Chemical synapses of the central nervous system

Neurons in the central nervous system communicate via specialized contacts called chemical synapses. At these contact sites electrical action potentials generated by a presynaptic nerve cell are translated into a chemical signal which is detected by the postsynaptic target cell via a specialized reception apparatus. The chemical signal, in form of neurotransmitter molecules like glutamate, glycine or GABA, is released by fusion of neuro-transmitter-filled synaptic vesicles with the presynaptic plasmamembrane. The neurotransmitter diffuses across the synaptic cleft and binds to the receptors in the postsynaptic membrane. Ionotopic neurotransmitter receptors are ion channels that open upon ligand binding. Thus, the chemical signal is transformed back into an electrical signal. To assure the precise spatial and temporal regulation of neurotransmission a highly structured protein apparatus is needed to keep all components of the synapse in register. The morphological correlate of this apparatus can be observed in the electron microscope as electron-dense cytomatrix apposed to the pre- and postsynaptic membrane (called presynaptic dense projection and postsynaptic density, respectively). The intimate contact between the two cells at synaptic junctions is underlined by the fact that biochemically intact synaptic junctions can be separated from other parts of the cells. In the last years identification of constituents of the cytomatrix led to an emerging picture of how crosslinking of receptors, cytoskeleton, cell adhesion molecules and signalling cascades may contribute to the function and plasticity of synapses and synaptic transmission.