Neurons contact and communicate with their target cells at structures called synapses. Most synapses are called chemical, because neurotransmitter molecules communicate information across the synapse. A few synapses are called electrical, where the neuron and its target cell are physically connected to allow membrane potential changes to flow directly from the neuron to the target cell. Chemical synapses are more complex. Chemical synapses have a small gap, called the synaptic cleft, that separates the membrane of the neuron, called presynaptic, and the membrane of the target cell, called postsynaptic. Inside the presynaptic membrane of the neuron are bubbles, called synaptic vesicles, which contain neurotransmitter. The postsynaptic membrane of the target cell has receptors for the neurotransmitter.
When an action potential reaches an axon terminal, neurotransmitter is usually released from the presynaptic membrane into the synaptic cleft to cross and bind to receptors on the postsynaptic membrane of the target cell. There are many types of neurotransmitter molecules, and many types of neurotransmitter receptors. The response of the target cell depends on the combination of the type of neurotransmitter that is released and the type of receptor that it binds to. The response of the target cell is usually an excitatory or inhibitory graded potential of some size and duration, or it may involve other changes in cell behavior. The neurotransmitter then diffuses out of the synapse, or it is actively removed by one of several methods, to reset the synapse so that it is ready to communicate more information.
The efficiency of information travel between neurons may increase or decrease based on experience, which is called neuroplasticity. Synapses that are used frequently get stronger, so that a bigger response occurs in the target cell with each action potential that reaches the synapse. The opposite is true of synapses that are used infrequently. More synapses may develop between frequently used axons and dendrites by sprouting of more axon terminals or dendritic branches, and the opposite is true of those used infrequently. This is often referred to as “neurons that fire together, wire together”.