The synapse is a unique junction that allows for the transfer of information from one neuron to the next or from a neuron to an effector cell. Synapses are usually between the axon of the sending cell and a dendrite (or membrane of an effector cell) of the recieving cell. Very rarely are there axon to axon synapses. There are two varieties of synapses, electrical and chemical. Electrical synapses are actually gap junctions, or rather a form of a gap junction called a bridge junction. A gap junction is a junction between two cells, via special protein channels, that connect the cytoplasm of the cells. This allows for the direct flow of ions from one cell to another and allows for rapid signal transmission between the cells. Generally this communication is unidirectional, but two way communication is also possible. Neurons coupled in this way are said to be electrically coupled. This form of synapse has an advantage in that it allows a simple and fast method of synchronizing all neurons that are so joined. These synapses are therefore largely found in smooth and cardiac muscle and in certain regions of the brain.

Due to the space between the pre and post synaptic neurons, current disspates so another form of communication must be utilized. In this situation unidirectional communication is achieved by the release of a messenger from the presynaptic neuron, and the response of the post synaptic neuron to that particular messenger. Chemical synapses are specially designed for the release, reception and, sometimes, uptake of these messengers, known as neurotransmitters. Neurotransmitters are molecules that are used as comunication agents between neurons. Each neurotransmitter sends a specific message to the receptor cell. The chemical synapse consists of three parts. The first part is the axonal terminal of the presynaptic neuron. These terminals are the storehouse of neurotransmitters, which are kept in sacs inside the cell called synaptic vesicles. Each vesicle contains thousands of neurotransmitter molecules. The synaptic cleft is the space between the pre and post-synaptic neurons. It is filled with fluid and approximately 30 to 50 nm wide. The receptor region is on the membrane of the message reciever and contains the receptor protein ion channels (more commonly reffered to as ligand gated ion channels)that were discussed in the section conerning graded potentials. The receptor and neurotransmitter, fit together like a lock and key. Thus the receptor will only bind, and respond to, the appropriate neurotransmitter. It is for this reason that the dendritic membrane may have many different receptor proteins, depending on the neuron.

In the axonal terminal of the presynaptic cells there is an abundance of voltage gated calcium (Ca²⁺) channels. As the action potential reaches the axonal terminals and the membrane becomes depolarized, not only are sodium channels opened, but calcium channels as well. This causes a rapid influx of calcium into the axonal terminals (synaptic knobs) . This increase in calcium concentration mediates a release of the contents of the synaptic vessicles by exocytosis. Basically the calcium causes the synaptic vessicles to fuse with the membrane of the synaptic knobs and release neurotransmitter into the synaptic cleft. The excess calcium is quickly removed by being taken up by mitochondria or ejected from the cell via an active calcium pump. The released neurotransmitter diffuses across the synaptic cleft and binds, reveribly, to protein receptors which are specific for that particular neurotransmitter. There is a time delay caused by the time it takes for the neurotransmitter to be released, diffuse across the cleft, and bind to the receptor that is known as the synaptic delay. It lasts from .3 to 5 ms. It is known as the rate limiting step in neural transmission. Once the neurotransmitter is bound the receptor proteins change their three dimensional shape allowing certain ions to flow into the cell. This causes either a de or hyper polarization of the dendritic membrane. This whole process is known as a synaptic potential.