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There are two main classes of neurotransmitter: excitatory and inhibitory. Excitatory neurotransmitters cause neurons to fire ‘action potentials’ – essentially an electrical signal – whilst inhibitory neurotransmitters prevent action potentials being fired. Action potentials play an important role in cell to cell communication; for example, in muscle cells, the firing of an action potential will eventually lead to contraction.

Different neurotransmitters will affect brain activity in different ways. There’s plenty more information on this page from the National Institute of Mental Health’s site.