On a neurological basis what is the difference between 'drunk' and 'stoned'?

It depends on what drug you're referring to, the term stoned is mostly applied to the high obtained from cannabis, though can also apply to opiates; though it's somewhat of an archaic term when it comes to opiates now.

Alcohol acts on the GABA receptors, specifically as an agonist to the GABA-A receptor, GABA is a neuroinhibitory molecule that slows the firing of neurons, depressing the central nervous system (CNS), alcohol also acts as an antagonist at the NMDA receptor. Benzodiazepines, barbiturates and other sedative-hypnotics like chloral hydrate also act at the GABA receptor.

Cannabis acts on the cannabinoid receptors, there are different cannabinoid receptors and each sub-receptor is responsible for somewhat different effects, different strains have different ratios of cannabinoids in them, which have different receptor activity; this is why different strains of cannabis can have somewhat different effects.

Opioids act on the opioid receptors, of which there are three types (mu, delta and kappa), the mu and delta receptors are most responsible for analgesia (relief of pain) and euphoria, the kappa receptor has some analgesic effect but kappa agonists also cause dissociative-hallucinogen effects and dysphoria; salvinorin A (active ingredient in salvia) is a Kappa specific opioid. Morphine, Heroin (diacetylmorphine/diamorphine), oxycodone and all other opioids used for pain relief are mainly mu and delta opioid receptor agonists.

This is just my opinion, I never really got drunk or stoned. Stoned can even mean someone is high and on drugs and experiencing an altered state of mind or even feeling high, being overstimulated.

In my opinion, drunk would mean you feel sleepy and numbed out, can't think clearly and you would feel tired, and sloppy when you're drunk, so there is some difference.

Stoned is more for drugs, drunk is alcohol.

drunk is for alcohol overuse, and stoned is for illegal drugs overuse,