Neurotransmitters are chemicals that communicate between nerves. In some ways, neurons are like computers: they receive messages, process those messages, and send out the results as new messages to other cells. In the case of neurons, messages consist of chemicals that interact with the outer surface of the cell membrane. This chemical interaction with the cell membrane causes chemical changes within the receiving neuron. The process in which when chemical massages are constantly being exchanged between neurons is called neurotransmission. A resting neuron has a negative charge. Therefore there are more negative ions inside the axon than outside the axon. However, the fluid outside the axon has a positive charge. Due to the different charges of the outside and inside of the axon, the axon is polarized. When a neuron is "fired", several events take place to create an electrical impulse called an action potential. This process depolarizes the axon, which changes the electrical charge inside the axon from negative to positive. In response to this action potential, the vesicles group at the "edge" of the axon and release neurotransmitters into the synapse. The neuron is again polarized and at rest, waiting to "fire" another action potential. When the neurotransmitters have been released, they float across the synapse until they attach to the dendrites of the next neuron. On each dendrite, neurotransmitters find molecules that are set to receive them. These molecules are called receptors. Neurotransmitters recognize specific receptors and "hook" onto them. This process is called binding. The neuron that originally released the neurotransmitter is the "sending" neuron. The neuron that binds the neurotransmitter is the "receiving" neuron. Each receptor accepts only certain neurotransmitters, much like a lock accepts only it's specific key. After the binding process is complete, the receptors release the neurotransmitters.