Dendritic spines are small protrusions on the branches (dendrites) of neurons, and most excitatory connections between neurons form on them. Counting spines is a common way to measure structural plasticity in animals.
Two studies are cited most. In 2018, a single injection of DMT significantly increased spine density on pyramidal neurons in the rat prefrontal cortex 24 hours later, and slice recordings suggested the new connections worked (Ly et al., 2018). In 2021, a Yale team imaged the same spines in living mice and found that one dose of psilocybin raised spine density and size by about 10% within 24 hours; about a third of the new spines were still there at 34 days, though only four mice were imaged then (Shao et al., 2021).
Not every study agrees. A 2026 rat study found no difference in synaptic density or spine types months after one dose of psilocybin, and put its lasting effects down to functional rather than structural change (Kramer et al., 2026). The 2021 authors also note it is not yet clear whether their results extrapolate to humans (Shao et al., 2021).
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