There are two types of synapses: electrical and chemical. At electrical synapses, the plasma membranes of the pre- and postsynaptic cells are joined by gap junctions. These allow the local currents resulting from arriving action potentials to flow directly across the junction through the connecting channels in either direction from one neuron to the neuron on the other side of the junction. This depolarizes the membrane of the second neuron to threshold, initiating an action potential. Communication between cells via electrical synapses is extremely rapid. Although numerous in cardiac and smooth muscles, electrical synapses are relatively rare in the mammalian nervous system.
In the chemical synapse, the axon of the presynaptic neuron ends in a slight swelling, the axon terminal, which holds the synaptic vesicles that contain the neurotransmitter. The postsynaptic membrane opposing the axon terminal has a high density of intrinsic and extrinsic membrane proteins that make up a specialized area called the postsynaptic density. Note that in actuality the size and shape of the pre- and postsynaptic elements can vary greatly. A 10- to 20-nm extracellular space, the synaptic cleft, separates the pre- and postsynaptic neurons and prevents direct propagation of the current from the presynaptic neuron to the postsynaptic cell. Instead, signals are transmitted across the synaptic cleft by means of a chemical messenger (a neurotransmitter) released from the presynaptic axon terminal. Sometimes more than one neurotransmitter may be simultaneously released from an axon, in which case the additional neurotransmitter is called a cotransmitter. These neurotransmitters have different receptors on the postsynaptic cell.
In general the neurotransmitter is stored on the presynaptic side of the synaptic cleft, whereas receptors for the neurotransmitters are on the postsynaptic side. Therefore, most chemical synapses operate in only one direction. One-way conduction across synapses causes action potentials to be transmitted along a given multineuronal pathway in one direction.