ACTIONS OF INHALED ANESTHETICS IN THE CENTRAL NERVOUS SYSTEM

Brain

Inhaled anesthetics may act by altering neuronal activity in selected regions of the CNS. Because the brainstem reticular formation plays a role in altering the state of consciousness and alertness and in regulating motor activity, it is often suggested that this structure is an important site of anesthetic action. Although the idea that anesthesia results from a decrease in "tone" in the ascending reticular system may be correct, it is most likely an oversimplification.^[18] The effect of anesthesia on neuronal activity in the reticular formation is variable and can be increased, unchanged, or decreased, depending on the agent and the neuronal unit examined.^[19]

General anesthetics interrupt transmission in the CNS at sites other than the reticular formation. Clinical concentrations of inhaled agents may alter spontaneous and evoked activity in the mammalian cerebral cortex and hippocampus. Although inhaled anesthetics usually depress excitability of brain neurons, situations can be found in which anesthetics enhance excitability ( Fig. 4-4 ).^[20] Inhibitory transmission may be influenced by volatile agents. For example, halothane, enflurane, isoflurane, and sevoflurane prolong γ-aminobutyric acid (GABA)-induced inhibition in the hippocampus^{[21] [22]} in an agent-specific manner. Neuronal pathways between various brain regions and consisting of excitatory and inhibitory components may also be influenced by inhaled agents. The transfer of sensory information from the thalamus to certain cortical regions may be particularly sensitive to anesthetics.^{[18] [23]}