Calcium ions and integration in neural circuits
Integration in the nervous system is achieved by signal processing within dynamic functional ensembles formed by highly complex neuronal-glial cellular circuits. The interactions between electrically excitable neuronal networks and electrically non-excitable glial syncytium occur through either chemical transmission, which involves the release of transmitters from presynaptic terminals or from astroglial cells, or via direct intercellular contacts, gap junctions. Calcium ions act as a universal intracellular signalling system, which controls many aspects of neuronal-glial communications. In neurones, calcium signalling events regulate the exocytosis of neurotransmitters and establish the link between excitation of postsynaptic cells and integrative intracellular events, which control synaptic strength, expression of genes and memory function. In glial cells metabotropic receptor mediated release of calcium ions from the intracellular endoplasmic reticulum calcium store provide specific form of glial excitability. Glial calcium signals ultimately result in vesicular secretion of ‘glio’ transmitters, which affect neuronal networks thus closing the glial-neuronal circuits. Cellular signalling through calcium ions therefore can be regarded as a molecular mechanism of integration in the nervous system.