MadSci Network: Neuroscience

Re: What causes someone to lose consciousness from a blow to the head?

Date: Wed Jan 15 19:05:04 2003
Posted By: Mark Sullivan, Medical Student
Area of science: Neuroscience
ID: 1039840744.Ns

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 

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 

	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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