Place the following events of excitation in the correct order. 1) ion channels on the sarcolemma open and sodium ions enter the muscle fiber 2) the entry of sodium ions into the muscle fiber depolarizes the sarcolemma locally 3) acetylcholine is released from vesicles in the motor neuron into the synaptic cleft 4) an action potential arrives at the axon terminal of a motor neuron 5) acetylcholine binds to ligand-gated sodium ion channels in the motor end plate

4, 3 5, 1, 2.

Explanation:

The events of muscle contraction begins from the presynaptic neuron, which cause the influx of Ca+, and the release of acetycholine neurotransmitter  into the synaptic cleft.

The Ach, binds with the receptors in the sarcolemma, leading to opening of Ligand gated Na+ channels. Therefore Na+diffuses in, causing depolarization, and propagation of action potential along the sarcolemma  spreading to the  T-tubules.

This is followed by opening of Calcium channels leading  exist of Ca+ from the sarcoplasmic reticulum.

The Calcium binds with the troponin, and  Tropomysoin exposed the myosin binding sites  on the actin filament.Thus cross-bridge is formed by the myosin head  with thin filaments which   shortened the sarcomere, leading to contraction of muscle fibres.

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