Unfortunately, the cellular mechanisms of the main constituents of marijuana are not well known, but, given the large amount of money allocated to research and the high numbers of researchers, the mysteries are likely to unravel quickly.  Despite the lack of definite information regarding the latter stages of the cellular pathways, scientists have made progress in determining the earlier stages.
 

Neurological Effects:     The psychoactive effects of marijuana are explained by the existence of specific receptors located mainly in the hippocampus, the cerebral cortex and the cerebellum.  When marijuana is injected, THC binds to cannabinoid receptors.  These receptors are G-protein linked, and are termed CB1.      In other words, the cannabinoid binds to the receptor, which then induces a conformational change in the receptor.  In turn, the change activates a G-protein, which activates adenylate cyclase (Kim and Thayer, 146).  Accordingly, a second messenger begins the cellular cascade.     With the cannabinoid receptors blocked by the marijuana-produced cannabinoid, scientists reason that THC "suppresses the neurons in the information-processing system of the hippocampus" (NIDA Notes, 2).  Since the hippocampus is involved in learning and memory, those abilities are impaired when THC is consumed.  Similarly, the binding of THC to the cannabinoid receptors in the cerebral cortex and the cerebellum impairs concentration and perception, and movement, respectively.

Photo courtesy of www.cannabinoid.com