There are a lot of myths out there about Post Traumatic Stress Disorder and Traumatic Brain Injury, and a lot of this comes from the fact that the majority of Americans simply can't understand why these disorders are so debilitating. I'll try to explain to you as much as possible both the physiological and psychological effects and symptoms of both and why they are both distinct and two separate disorders.
lets talk about PTSD first, because that's the one that I know so well. First off you need to understand the adrenal system, from which we get adrenalin (among other things). Now when a person is faced with any serious danger the adrenal gland will flood your system with adrenalin. The blood pressure will go up the Bronchioles will dilate, and the heart will start pumping faster among other thing (I won't go into 90% of the other symptoms but its actually pretty interesting.) . Now why is this important? Well its designed to help you Fight, your muscles are primed and your nerves will be firing on overdrive, or Flight, your airways are dilated and again your muscles are primed. In a lot of laymen's terms its a lot like having a sports car and flooring the accelerator. There's a small catch, this car doesn't have brakes. so if you floor the accelerator, go to 120 MPH and wait for gravity to coast you to a slow down. A lot of time though, every time there is a new threat presented the previous levels of adrenalin are not taken to account so you're basically flooring the accelerator of this metaphotical car.
Go experience a roller coaster and you will feel some of this. Notice how its a little difficult to stand afterwards and that your hands are shaking. Well imagine that fear you felt on that roller coaster magnified several times over. You also have to understand that memories while experiencing that kind of fear are extremely sharp, and clear. It is clearer than when your first child is born , when you get married, or when you buy your first house. Met a celebrity? Make a presentation before hundreds of people? The reason those memories are so powerful is a *small* amount of adrenalin. Now imagine how powerful those memories would be if you had about ten times the adrenalin flooding through your system. It can actually get to the point that you overload on Adrenalin and this is when you "freeze", you are simply unable to handle the physiological effects.
Now add in the natural grief that one would feel about the death of someone close to them with the fear one might feel for their own life and really ASR (Acute Stress Reaction) actually makes a lot of sense. You tend to "shut down" as a way to cope with the extremes of the memories of what has happened. PTSD comes when you don't successfully process the events that happened to you. Despite the fact that PTSD does have a physiological cause it is a primarily psychological disorder and can be treated, but it typically will always stay with a person until the day they die.
TBI or Traumatic Brain Injury is a little bit more complicated to explain. You need to know some things about the brain to really understand this injury. First off blood is actually acidic, and despite the fact that the blood does go to the cranium, there is something called the blood to brain barrier, where the oxygen and nutrients are traded. No actual blood touches the grey or white mater (the squishy stuff that makes up your brain) because if it did it would actually kill the nerves that make up the brain, so anytime there is blood in the cranium that is a very bad thing.
More than that, though, the brain is one of the few organs that is simply not designed to withstand any level of injury. There's a reason your skull is so thick. Even your heart, which is just as vital, is not nearly protected as well as the brain. That should tell you all you need to know about how easily the brain is injured.
The next thing you need to know is physics. Newton's Third Law means that the more extreme the action, the more extreme the reaction. So if say, a 500 LB IED goes off a few meters away and I'm inside a vehicle, my brain is going to be thrown back (just like everything else in my body) and rebound off the back of my skull and back and forth till all the energy has dissipated. But there's also another force at work here, cavitation. The "waves" the blast sends out of compressed air,will translate their energy into everything they touch. A bullet, fired into ballistics gel will give you an idea of what it looks like. Go watch Mythbusters sometime they're shooting ballistics gel, then imagine something with several thousand times the energy of the bullet they fire, has gone off and that ballistics gel is your brain. That you are able to think at all after an injury like that is something of a minor miracle.
Here's the really hard part about TBI and PTSD, the symptoms are very similar. It has actually happened that a person with mild or even severe TBI has been misdiagnosed, and thus received the wrong treatment. In some cases the TBI is coupled with PTSD, which only makes the treatment more complicated. Perhaps worst of all, we simply don't know enough about the brain to really know the long term effects such an injury will have. We don't know how the brain copes, we don't know the repair rate or even if certain areas can be repaired, we simply don't know. A lot of research is being done, and hopefully will progress apace, but for the time being there is a vast unknown on this.
What I can tell you is that if you have a friend, or loved one that is suffering from either or both PTSD or TBI, you need to understand that you'll never understand what they went through. If they tell you, it will be on their terms, not yours. The healing process may be long, and above all you need to be patient, and encouraging. Don't forget that you will need support as well. The more sever the case the more support they (and in turn you) will need. Don't be afraid to tap into the resources available.
Remember these illnesses are treatable. They are not the death sentence a lot of people make them out to be. A person can still live a full and complete life after receiving mTBI, or even sTBI, and PTSD is not the black plague, and should not be treated as such. In these cases, sometimes the best medicine is compassion. I might also point out that pity is absolutely the wrong thing to show these people.