Any cell receptor that binds and is activated by the catecholamines adrenaline or noradrenaline. Adrenoceptors are therefore crucial in mediating the effects of catecholamines as neurotransmitters or hormones. There are two principal types of adrenoceptor, alpha (α‎) and beta (β‎), and various subtypes of each, with differing sensitivities to the catecholamines and to certain drugs. Adrenaline acts equally on both α‎- and β‎-adrenoceptors, whereas noradrenaline acts more strongly on α‎-receptors. All types are coupled to a G protein, which initiates the signal transduction pathway inside the cell. The alpha adrenoceptors fall into two main subtypes: α‎₁-adrenoceptors, which mediate the contraction of smooth muscle and hence, for example, cause constriction of blood vessels due to contraction of their muscular walls; and α‎₂-adrenoceptors, which occur, for example, in presynaptic neurons at certain nerve synapses, where they inhibit release of noradrenaline from the neuron. As regards cell signalling, α‎₁-receptors activate phospholipase C, which causes the synthesis of second messengers, whereas α‎₂-receptors inhibit adenylyl cyclase, thereby decreasing levels of cyclic AMP. The beta adrenoceptors also have two main subtypes: β‎₁-adrenoceptors, which stimulate cardiac muscle causing a faster and stronger heartbeat; and β‎₂-adrenoceptors, which mediate relaxation of smooth muscle in blood vessels, bronchi, the uterus, bladder, and other organs. Both act through a G protein that stimulates adenylyl cyclase, thus increasing intracellular levels of cyclic AMP. Activation of β‎₂-adrenoceptors thus causes widening of the airways (bronchodilation) and blood vessels (vasodilation). See also beta blocker; signal transduction.