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What Does Anesthesia Represent & Where Does It Work?
Anesthetic action has three principal components: immobility, amnesia, and unconsciousness.
Immobility is the easiest anesthetic end point to measure. Edmond Eger and colleagues introduced the concept of minimal alveolar concentration (MAC) to quantify the potency of an inha-lational anesthetic. They defined 1.0 MAC as the partial pressure of an inhalational anesthetic in the alveoli of the lungs at which 50% of a population of nonrelaxed patients remained immobile at the time of a skin incision. Anesthetic immobility is mediated primarily by neural inhibition within the spinal cord.
The ablation of memory arises from several locations in the CNS, including the hippocampus, amygdala, prefrontal cortex, and regions of the sensory and motor cortices. Memory researchers differentiate two types of memory: (1) explicit memory, ie, specific awareness or consciousness under anesthesia, and (2) implicit memory, the unconscious acquisition of information under ade-quate levels of anesthesia. Their studies have found that formation
of both types of memory is reliably prevented at low MAC values (0.2–0.4 MAC). Prevention of explicit memory (awareness) has spurred the development of monitors such as the bispectral index, electroencephalogram, and entropy monitor of auditory evoked potentials to recognize inadequate planes of anesthesia.
The ability of anesthetic drugs to abolish consciousness requires action at anatomic locations responsible for the formation of human consciousness. Leading neuroscientists studying con-sciousness identify three regions in the brain involved in generat-ing personal awareness: the cerebral cortex, the thalamus, and the reticular activating system. These regions seem to interact as a cortical system via identified pathways, producing a state in which humans are awake, aware, and perceiving.
Our current state of understanding supports the following framework: sensory stimuli conducted through the reticular for-mation of the brainstem into supratentorial signaling loops, con-necting the thalamus with various regions of the cortex, are the foundation of consciousness. These neural pathways involved in the development of consciousness are disrupted by anesthetics.
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