Have you ever wondered how the cells of the body recognize "signals" in the form of chemical messengers such as the catecholamines epinephrine (adrenaline) and norepinephrine (noradrenaline)? It is through interactions with receptors.
Receptors are structures that are located either on the surface of cell membranes or within cells themselves.
The sympathetic division of the autonomic nervous system has a number of catecholamine receptors called adrenoceptors and a Greek letter and a number designate these. For example β1 (beta one) adrenoceptors are receptors located specifically within the heart. When activated the heart rate increases.
In contrast β2 (beta two) adrenoceptors are widespread within smooth muscle of the body including the airway. When β2 adrenoceptors are activated the smooth muscle of the airway relaxes and the airway dilates.