A general anaesthetic is a drug that brings about a reversible loss of consciousness.* ^{[ see note below ]}

In formal use since 1842 - the first public demonstration used 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."

Source :Wikipedia

An in-depth article by Philip Ball in New Scientist, March 1st 2017 covers the yet-to-be-resolved mysteries surrounding general anaesthetics.

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.

* Notes :

1) Tracing the unknown bio-chemical processes are further complicated by the fact that Consciousnessplugin-autotooltip__plain plugin-autotooltip_bigConsciousness

inexplicable

Does an ant carrying a piece of leaf across the forest floor ‘know’ it’s doing so? Is it conscious? If science could construct a machine as complex as the human brain, would it ‘know’ it existed? Will it oneday be possible to explain consciousness via the laws of physics?.itself is poorly defined and remains unexplained.

2) Charles Darwin, and many other subsequent scientists have experimented with a wide range of anaesthetics on plants which rapidly move - such as Mimosa (Pudica) and the Venus Flytrapplugin-autotooltip__plain plugin-autotooltip_bigVenus Flytrap mechanism

The Venus Flytrap (Dionaea muscipula) is a well-known carnivorous plant native to the sub-tropical wetlands of the US.

The 'trap' structure snaps shut when insects or spiders repeatedly touch specialised 'trigger hairs' on the trap's upper surface. (Dionaea muscipula). Many of the compounds tested do indeed (reversibly) immobilize the plants. Again, an exact overall explanation is lacking. See : Plant Signaling & Behavior Volume 7, Issue 9

Also see : Intraoperative Awarenessplugin-autotooltip__plain plugin-autotooltip_bigIntraoperative Awareness

Intra-operative Awareness (a.k.a. Anesthesia awareness) sometimes occurs as a complication during general anesthesia, where the intended state of complete unconsciousness is not maintained throughout the whole procedure.

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