[Immobility: essential inhalational anesthetics action.]

Immobility is an essential component of general anesthesia and should be looked for and maintained throughout anesthesia. Anesthetic potency, called Minimum Alveolar Concentration (MAC), results from the inhibition of movement response to noxious stimulation. However, although spinal cord is recognized as the primary mediator of surgical immobility, cellular and subcelular mechanisms of action of inhaled anesthetics to produce immobility are not yet totally known. Considering major research advances on mechanisms of action of inhaled anesthetics and resulting wide variety of information, this review aimed at critically evaluating clinical and experimental studies performed to identify sites of action and mechanisms of inhaled anesthetics to promote immobility in response to noxious stimulations.

Complex mechanisms of action of inhaled anesthetics on central nervous system may be divided into three levels: macroscopic, microscopic, and molecular. Macroscopically, behavioral studies have shown spinal cord to be the primary anesthetic site of action to promote immobility in response to noxious stimulations. At cellular level, excitability of motor neurons, nociceptive neurons and synaptic transmission are involved in the anesthetic action. At molecular level, several receptors are affected by inhaled anesthetics, but only a few may directly mediate anesthetic action, among them: glycine, glutamate AMPA and 5-HT2A receptors, in addition to voltage-gated sodium channels.

Inhaled anesthetics-induced immobility is primarily mediated by an action on the spinal cord, as a consequence of anesthetic action upon motor neurons excitability and upon nociceptive neurons of the spinal cord dorsal horn. Actions on specific receptors have an effect on their synaptic transmission.