In the brain, there are receptors whose only job is to wait for cannabinoids to show up, so they can bind to them and form a molecule that signals changes throughout the brain, either by getting in the way of other chemicals or helping those chemicals be more effective. In the hippocampus, the memory storing portion of the brain, THC sticks its molecular butt into pathways, blocking their signals. One example is the pathway that forms short-term memory. This might explain why, the last time you got high, you’re pretty sure you had the same conversation with the same person at least three different times on the same night.
Marijuana also causes a significant decrease in blood flow to the motor control part of the brain, the cerebellum. Blood flowing anywhere in the body brings signals and essential molecules that help that part of the body function, and the brain is no different. By decreasing blood flow to the cerebellum, marijuana lowers the power of this site. That glazed-over look you see in people who are high is from the cerebellum entering a hazy state where it has slowed down. This down-regulation impairs reflexes, coordination, and movement. That’s fine if you are sitting on your couch watching your roommate play Halo, but when you get behind the wheel of a car, you might find you can’t seem to hit the break quickly enough to avoid rear-ending the person in front of you.
Research has shown that long-term use of weed may cause permanent damage to the brain. Because THC does such a great job of slowing everything down, with a lot of use, it can lead to long term depression in brain function. More specifically, you may find it is harder to access previously learned information for problem solving and complex concepts. Which might make it a bit hard to remember all those formulas on your next physics exam…