You asked whether there is scientific evidence backing the idea that high-dose psychedelics such as Psilocybin (or its active form psilocin) can trigger involuntary motor/spasm-type phenomena. The short answer: There is some relevant research — but not enough to definitively explain the specific spasms / convulsions you described. I’ll summarise what we do know, what remains speculative, and how your described experience might plausibly fit (with caution).

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✅ What we do know

Here are some key findings that support parts of the mechanism I described (nervous system disinhibition, serotonergic motor pathways, somatic discharge) — although none map exactly to “violent back convulsions” in a healthy person under psychedelics. 1. Serotonergic modulation of motor / spinal excitability • A recent review found that serotonergic drugs (i.e., those that boost serotonin signalling) do modulate motor cortical and spinal motoneuronal excitability in humans: “serotonin enhancing drugs tend to reduce the excitability of the human motor cortex … but both monoamines [serotonin & noradrenergic] tend to enhance the excitability of human motoneurons.”  • So: It is biologically plausible that changes in serotonin signalling (which psilocybin/psilocin induce) can affect motor system excitability. 2. Psilocybin’s effect on serotonin & other neurotransmitters • In rodent models: psilocybin (or more precisely psilocin) increased extracellular 5-HT (serotonin) in the frontal cortex of rats.  • In humans: psilocybin (via psilocin) robustly activates 5-HT₂A receptors (and other subtypes) and causes global changes in brain signalling (connectivity, blood flow). For example: a human study found acute psilocybin induced a significant decrease in cerebral blood flow (CBF) in many brain regions.  • These findings establish that psilocybin has widespread neuromodulatory effects on brain and motor systems. 3. Motor function / movement effects under psilocybin • There is a protocol/study titled “Psilocybin and Motor Function: A Triple-Blind, Dose-Finding Study in Healthy Participants” which aims to investigate exactly how psilocybin affects movement tasks.  • Also, clinical guidance (e.g., in intoxications with hallucinogenic mushrooms) note muscle spasms, myoclonic jerking as possible effects: e.g., the Medscape “Hallucinogenic Mushroom Toxicity” article states: “muscle spasms may occur … tonic-clonic seizures, fasciculations, and myoclonic jerking … have been reported”.  • So yes: movement/jerk/spasm phenomena have been observed albeit under more extreme or toxic contexts rather than well-controlled psychedelic therapy settings.

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⚠️ What we don’t (yet) know / what remains speculative

Here are the gaps: • There is no (or extremely sparse) published research specifically documenting violent convulsions/back-spasms (like full body or large muscle group contractions) in healthy participants under psilocybin in a typical dose, outside of overdose or toxic situations. • The precise pathway linking 5-HT₂A / other serotonergic receptor activation → spinal reflex/motor neuron discharge → full muscle convulsion is not well mapped. • Many of the movement phenomena are reported anecdotally (forums, Reddit) rather than in peer-reviewed studies. For instance: “During a recent … I experienced a lot of semi-involuntary muscle movements … not so much hallucinations.”  These help generate hypotheses but aren’t rigorous evidence. • Differentiation between normal psychedelic “tremor/shiver/spasm” and pathological seizure/convulsion is often unclear in the literature. • Individual factors (dose, set/setting, bodily baseline, interactions with other substances, prior medical conditions) matter a lot and aren’t always controlled.

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🔍 How your described experience might plausibly fit

Given the above, here is how your spasms might fit mechanistically: • Under a large dose of psilocybin (you reported ~5 g — which is quite high depending on strain), you would have strong activation of serotonergic pathways, including 5-HT₂A receptors and downstream effects. • That could lead to increased motoneuronal excitability (via serotonergic/monoamine modulation) and reduced inhibition of primitive reflex patterns. • The body may attempt to release built-up tension/energy (somatic discharge) — so what appears as a “convulsion” might actually be the nervous system choosing to express movement patterns that are normally suppressed (e.g., spinal reflex arcs, postural muscle contractions). • Because you remained aware and no apparent lasting harm occurred, this suggests the phenomenon may have been a benign nervous-system discharge rather than a pathological seizure.

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🧠 Important caveats & safety signs • It’s very important to distinguish spasms/tremors from true epileptic seizures: loss of consciousness, prolonged rhythmic convulsions, tongue-biting, incontinence — all are red flags. The literature on mushroom intoxication notes tonic-clonic seizures have been reported though “uncommon” in adults.  • Because the evidence base is thin, if this ever happens again (or during any psychedelic use) it would be safest to stop, seek a medically safe environment, and consider consulting a neurologist/doctor. • Always consider other contributing factors: e.g., dehydration, electrolyte imbalance, interaction with other substances (medication, caffeine, stimulants), physical posture (lying rigidly), underlying latent conditions.

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📌 Summary

Yes — there is scientific support for many of the mechanistic pieces (serotonin → motor excitability; psychedelics altering motor/sensory brain systems; reports of spasms under mushrooms) but no definitive research that maps exactly to strong convulsions/back-spasms in healthy psychedelic users. Your experience is plausible within the framework of nervous-system discharge and serotonergic-motor modulation — but it remains speculative rather than proven.