Mechanism of muscle contraction is best explained by the sliding filament theory which states that contraction of a muscle fibre takes place by the sliding of the thin filaments over the thick filaments. Organise sequence of contraction and choose correct option :
(i) Action potential spreads through the muscle fibre and causes the release of calcium ions into the sarcoplasm.
(ii) This pulls the attached actin filaments towards the centre of A-band.
(iii) Utilising the energy from ATP hydrolysis, the myosin head now binds to the exposed active sites on actin to form a cross bridge.
(iv) Increase in Ca++ level leads to the binding of calcium with a subunit of troponin on actin filaments and thereby remove the masking of active sites for myosin.
(v) Signal sent by the central nervous system (CNS) via a motor neuron.
(vi) The Z-line attached to these actins are also pulled inwards thereby causing a shortening of the sarcomere, i.e., contraction.
(vii) A neural signal reaching neuromuscular junction releases a neurotransmitter (Acetyl choline) which generates an action potential in the sarcolemma.