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]

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 th

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ack

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hole

mus

cle…

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/eccentric contraction: Cross-bridges continue to form and “reach” even if opposing force is greater