A general anaesthetic is a drug that brings about a reversible loss of consciousness. In formal use since 1842 - with the first public demonstration of the use of Diethyl ether to carry out surgery. (Though it's likely that alcohol, another chemical classed as an anaesthetic, was previously used, extensively, for similar purposes).
Since then, many other compounds with similar effects have been discovered, and several are now in routine use as general anaesthetics. But the neurological pathways by which general anaesthetics work are still unclear.
“It has always been believed that general anaesthetics exert their effects (analgesia, amnesia, immobility) by modulating the activity of membrane proteins in the neuronal membrane. However, the exact location and mechanism of this action are still largely unknown although much research has been done in this area.”
A 2021 study published in the journal Anesthesiology points to a discovery regarding the presence (or lack) of the neurotransmitter acetylcholine in the brain.
Although the precise neural correlates of consciousness have not yet been identified, there is one neurochemical marker that appears to track with our capacity to experience something: acetylcholine concentration in the cerebral cortex. Cortical acetylcholine concentration is high during wakefulness, decreases during slow-wave sleep, and increases again during rapid eye movement sleep, when we can have the conscious experience of dreaming.
Source : Anesthesiology editorial, Feb. 2021
A further anomaly has recently gained attention with regard to xenon. The (extremely inert) gas works efficiently as a general anaesthetic, with low toxicity. It's known to be an NMDA inhibitor but :
“It is not possible to state categorically which […] effects on ligand-gated and receptor-gated ion channels are causally linked to the anaesthetic action of xenon.”
See: Xenon: no stranger to anaesthesia British Journal of Anaesthesia 91 (5)
Given its rarity, xenon is considered prohibitively expensive for medical use - but with the appropriate machinery, it can be recovered (almost 100%) from exhalations.
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