This is a THC molecule:
It is formed in cannabis plants and is similar in both structure and function to the endocannibinoids produced in the human body. It is introduced to the human body via smoke (entering the blood stream in the lungs) or orally, although the physical reaction to the latter is delayed due to slower uptake via the stomach and the fact that when taken orally, THC is usually diluted by food-food.
Not now Pinkie Pie. (Click to embiggen.)
This is the synapse:
Allow me to render this situation in a more symbolic manner:
Here we see the synaptic cleft. On the right we can see the inhibitory neural transmitters, (represented by dogs). On the left is Dopamine, (represented by cat heads), and at the bottom the dopamine receptors (the headless cat bodies).
The release of dopamine, is it important to note, is associated with feeling of relaxation and a dramatic decrease in one's ability to feel physical pain. It is also worth noting that a lack of dopamine in the brain is what causes Parkinson's disease.
Now, normally, the synaptic cleft looks like this:
Without the presence of a cannabinoid (either a self-produced Endocannabinoid, or one from an outside source) the inhibitory neurotransmitters prevent the release of dopamine into the synaptic cleft. As such, the dopamine remains contained.
However, if your body is injured or feels the need to react, your brain will allow the release of an inhibitory chemical like (GABA) into the synapse. If you were hit by a car, for example, a massive surge of dopamine is one of the chemicals responsible for keeping you alive and not-incapacitated by pain. E.g. If you don't feel the shrapnel in your leg, you can crawl off the highway.
Here represented by the leash, the inhibitory-transmitter-inhibitor (here, the cannabinoid) prevents the release of the inhibitory neurotransmitters and in turn Dopamine is released into the synaptic cleft where it binds with the dopamine receptors of the proximal neuron. While dopamine is constantly being released in healthy brains, a sudden increase in dopamine levels results in a decrease in activity levels and an increase in feelings of relaxation which allow the organism to temporarily cope with injury.
Naturally-produced inhibitory-transmitter-inhibitors break down fairly quickly in the body, which is why you don't usually get a lasting high when you get hurt. The leashes get worn out.
Unlike chemicals like Anadamine (leash), THC is more like one of those metal stake things that you dig into the ground when you're mad at the dog for pissing on you while you slept.
The pathways stay open, your brain is continually being flooded with dopamine, and you end up feeling relaxed, blood pressure increases (but pressure on your eyes decreases), and become less able to feel as much physical pain; your appetite increases (due to the fact that THC mimics an endocannibinoid responsible for hunger) and you become less likely have muscle spasms.
This is why marijuana is so effective in treating chronic pain, it causes the body to react as if whatever is causing the pain were a new injury, and makes your body hurt less, so you can go on living.