Many neurotransmitters are removed from the synaptic gap, after they have activated their specific receptors, by transport proteins residing in neuronal and glial plasma membranes. At cholinergic synapses where acetylcholine (ACh) is the neurotransmitter, the enzyme acetylcholinesterase rather than a transport protein is responsible for removing the ACh. It is important to remove neurotransmitters from the synaptic gap so that they do not continue to stimulate or inhibit the firing of the postsynaptic neuron.
Neurotransmitters may be either excitatory or inhibitory; that is, they may be of a type that fosters the initiation of a nerve impulse in the receiving neuron, or they may inhibit such an impulse (more at synapse). Most are small molecules that are amino acids or are derived from amino acids. GABA and glycine are well-known inhibitory neurotransmitters.
There are many neurotransmitters; some of the important ones are: