[Physiology of nodal tissue and effect of products used in anesthesia]

The three fundamental properties of the nodal tissue, i.e.: excitability, automaticity and conduction of the depolarisation wave, are directly dependent on ionic transmembrane currents. These currents, either active or passive, determine the characteristics of the membrane action potential. Passive ionic movements cross gated transmembrane channels which activation is voltage-dependent, sometimes time-dependent and, for Ca2+ channels, dependent on intra-cellular cyclic AMP content. Both intra and extra-cellular ionic concentrations (K+, Ca2+, Mg2+) alter these passive movements. Active ionic movements require ATP hydrolysis and depend only on cellular metabolism. The neurovegetative system and its transmitters modulate the nodal tissue function altering passive ionic currents. Drugs employed during anesthesia change this function either by direct action on the ionic currents or by indirect action on the neurovegetative system or on histamine liberation. Thus: local anesthetics block selectively fast sodium channels; halogenated volatile anesthetics inhibit slow calcium channels: and narcotics are quite always vagomimetics, sometimes sympathicolytics, they often release histamine. Nevertheless, mechanism of action of various drugs remain unclear.