In its resting state, a neuron is polarized, with its inside at about −70 mV relative to its surroundings. When an excitatory neurotransmitter is released by the presynaptic neuron and binds to the postsynaptic dendritic spines, ligand-gated ion channels open, allowing sodium ions to enter the cell. This may make the postsynaptic membrane depolarized (less negative). This depolarization will travel towards the axon hillock, diminishing exponentially with time and distance. If several such events occur in a short time, the axon hillock may become sufficiently depolarized for the voltage-gated sodium channels to open. This initiates an action potential that then propagates down the axon.