sliding filament theory

Resting phase

The muscle is not being stimulated by the motor neuron so the muscle is at rest

Excitation phase

Impulse reaches the neuromuscular junction.

This causes acetylcholine to be released across the synaptic cleft.

This creates nervous stimulation of the muscle tissue and triggers an action potential.

Contraction phase (1)

The action potential causes calcium ions to be released which bind to the troponin causing the tropomyosin to be moved away from (exposing) the binding sites on the actin.

Contraction phase (2)

If ATP is present a cross bridge forms (myosin heads attach to actin binding sites).

The myosin heads pull on the actin filament (power stroke) with a ratchet like mechanism.

This pulls the Z lines on the sarcomere towards the M line and the sarcomere shortens.

Recharge phase

Muscular contraction is a process that can be repeated depending on factors such as

• calcium being available to bind to troponin

• ATP being available to provide the energy

If these factors are both present, the myosin heads will move further down the actin filament to a new binding site and pull again.

Relaxation phase

When the motor neuron stops firing the impulse calcium moves back to the sarcoplasmic reticulum.

This means the binding sites on the actin are no longer available.

The muscle relaxes and returns to its resting length.