stimulating various fibers is under involuntary control