What is the role of neurotransmitters at synpatic junctions?

Firstly, it is important to understand what a synaptic junction is and its purpose. A synaptic junction is a type of gap junction between the pre-synaptic and post-synaptic membranes of two neurones. Neurotransmitters (for example dopamine and GABA) are needed to communicate the electrical current (the action potential) from one neurone to another. This communication occurs as the pre-synaptic terminal releases the neurotransmitter by exoctyosis, which then diffuses across the synaptic cleft and interacts with a receptor on the post-synaptic membrane. This will induce opening of ion gated channels, allowing an influx of ions and a change in the membrane potential of the post-synaptic neurone. The neurotransmitter is either broken down by enzymatic action or taken back up by the pre-synaptic neurone - this ensures a signal is not continuously sent to the post-synaptic neurone. 

In summary, neurotransmitters are important as signalling molecules to allow the transmission of an action potential between 2 neurones at the synaptic cleft.