Contraction

Q: Actin-Myosin Cross Bridges

The influx of Ca 2+ ions to the sarcoplasm allows myosin and actin filaments to bind, creating an actin-myosin cross bridge. The roles of the cross bridges in muscle contraction are:

Test yourself on Contraction

After reading these notes, test your knowledge with free interactive questions on Seneca — used by over 10 million students.

Role of Calcium Ions

When muscle cells are stimulated, there is an influx of calcium ions. The ions play an important role in initiating muscle contraction. The steps involved are:

Depolarisation

Muscle contraction is initiated when an action potential arrives at a neuromuscular junction from a motor neurone.
The action potential causes depolarisation of the sarcolemma.
Depolarisation spreads along the T tubules and into the sarcoplasm.

Influx of calcium ions

Depolarisation of the T tubules stimulates the sarcoplasmic reticulum (SR).
The SR releases Ca²⁺ ions into the sarcoplasm.

Tropomyosin

Ca²⁺ ions bind to a protein attached to tropomyosin.
- Tropomyosin is a protein that blocks the actin-myosin binding site.
Binding of Ca²⁺ ions causes the protein to change shape.
Altering the protein causes tropomyosin to be moved. The actin-myosin binding site is no longer blocked by tropomyosin.

Actin-myosin cross bridge

The myosin head can now bind to the actin filament.
The bond between actin and myosin is called the actin-myosin cross bridge.

ATP hydrolase

Ca²⁺ ions also activate ATP hydrolase.
ATP hydrolase is an enzyme that hydrolyses ATP to ADP and inorganic phosphate. This process releases energy that can power muscle contraction.

Actin-Myosin Cross Bridges

The influx of Ca²⁺ ions to the sarcoplasm allows myosin and actin filaments to bind, creating an actin-myosin cross bridge. The roles of the cross bridges in muscle contraction are:

Bending of myosin heads

When Ca²⁺ ions activate ATP hydrolase, ATP is hydrolysed and energy is released.
The energy released from this reaction causes the myosin head to bend.
The movement of the myosin head causes the actin filament to slide past the myosin filament.
The actin filament is pulled by the myosin head because of the actin-myosin cross bridge.

Breaking the cross bridge

After the actin filament has slid past the myosin filament, the actin-myosin cross bridge is broken. This is driven by energy from ATP.
The myosin head is no longer attached to the actin filament.

Forming a new cross bridge

The myosin head bends back to its original position after it is released from the actin binding site.
The myosin forms a new cross bridge with a binding site further along the actin filament.

Contraction

The cycle of forming and breaking actin-myosin cross bridges occurs quickly and continuously.
As actin filaments are pulled past the myosin filaments, the overall result is the shortening of the sarcomere.
Shortening of the sarcomere causes muscle contraction.

Halting Contraction

Muscle contraction is stopped when the muscle cells are no longer stimulated. The steps involved are:

Removal of calcium ions

If action potentials are no longer stimulating the muscle cells, the release of Ca²⁺ ions by the sarcoplasmic reticulum (SR) will stop.
The Ca²⁺ ions are transported back into the SR by active transport.

Movement of tropomyosin

Removal of Ca²⁺ ions means that the protein attached to tropomyosin undergoes a conformational change.
The protein changes shape. This causes tropomyosin to shift so that it is blocking the actin-myosin binding sites.
Myosin heads can no longer bind to actin filaments.