Keeping in mind that I've never tried it myself, from what I gather from looking into this route, it would be the THC component that would cause the high from marijuana, as well as being the cause of the anxiety or paranoia that some users experience. The other major component CBD would not result in the high, the anxiety/paranoia, and it also does not raise the heart rate like THC does.

Every marijuana strain would carry it's own THC to CBD ratio, and even within the strains themselves each plant would vary - sometimes dramatically. Most strains these days have higher THC levels (due to being bred for the high effect), but there are some strains with a 1:1 ratio, and there's a growing trend (no pun intended) towards developing medicinal-oriented high CBD strain with low THC levels.

The reason marijuana is being looked at to assist with PTSD is because of what research is suggesting happens when someone has PTSD. The brain has a type of receptor called CB1 receptors (Cannabinoid type 1 receptors), and in people with PTSD there seems to be more of these receptors than normal. Working with those CB1 receptors is a type of neurotransmitter called anandamide, and it supposedly plays a role in transmitting the signals that help a person manage and handle mood, stress, memory, etc.

However, the people with PTSD don't have enough anandamide, and so it causes those CB1 receptors to not be able to help manage those areas, leading to the PTSD symptoms.

The THC and CBD in marijuana have a similar structure to anandamide, so the thinking seems to be that when they use marijuana, it goes and binds to those CB1 receptors that are lacking anandamide, in effect tricking the receptors into thinking everything is normal. The result being the PTSD symptoms don't get triggered as strongly.

It wouldn't be a long term fix though because frequent marijuana use (daily smokers) is linked with a decreased number of CB1 receptors. This seems to be due to the marijuana tricking the brain into thinking it's getting plenty of anandamide, so CB1 receptors are decreased (the brain thinks it doesn't need so many to catch it since there's so much available). That being said, the brain does seem to work on rebalancing itself once the marijuana is stopped.

At least that's how I understand what I've come across. I'm no neuroscientist, and as I mentioned I've never tried marijuana, so I'm just going off of what I've read.