ABSTRACT
Two types of transmission occur in the nervous system, electrical and chemical. Electrical transmission is mediated by electrical synapses – gap junctions between adjacent neurons. Gap junctions are arrays of paired hexameric ion channels called connexons. The vast majority of synapses are chemical. At most central synapses chemical neurotransmission happens in the following way: the arrival of an action potential at the axon terminal may result in the release of neurotransmitter from a single presynaptic vesicle. Neurotransmitter receptors belong to two superfamilies. The ligand-gated ion channel receptors or ionotropic receptors have ion-selective channels as part of the receptor. The second superfamily is the G protein-linked receptors, also referred to as metabotropic receptors. Transmitters are rapidly cleared from the synaptic cleft after release by one of three methods: passive diffusion away from the cleft, reuptake into surrounding neurons or glia, or enzyme degradation.
