Definition
The mechanisms by which anesthesia occurs are complicated and not fully understood. In general, however, several ways through which anesthetics affect the CNS to induce unconsciousness, immobility, and analgesia have been identified. Some key mechanisms are effects on neurotransmitters and receptors.
Effects on Neurotransmitters and Receptors:
GABA Receptors:
Many anesthetic drugs enhance the action of the neurotransmitter gamma-aminobutyric acid (GABA), an inhibitory neurotransmitter. They interact with GABA-A receptors and enhance the influx of chloride ions into the neurons, which diminishes the excitability of neurons responsible for sedation and unconsciousness.
NMDA Receptors:
Some anesthetics, like ketamine, are N-methyl-D-aspartate (NMDA) receptor antagonists. The receptors participate in excitatory neurotransmission and pain perception. Inhibition of these receptors is particularly relevant for anesthesia by way of pain control and dissociative anesthesia.
Glycine Receptors:
Similar to GABA, glycine is an inhibitory neurotransmitter in the spinal cord and brainstem. Perhaps the activity of glycine receptors is enhanced by anesthetics as part of a mechanism to inhibit movement and relax muscles.
Effects differ by type of anesthetic:
Volatile Anesthetics (for example, sevoflurane, isoflurane): 
These are inhalation agents that act primarily by potentiating GABA and glycine receptors and also inhibit excitatory synaptic transmission.
Intravenous Anesthetics (for example, propofol, ketamine): 
Propofol acts via the GABA-A receptor but ketamine has a distinctive action at the NMDA receptor, leading to an overall state of dissociation rather than unconsciousness per se.
Effects on Neural Networks and Consciousness:
Anesthesia interferes with the connectivity between brain regions involved in consciousness, sensory experience, and motor outputs.
Studies involving EEG and fMRI have shown that anesthetics dilute the coupling between the thalamus and the cortex, both of which mediate consciousness and sensory processing.
Anesthesia and Modulation of Brain Wave Activity:
During anesthesia, brain waves change to the patterns determined by the level of anesthesia. Specifically, alpha and delta waves dominate in the presence of deep anesthesia as recorded with EEG.
This modulation of brain waves offers insight into how anesthetics alter consciousness and has clinical utility in determining the depth of anesthesia.
Continuing Research and Theories:
Continued investigations and discoveries are indeed helping to reveal mechanisms of anesthetic agents such as proteins from TREK-1 channels to molecular pathways mediating effects of anesthetics. These pathways will form a pathway to creating safer, more potent anesthetic agents.
The various mechanisms of each anesthetic agent often overlap and include common pathways with other agents, making anesthesia a multi-dimensional process involving many complex interactions. Thus, understanding these mechanisms improves the anesthetic care process and minimizes side effects.
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