Since the time of the initial studies of the nervous system, neurons were recognized as the cellular elements responsible for the information processing of the nervous system, while glial cells were considered as playing simple supportive roles to neurons. The fundamental attribute of neurons is their cellular electrical excitability, which is based on the expression of a plethora of ligand- and voltage-gated membrane channels that give rise to prominent membrane currents and membrane potential variations, which represent the biophysical substrate underlying the integration and transfer of information at the cellular level in the Central Nervous System (CNS). By contrast, glial cells are not electrically excitable. Although they are able to express some of the ion channels that are expressed by neurons, the level of expression of some key channels is not sufficiently high to support the generation of active electrical behaviors in response to different stimuli. Nevertheless, glial cells display a form of excitability that is based on variations of the Ca2+ concentration in the cytosol rather than electrical changes in the membrane potential.
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We thank Dr. W. Buño for helpful comments on the manuscript. This work was supported by Ministerio de Educación y Ciencia (BFU2004-00448), Spain. GP is a CSIC predoctoral fellow.