The transmission of a nerve impulse starts when the calcium gates open. At the end of the axon from which the impulse is coming, the membrane depolarizes, gated ion channels open, and calcium ions are allowed to enter the cell. When the calcium ions rush in, a neurotransmitter is released into the synapse. The chemical that serves as the neurotransmitter moves across the synapse and binds to proteins on the neuron membrane that is about to receive the impulse. The proteins serve as the receptor, and different proteins serve as receptors for difference neurotransmitters. Whether excitation or inhibition occurs depends on what chemical served as the neurotransmitter and the result it had. For example, if the neurotransmitter causes the sodium