Noradrenaline (NA) is a catecholamine neurotransmitter at postganglionic sympathetic fibres, produced following a series of steps from the essential amino acid phenylalanine. Terminal postganglionic sympathetic fibres have varicosities (where NA is synthesised and stored) which form synapses with the effector organ. The adrenal medulla is a major source of catecholamines (such as NA and adrenaline), however, most of the NA produced in the adrenal medulla is converted into adrenaline. NA is key in the fight-or-flight response and also directly increases heart rate, energy release from fat, and muscle readiness.