What is a Concussion?
In order to understand how concussions affect the brain, it is first necessary to understand the biomechanical forces that lead to a concussion. An impact injury occurs when an object strikes the skull with such force that it imparts kinetic energy to the skull and the brain (think of this part as waves traveling through water). The energy of the blow accelerates first the skull then the brain. There are two types of forces that occur: linear and rotational. Linear force is what occurs during a car accident where the car hits an object, stopping suddenly. The person's head moves forward rapidly hitting the steering wheel with his/her forehead. As the skull accelerates forward and comes to a sudden stop, the brain also moves forward rapidly and is suddenly stopped when it hits the skull. This is known as a coup injury and typically affects the frontal lobes. The initial forward is movement followed by a backwards movement of the head resulting in the rear portion of the brain hitting the cranium, the contra coup injury. For a concussion to occur via linear force, acceleration/deceleration is necessary as in a car accident. A linear force imparted, for example, by a bullet, may crush and penetrate the skull, causing mainly local and even fatal damage, but will not likely result in concussive symptoms.
Two types of injuries result from rotational forces: shearing/tearing of brain tissue and stretching of tissue. In a rotational injury, the inertial forces are imparted to the skull and brain so that an angular acceleration occurs around the mid-line axis. This is the type of injury that typically occurs when you are hit on the side of your head resulting in a rapid turning of the head/neck. The injury resulting from rotational force is most prominent at the juncture between gray and white matter i.e., where neurons and axons from other neurons meet. The danger of rotational force injuries is why the NFL made helmet to helmet hits illegal.
Now that you understand the mechanics of concussion, here comes the physiology...
Loss of consciousness (LOC) occurs in approximately 5% of sport-related concussions. LOC is associated with brain stem trauma, specifically affecting the ascending reticular activating system, which is involved in maintaining conscious awareness. The flexion that occurs to the brainstem following a concussion is responsible for LOC when it occurs, but does not explain other cognitive symptoms, including amnesia.
The impairments associated with concussion are the result of what is referred to as a neurotoxic cascade. At the initial time of impact there is an immediate increase in blood pressure and decrease in cerebral blood flow. This is where things become complex. The pressure wave within the skull results in shearing of tissue that results in changes in the activity of the neurons, specifically the opening of normally voltage dependent ion channels (this means that neuron ion channels that typically do not open until a certain level of activation is reached become over active). The ionic flux results in massive depolarization, leading to the release of a large amount of excitatory amino acid transmitters. These amino acids signal to the neurons to maintain the ionic flux, especially of the release of potassium ions and taking in of calcium ions. The amount of activity results in a hypermetabolic state that requires an incredible amount of oxygen and adenosine triphosphate (ATP; serves as an energy source for the brain). Because of this hypermetabolic state, a large amount of glucose is needed by the brain, but because of the decreased cerebral blood flow, glucose supplies run low. In addition, the calcium influx disrupts mitochondrial function (these are the energy plants of the cell), so energy production plummets at a time when an increase is needed.
This process is a general one and does not affect a specific brain region. In rats, and likely in humans, the hypermetabolic state lasts about 6 hours after which a state of hypometabolism (decreased cellular activity results) and can last about 10 days. This process of hyper- followed by hypometabolism is responsible for the symptoms of concussion.