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THE ANATOMY OF A HEAD INJURY
| The effects of a blow to the head on brain function arise from the structural characteristics of the skull and the brain and the direction and size of the forces acting on the head. The brain, a rather soft tissue with the consistency somewhere between egg white and jello, is covered by three membrane layers. The outer-most layer, called the dura mater, is connected to the inside of the skull at various suture points which serve to suspend the brain within the skull. The brain sits atop the brain stem and the spinal cord extends from there, out the base of the skull through a hole called the foramen magnum. Brain injuries arise from three characteristics of this brain-skull anatomy: the rigidity and internal contours of the skull, the incompressibility of brain tissue and the susceptibility of the brain to shearing forces. |
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The first two characteristics give rise to contusions or hematomas (i.e. bleeding) on the surface of the brain, one of the most common injuries. There are usually two contusion sites in a brain injury. One occurs at the site of the blow to the brain and is called the coup injury. The other arises where the brain bounces off the skull when it has been moved away from the site of the original blow. The contusion here is termed the contre coup injury. Some bleeding may also arise at the suture points when the dura mater is torn away from the inside of the skull.
The third characteristic, susceptibility to shearing forces, plays a role primarily in injuries which involve rapid and forceful movements of the head, such as in motor vehicle accidents. In these situations rotational forces such as might occur in whiplash-type injuries are particularly important. These forces, associated with the rapid acceleration and deceleration of the head, are smallest at the point of rotation of the brain near the lower end of the brain stem and successively increase at increasing distances from this point. The resulting shearing forces cause different levels in the brain to move relative to one another. This movement produces stretching and tearing of axons (diffuse axonal injury) and the insulating myelin sheath, injuries which are the major cause of loss of consciousness in a head trauma. Small blood vessels are also damaged causing bleeding (petechial hemorrhages) deep within the brain.
Collectively these injuries can result in swelling of the brain. If the pressure within the skull is not relieved through surgery, cooling or medication, the brain will gradually be pushed down through the opening at the base of the skull, the foramen magnum. Nuclei in the brain stem controlling breathing and cardiac function will eventually be compressed resulting in death.
(Source: extract from a paper by Eric A. Roy, Ph.D., C.Psych. Professor and Director, Centre for Habilitation Education and Research)
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