Sliding Filament Model of Muscle Contraction Larry M. Frolich, Ph.D. April 15, 2010 HOOK Muscle is only biological cell/tissue that can cause rapid, large-scale movement Role of filamentous proteins understood as great and early breakthrough in cell/molecular biology— lots of protein available, (like Hemoglobin) I normally cover neurons and muscle together as part of unit on movement—see website

Sliding Filament Model of Muscle Contraction OUTLINE  Motor Unit  Muscle Cell Architecture and Function  Sliding Filamentous Proteins  Muscle Force Properties

Muscle Cells and Neurons are unique to animals have “excitable” membranes that transmit action potentials allow for rapid large-scale movements Motor Unit is one motor neuron plus the muscle cells that it stimulates (or synapses with)--the minimal construct that allows for movement in our body

Muscle fibers are cells—visible to naked eye as fibers in meat, chicken, fish Sarcolemma is muscle cell membrane—”excitable” so has action potentials just like neurons Because cell is large, T-tubules carry action potential— ionic depolarization—into internal parts of cell Sarcoplasmic reticulum releases calcium which triggers actin-myosin protein filaments to contract Muscle cells Sequence of events Motor Neuron to Muscle contraction at cellular level (from the Brain Top to Bottom) [link]link

Muscle cell or muscle “fiber” is composed of myofibrils which contain sarcomeres or contractile “units” Myo- Sarco- (= muscle)

Molecular Basis of Muscle Function Actin-Myosin “sliding filament” model Explains – Muscle movement or shortening – Muscle force generation or “contraction” Actin and myosin filamentous proteins are packed parallel in sarcomeres

How does the actin-myosin complex (sarcomere) shorten and contract the muscle? Actin = thin filament “lattice-work” Myosin = thick filament “core” Ca release triggers the formation of molecular cross-bridges from myosin to actin Cross-bridges “row” or “reach” for more adjacent binding site on actin.

Put the sliding filaments back into a whole muscle… And the result is muscle movement

Details, details, details… Tropomyosin and troponin create binding site on actin filament Presence of Ca++ exposes binding site “Cocked” cross-bridge on myosin (uses ATP) then attaches to binding site and pulls or “rows” actin filament Cross-bridge linkage is broken and re-cocks to link with next binding site Details Video

Sliding Filament Model explains Why muscle has peak force at certain length: (ideal actin-myosin overlap for cross-bridge formation)— BUCKET DEMO More muscle cells means more muscle force: (more cross-bridge formation)— EMG, Isolated muscle online lab Concentric/isometric/eccent ric contraction: Cross- bridges continue to form and “reach” even if opposing force is greater