Muscles and Motor Locomotion

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