Muscle contraction is triggered by the release of high levels of oxygen and sugar by the sarcolemma. It is also triggered by the release of a surplus of ATP by the nerve motor unit. Additionally, muscle contraction can be triggered by the release of a neurotransmitter at the synapse, which directly causes actin and myosin to slide. Another way muscle contraction can be triggered is by the nerve releasing a neurotransmitter, which triggers the flow of calcium that attaches to actin filaments and exposes the myosin binding sites. Finally, muscle contraction can be triggered by the nerve releasing a neurotransmitter, which triggers the flow of calcium that releases ATP and begins the actin filaments sliding across the myosin binding sites.