When calcium ions enter the synaptic terminal, they cause vesicles containing neurotransmitter molecules to fuse to the plasma membrane of the sending neuron. When neurotransmitter molecules bind to receptors in the plasma membrane of the receiving neuron, ion channels in the plasma membrane of the receiving neuron open. If a signal from a sending neuron makes the receiving neuron more negative inside, the receiving neuron is less likely to generate an action potential.