|MadSci Network: Neuroscience|
That is a very interesting question; one, which I must admit, is difficult for me to answer concretely. However, I did do some investigating into some different ideas I had and have come up with some possibilities that will hopefully satisfy you. Now we have to look at the concept of loss of consciousness (LOC) in a couple different ways to explore this fully. First is what is called a syncopal episode which can cause LOC by a number of different patho- physiological mechanisms. One would be cardiac cause such as a heart attack, which would result in a decreased amount of blood flow to the brain, depriving it from sufficient blood flow to provide oxygen. Another type is called a vaso-vagal response where the nervous system reflexively decreases the amount of blood flow to the brain as a protective mechanism. Too much pressure to the brain can cause a rupture in a blood vessel known as a stroke, so the body tells the heart to slow down and not pump as hard. Some people have this as an unfortunate response when they see needles, or blood. So that is just a cause of LOC that we have to consider in this whole investigation. Quick anatomy: the brain is a soft organ that is suspended in the skull by a water-like solution for cushioning and a couple of sheet-like membranes that cover it and help anchor it somewhat in place. The brain is connected to the spinal cord by the brainstem and exits the skull at the base to continue down into the spine as the spinal cord. Now, when we are hit in the head the brain is "jarred", if you will, by the forces that are applied and, as a result, will move within the skull. Imagine how an ice-cube clinks against the side of the glass when you move your ice water suddenly. There are 3 different force mechanisms that we must consider, and when applied to the head can cause Traumatic Brain Injury (TBI) in different ways. First is Acceleration, which occurs when a moving object strikes a stationary head like a straight punch to the head. Linear acceleration is considered the least injurious force and typically is associated with superficial contusions (bruises) or, in some cases, subdural hematomas (bleeding inside the brain). Second is Deceleration, which results when a moving head strikes a stationary surface like running into a brick wall headfirst. Sudden deceleration is thought to be responsible for most severe brain stem injuries. Last is Rotation of the brain and occurs when the head is struck in an asymmetric manner such as a punch that turns the head. Rotational acceleration-deceleration can induce shearing forces, disruption of the white matter (neurons), and widespread injury that most likely is vasogenic (from bleeding). OK, so we know the types of forces and the type of injury they cause in the brain, but what does all that mean really in terms of LOC? Without going into too much detail about the structure of the brain and all the inner workings thereof I will try to make some sense of this. The brain, as you know, is the central processing computer for us, and it takes in all sorts of information from our senses so we can make decisions about everything. For example, sensory neurons in the hand relay information to the brain that the water touching my hand is too hot. The brain processes the info then dictates: remove hand from water! A part of what allows us to perform this feat is a relay circuit known as the Reticular Formation, which relays information about pain, and helps control non-voluntary systems, and is involved in sleep and wakefulness. This pathway runs right through the brainstem which is the section of brain that controls things like heart rate, blood-pressure and breathing that you are completely unaware of most of the time, and that you donít have to actively think about (non-voluntary) to make changes, your body does it automatically. Imagine if we had to consciously control our own heart rate, breathing rate, blood pressure all the time, we'd never get any sleep! So we have a part of the brain that relays all sorts of information, that is able to change our blood pressure and heart rate reflexively, that is also involved in sleep and wakefulness and runs right through the brainstem. Remember those forces that I talked about above? Rotation probably hits the brainstem most, and, as a result, can affect all these functions. Perhaps it is an immediate decrease in blood pressure, or maybe just triggering of the sleep response, or perhaps the pain sensation from a hit is too much and this sets off the sleep response. One source I looked at which was studying concussions indicated that "inertial forces from the traumatic injury [that] lead to shear strain. This strain results in increased energy demands of the brain and transient diffuse cerebral dysfunction, which involves the reticular formation in the brain stem." So basically the sudden jolt of movement to the brain inside the skull was a real shock to the system and caused a widespread shutdown for a brief period of time. The amount of injury to the brain depends on the type of force and how much is applied. So you can see that the injury resulting from a punch to the head from the average guy should be much less than say getting slammed in the head by Mike Tyson. The first example may lead to LOC, the second has been proven time and again by Mike to result in LOC. I would advocate not getting punched or hit in the head altogether though because successive hits to the head can result in accumulative brain injury, which is just plain bad in the long run. OK, hope that wasn't too confusing, but that was a subject that required a lot of anatomy and neuro- anatomy mumbo-jumbo. If I get into it anymore than I already have I am sure you would lose consciousness. A book that is pretty exhaustive on the subject is "The Human Brain. An introduction to its functional anatomy" by John Nolte. Good luck and don't get hit! Mark Sullivan
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