Information can only be transmitted from a presynaptic to a postsynaptic nerve ending, ensuring that the signal only goes one way. This is because neurotransmitters are only created in the presynaptic neuron, and can only be received by receptors in the postsynaptic neuron.
For any electrical current to be transmitted in the body, it must cross the synaptic gap or cleft between two neurons. Neurotransmitters are built up in the presynaptic neuron, and when they are stimulated by an electrical impulse, vesicles containing these neurotransmitters are released across the cleft. When these reach the other side of synapse they bind with the receptor site on the postsynaptic nerve. This alters the postsynaptic cell's level of excitement and can make it either more or less likely to reach an action potential. To reach this potential, a certain threshold of excitement must be present, and if this is obtained then the signal will carry on.
The human body uses a range of different neurotransmitters and neuroreceptors, which have different functions. Synapses are an important part of the electrical system in the body for several reasons. They create a filtering system for background stimuli that would otherwise overwhelm the system, they allow dispersion of the impulse down different paths, they allow grading of the signal, and they also ensure the signal travels in one direction only.