What is the Mechanism of 5-HT2A-mediated psychedelia? Drugs that activate the serotonergic 5-HT2A receptor (e.g. LSD, psilocybin, mescaline) are notorious for causing psychedelic experiences and I would like to know the neurochemical mechanism behind this.
Broadband cortical desynchronization "begins at the cellular level with
excitation of layer 5 pyramidal neurons, and extends to the macroscopic
level with cortical desynchronization and decreased
brain network integrity."

"Stimulation of the 5-HT2A receptor has been shown to
enhance spontaneous EPSPs/EPSCs (excitatory postsynaptic potential / 
excitatory postsynaptic current) in neocortical layer 5 pyramidal
cells by reducing outward potassium currents (Aghajanian and Marek, 1997; Lambe et al., 2000) and to cause increased glutamatergic recurrent activity in layer 5 of the cortex."

"Speculatively, if deep-layer pyramidal cells were to become hyperexcitable during the psychedelic state, information processing would be biased in the direction of inference—such that implicit models of the
world become spontaneously manifest—intruding into consciousness without prior invitation from sensory data. This could explain many of the subjective effects of psychedelics."

http://www.jneurosci.org/content/33/38/15171.short