 In humans , this signalling role is important in both the central and peripheral nervous system . Activity @-@ dependent release of ATP from synapses , axons and glia activates purinergic membrane receptors known as P2 . The P2Y receptors are metabotropic , i.e. G protein @-@ coupled and modulate mainly intracellular calcium and sometimes cyclic AMP levels . Though named between P2Y1 and P2Y15 , only nine members of the P2Y family have been cloned , and some are only related through weak homology and several ( P2Y5 , P2Y7 , P2Y9 , P2Y10 ) do not function as receptors that raise cytosolic calcium . The P2X ionotropic receptor subgroup comprises seven members ( P2X1 – P2X7 ) , which are ligand @-@ gated Ca2 + -permeable ion channels that open when bound to an extracellular purine nucleotide . In contrast to P2 receptors ( agonist order ATP > ADP > AMP > ADO ) , purinergic nucleoside triphosphates like ATP are not strong agonists of P1 receptors , which are strongly activated by adenosine and other nucleosides ( ADO > AMP > ADP > ATP ) . P1 receptors have A1 , A2a , A2b , and A3 subtypes ( " A " as a remnant of old nomenclature of adenosine receptor ) , all of which are G protein @-@ coupled receptors , A1 and A3 being coupled to Gi , and A2a and A2b being coupled to Gs . All adenosine receptors were shown to activate at least one subfamily of mitogen @-@ activated protein kinases . The actions of adenosine are often antagonistic or synergistic to the actions of ATP . In the CNS , adenosine has multiple functions , such as modulation of neural development , neuron and glial signalling and the control of innate and adaptive immune systems . 
