Muscle Movement

• Muscles do work by contracting

  • Skeletal muscles come in antagonistic pairs

    • Flexor vs. extensor

  • Contracting = shortening

    • Move skeletal parts

  • Tendons

    • Connect bone to muscle

  • Ligaments

    • Connect bone to bone

Structure of Striated Skeletal Muscle

• Muscle fiber

  • Muscle cell

    • Divided into sections = sarcomeres

• Sarcomere

  • Functional unit of muscle contraction

  • Alternating bands of thin (actin) and thick (myosin) protein filaments

Actin

• Complex of fibers

  • Brain of actin molecules and tropomyosin fibers

    • Tropomyosin fibers secured with troponin molecules

Myosin

• Single protein

  • Myosin molecule

    • Long protein with globular head

Thick and thin filaments

• Myosin tails aligned together and heads pointed away from center of sarcomere

Interaction of Thick and Thin Filaments

• Cross bridges

  • Connections formed between myosin heads and actin

  • Cause muscle to shorten

Muscle Cell Organelles

• Sarcoplasm

  • Muscle cell cytoplasm

  • Contains many mitochondria

• Sarcoplasmic reticulum

  • Organelle similar to ER

    • Network of tubes

• Stores Ca2+

  • Ca2+ released from sarcoplasmic reticulum through channels

  • Ca2+ restored to sarcoplasmic reticulum by Ca2+ pumps

    • Pump Ca2+ from cytosol

    • Pumps use ATP

Muscle at Rest

• Interacting proteins

  • At rest, troponin molecules hold tropomyosin fibers, so that they cover the myosin-binding sites on actin

    • Troponin has Ca2+ binding sites

Motor Neurons

• Motor neuron triggers muscle contraction

  • Release acetylcholine (Ach) neurotransmitter

Nerve Trigger of Muscle Action

• Nerve signal travels down T-tubule

  • Stimulates sarcoplasmic reticulum of muscle cell to releases stored Ca2+

  • Flooding muscle fibers with Ca2+

Ca2+ Triggers Muscle Action

• At rest, tropomyosin blocks myosin-binding sites on actin

  • Secured by troponin

• Ca2+ binds to troponin

  • Shape change accuses movement of troponin

  • Releasing tropomyosin

  • Exposes myosin-binding sites on actin

How Ca2+ Controls Muscle

• Sliding filament mode

  • Exposed actin binds to myosin

  • Fibers slide past each other

    • Ratchet system

  • Shorten muscle cell

    • Muscle contraction

  • Muscle doesn’t relax until Ca2+ is pumped back into sarcoplasmic reticulum

    • Requires ATP

How It All Works

• Action potential causes Ca2+ release from sarcoplasmic reticulum

  • Ca2+ binds to troponin

• Troponin moves tropomyosin, uncovering myosin binding site on actin

• Myosin bonds actin

  • Uses ATP to ratchet each time

  • Releases and bonds to next actin

• Myosin pulls actin chain along

• Sarcomere shortens

  • Z discs move closer together

• Whole fiber shortens

  • Contraction

• Ca2+ pumps restore Ca2+ to sarcoplasmic reticulum

Fast Twitch and Slow Twitch Muscles

• Slow twitch muscle fibers

  • Contract slowly, but keep going for a long time

    • More mitochondria for aerobic respiration

    • Less sarcoplasmic reticulum

      • Ca2+ remain in cytosol longer

  • Long distance runner

  • “Dark” meat = more blood vessels

• Fast twitch muscle fibers

  • Contract quickly, but get tired rapidly

    • Store more glycogen for anaerobic respiration

  • Sprinter

  • “White” meat

Muscle Limits

• Muscle fatigue

  • Lack of sugar

    • Lack of ATP to restore Ca2+ gradient

  • Low O2

    • Lactic acid drops pH which interferes with protein function

  • Synaptic fatigue

    • Loss of acetylcholine

• Muscle cramps

  • Build up of lactic acid

  • ATP depletion

  • Ion imbalance

    • Massaging or stretching increases circulation

Diseases of Muscle Tissue

• ALS

  • Amyotrophic lateral sclerosis

  • Lou Gehrig’s disease

  • Motor neurons degenerate

• Myasthenia gravis

  • Autoimmune

  • Antibodies to acetylcholine receptors

Botox

• Bacteria Clostridium botulinum toxin

  • Blocks release of acetylcholine

  • Botulism can be fatal

Rigor Mortis

• No life, no breathing

• No breathing, no O2

• No O2, no aerobic respiration

• No aerobic respiration, no ATP

• No ATP, no Ca2+ pumps

• Ca2+ stays in muscle cytoplasm

• Muscle fibers continually contract

• Eventually, tissues breakdown and relax

  • Measure of time of death