Put the steps of impulse transmission at a synapse in the correct order.1.Action potential promotes the entry of calcium ions into the synaptic knob.2.Neurotransmitter diffuses across the synaptic cleft.3.The presynaptic membrane releases neurotransmitter by exocytosis.4.Nerve impulse travels along the axon to the synaptic terminal.5.Neurotransmitter binds to receptors on the postsynaptic membrane.6.Secretory vesicles containing neurotransmitter fuse with the presynaptic membrane.