In the central nervous system (CNS), glycine-mediated inhibitory neurotransmission is essential to voluntary motor control and reflex responses. Glycine binds to glycine receptors (GlyR) in the postsynaptic neuronal membranes. GlyR, gamma-aminobutryic acid, serotonin and acetylcholine comprise an evolutionally conserved superfamily of ligand-gated ion channels. The pentameric subunit structure of GlyR consists of two types of glycosylated membrane proteins, alpha1 through alpha4 and beta, and an associated peripheral membrane protein, which combine to form a chloride-selective ion channel. In humans, the composition of the pentamer changes from ?2 subunits in the fetal CNS to alpha1 and beta subunits in the adult CNS. Fast potentiation of GlyR by intracellular Ca2+ in the brainstem and midbrain indicate an important role for Ca2+ in modulation of glycinergic synapses.