The more recent advances involve understanding at the organic molecular level; biochemical action of the endogenous ligands, enzymes, receptor proteins, etc. The critical changes affecting cell firing occur when the signalling neurotransmitters from one neuron, acting as ligands, bind to receptors of another neuron. Many neurotransmitter systems and receptors are well known, and research continues toward the identification and characterization of a large number of very specific subtypes of receptors. For the six more important neurotransmitters Glu, GABA, Ach, NE, DA, and 5HT (listed at "neurotransmitter") there are at least 29 major subtypes of receptor. Further "sub-subtypes" exist together with variants, totalling in the hundreds for just these 6 transmitters. "- (see serotonin receptor for example.)" It is often found that receptor subtypes have differentiated function, which in principle opens up the possibility of refined intentional control over brain function.