Excitatory neurotransmitters (e.g. noradrenaline) make the post-synaptic cell more likely to fire: if an excitatory neurotransmitter like noradrenaline binds to the post-synaptic receptors it will cause an electrical charge in the cell membrane which results in an excitatory post-synaptic potential (EPSP), which makes the post-synaptic cell more likely to fire.