sliding filament theory of muscle contraction: an impulse travels down a motor neuron axon, the motor neuron releases the neurotransmitter ACh, the ACh binds to its receptors in the muscle fiber membrane, the sarcolemma is stimulated and the impulse reaches the sarcoplasmic reticulum, calcium ions diffuse from the sarcoplasmic reticulum, tropomyosin molecules expose sites on actins for myosin heads, a cross bridge forms, the thin filaments are pulled towars sarcomere by the cross bridge, and then the muscle fibers pulls on its attachments as a contraction occurs.