An action potential is an electrical impulse that travels down the axon of a stimulated neuron. Action potentials are unidirectional (travel in only one direction down the axon) because of the anatomical and functional structure of neurons. A neuron is stimulated via connections with other neurons when a neurotransmitter from the pre-synaptic neuron binds to the receptor cells on the dendrites of the post-synaptic neuron. The synapse is the extracellular junction between two neurons. When the neurotransmitter binds to the post-synaptic neuron ion channels open allowing for the flow of charged particles into and out of the cell body. This process causes the electrical impulse which then travels down the axon to the terminal buttons on the other end of the neuron. The impulse causes the release of neurotransmitters into the synapse between neurons, and the process begins again in the next neuron. Electrical impulses are not created in the terminal buttons so there is no electrical impulse to travel in the opposite direction.