Synapses are the connections between neurons by which they communicate with each other. Chemical synapses use neurotransmitters as a chemical signal for communicating information, while electrical synapses communicate information by directly transferring an action potential to another cell.
The activation of a synapse generally affects the postsynaptic cell in one of two ways, it either excites the neuron, increasing the chances that it fires, or it inhibits the neuron, decreasing the chances that it fires. Chemical synapses can be either excitatory or inhibitory, but electrical synapses are always excitatory.
The transmission of a signal through a chemical synapse is a four step process.
First an action potential reaches the synapse in the presynaptic cell. This causes Ca2+ channels to open allowing Ca2+ to flow into the cell.
The increase in Ca2+ allows neurotransmitter vesicles to fuse with the cell membrane, releasing neurotransmitters into the synaptic cleft.
The neurotransmitters then bind with the receptor channels on the postsynaptic terminal, which either depolarizes or hyperpolarizes the cell membrane.
Then the left over neurotransmitters are eliminated from the synaptic cleft.
Neurotransmitters are eliminated from the synaptic cleft in one of three ways. They can be reabsorbed by the presynaptic cell, in which case they will be reused later, or by a glial cell. They may also be diffused into intercellular fluid or broken down by enzymes in the synaptic cleft.
Electrical synapses also transfer signals from one cell to another, but instead of using chemical signals, there is a physical connection between the cytoplasm of the two cells, allowing the action potential to be instantaneously transferred from the presynaptic cell to the postsynaptic cell.
The physical connection is called a gap junction, and because of the size of the gap junction, ionic current is able to flow freely from one cell to the other. While chemical synapses are unidirectional, electrical synapses are bidirectional, so either cell could transfer a signal to the other cell.
Because electrical synapses are so fast they are often found in situations where it is necessary to synchronize electrical activity for many cells.