question: Why does a hard hit to the head knock us out?
comment: Take whatever answers you get about the brain with a grain of salt. The human brain is the most complex object in the known universe and we know very little about how it works. Most of the responses you will get will be, "makes sense in my head" responses but not scientifically accurate.
passages: Section::::Causes.
  
  Brain injuries can result from a number of conditions including:
  
  BULLET::::- trauma; multiple traumatic injuries can lead to chronic traumatic encephalopathy. A coup-contrecoup injury occurs when the force impacting the head is not only strong enough to cause a contusion at the site of impact, but also able to move the brain and cause it to displace rapidly into the opposite side of the skull, causing an additional contusion.
  
  BULLET::::- open head injury
  
  BULLET::::- closed head injury
   The brain is surrounded by cerebrospinal fluid, which protects it from light trauma. More severe impacts, or the forces associated with rapid acceleration, may not be absorbed by this cushion. Concussion may be caused by impact forces, in which the head strikes or is struck by something, or impulsive forces, in which the head moves without itself being subject to blunt trauma (for example, when the chest hits something and the head snaps forward).
   Brain injury can occur at the site of impact, but can also be at the opposite side of the skull due to a "contrecoup" effect (the impact to the head can cause the brain to move within the skull, causing the brain to impact the interior of the skull opposite the head-impact). If the impact causes the head to move, the injury may be worsened, because the brain may ricochet inside the skull causing additional impacts, or the brain may stay relatively still (due to inertia) but be hit by the moving skull (both are contrecoup injuries).
   Little is known about what exactly causes one to be knocked unconscious, but many agree it is related to trauma to the brain stem. This usually happens when the head rotates sharply, often as a result of a strike. There are three general manifestations of such trauma:
  
  BULLET::::- a typical knockout, which results in a sustained (three seconds or more) loss of consciousness (comparable to general anesthesia, when the recipient emerges and has lost memory of the event),
   Even in the absence of an impact, significant acceleration or deceleration of the head can cause TBI; however in most cases a combination of impact and acceleration is probably to blame. Forces involving the head striking or being struck by something, termed "contact" or "impact loading", are the cause of most focal injuries, and movement of the brain within the skull, termed "noncontact" or "inertial loading", usually causes diffuse injuries. The violent shaking of an infant that causes shaken baby syndrome commonly manifests as diffuse injury. In impact loading, the force sends shock waves through the skull and brain, resulting in tissue damage. Shock waves caused by penetrating injuries can also destroy tissue along the path of a projectile, compounding the damage caused by the missile itself.
   Section::::Mechanism.
  
  Section::::Mechanism.:Physical forces.
  
  The type, direction, intensity, and duration of forces all contribute to the characteristics and severity TBI. Forces that may contribute to TBI include angular, rotational, shear, and translational forces.
   Damage may occur directly under the site of impact, or it may occur on the side opposite the impact (coup and contrecoup injury, respectively). When a moving object impacts the stationary head, coup injuries are typical, while contrecoup injuries are usually produced when the moving head strikes a stationary object.
  
  Section::::Mechanism.:Primary and secondary injury.
   The parts of the brain most affected by rotational forces are the midbrain and diencephalon. It is thought that the forces from the injury disrupt the normal cellular activities in the reticular activating system located in these areas and that this disruption produces the loss of consciousness often seen in concussion. Other areas of the brain that may be affected include the upper part of the brain stem, the fornix, the corpus callosum, the temporal lobe, and the frontal lobe. Angular accelerations of 4600, 5900, or 7900 rad/s are estimated to have 25, 50, or 80% risk of MTBI respectively.
   The exact mechanism for the injuries, especially contrecoup injuries, is a subject of much debate. In general, they involve an abrupt deceleration of the head, causing the brain to collide with the inside of the skull. It is likely that inertia is involved in the injuries, e.g. when the brain keeps moving after the skull is stopped by a fixed object or when the brain remains still after the skull is accelerated by an impact with a moving object. Additionally, increased intracranial pressure and movement of cerebrospinal fluid following a trauma may play a role in the injury.
  
  Section::::Mechanisms.
   One exception to this is when a very powerful temporary cavity intersects with the spine. In this case, the resulting blunt trauma can slam the vertebrae together hard enough to either sever the spinal cord, or damage it enough to knock out, stun, or paralyze the target. For instance, in the shootout between eight FBI agents and two bank robbers in the 1986 FBI Miami shootout, Special Agent Gordon McNeill was struck in the neck by a high-velocity .223 bullet fired by Michael Platt. While the bullet did not directly contact the spine, and the wound incurred was not ultimately fatal, the temporary cavitation was sufficient to render SA McNeill paralyzed for several hours. Temporary cavitation may similarly fracture the femur if it is narrowly missed by a bullet.
   Impact damage is the most common type of TBI, estimated at 75% of all TBI injuries, and results from the brain making physical contact with the skull. Impact-induced TBI is localized to the region of impact, although the depth varies by person and force of impact. While cerebrospinal fluid normally acts as a buffer between the brain and skull, during moments of extreme force (i.e. car collision or physical contact sports), this barrier can be overcome, resulting in an impact as the brain rams into the skull. During the moment of impact, some neurons die immediately from being crushed, while other neurons may be damaged to varying degrees and undergo apoptosis. Concussions are the most common and least damaging form of impact-induced TBI.
   There are five major features in conjunction with the definition:
  
  BULLET::::- Concussion may be caused either by a direct blow to the head, face, or neck or elsewhere on the body with an "implosive" force transmitted to the head.
  
  BULLET::::- Concussion typically results in the rapid onset of short-lived impairment of neurologic function that resolves spontaneously.
  
  BULLET::::- Concussion may result in neuro-pathological changes, but the acute clinical symptoms largely reflect a functional disturbance rather than a structural injury.
   Cerebral contusions are bruises to the brain caused by a direct blow to the head causing the brain to bounce against the inside of the skull and bruise brain tissue. The force of the blow causes either a tearing or twisting of the structure and blood vessels which hinders the ability of the receptors to send feedback to the brain. With the tearing or twisting of structure, the brain begins to swell and bleed. Since the brain cavity has no room to expand due to the swelling, bruises begin to form. Due to the nature of the injury, most of the contusive damage is found deeper in the brain.
   Changes indicative of SIS may begin occurring in the injured brain within 15 seconds of the second concussion. Pathophysiological changes in SIS can include a loss of autoregulation of the brain's blood vessels, which causes them to become congested. The vessels dilate, greatly increasing their diameter and leading to a large increase in cerebral blood flow. Progressive cerebral edema may also occur. The increase of blood and brain volume within the skull causes a rapid and severe increase in intracranial pressure, which can in turn cause uncal and cerebellar brain herniation, a disastrous and potentially fatal condition in which the brain is squeezed past structures within the skull.
   Studies on animals have shown that the brain may be more vulnerable to a second concussive injury administered shortly after a first. In one such study, a mild impact administered within 24 hours of another one with minimal neurological impairment caused massive breakdown of the blood brain barrier and subsequent brain swelling. Loss of this protective barrier could be responsible for the edema found in SIS.
  
  Animal studies have shown that the immature brain may be more vulnerable to brain trauma; these findings may provide clues as to why SIS primarily affects people under age 18.
  
  Section::::Diagnosis.
   Often caused by a blow to the head, contusions commonly occur in coup or contre-coup injuries. In coup injuries, the brain is injured directly under the area of impact, while in contrecoup injuries it is injured on the side opposite the impact.
   Coup injury may be caused when, during an impact, the brain undergoes linear acceleration and deceleration forces or rotational forces, causing it to collide with the opposite side of the skull. The injuries can also be caused solely by acceleration or deceleration in the absence of an impact. Contrecoup injury may be produced by tensile forces. These forces directly disrupt neurons, axons, other neural and meningeal structures, and blood vessels in local or diffuse patterns, typically leading to various cellular, neurochemical and metabolic effects.
  
  Section::::Features.
   For a period of minutes to days after a concussion, the brain is especially vulnerable to changes in intracranial pressure, blood flow, and anoxia. According to studies performed on animals (which are not always applicable to humans), large numbers of neurons can die during this period in response to slight, normally innocuous changes in blood flow.
   Studies have found that people who have received one concussion are at a higher risk to receive a concussion in the future.
  
  Section::::Pathophysiology.
  
  Concussion temporarily changes the brain's function.
  
  It is believed that the brain is left in a vulnerable state after concussion and that a second blow is linked to SIS. The actual mechanism behind the catastrophic brain swelling is controversial. A second injury during this time is thought to unleash a series of metabolic events within the brain.
   Head injuries can be caused by a large variety of reasons. All of these causes can be put into two categories used to classify head injuries; those that occur from impact (blows) and those that occur from shaking. Common causes of head injury due to impact are motor vehicle traffic collisions, home and occupational accidents, falls, assault, and sports related accidents. Head injuries from shaking are most common amongst infants and children.
   Contusions are identified with two forms of diagnosis: acceleration of the brain and direct trauma. A direct trauma injury is much more severe than an acceleration injury (in most cases) and requires much more intensive diagnosis and testing. The full extent of the injury may not be known until testing done in a hospital is complete.
   DAI can occur across the spectrum of traumatic brain injury (TBI) severity, wherein the burden of injury increases from mild to severe. Concussion may be a milder type of diffuse axonal injury.
  
  Section::::Mechanism.
  
  DAI is the result of traumatic shearing forces that occur when the head is rapidly accelerated or decelerated, as may occur in car accidents, falls, and assaults. Vehicle accidents are the most frequent cause of DAI; it can also occur as the result of child abuse such as in shaken baby syndrome.
   A skull fracture occurs when a bone in the skull breaks, and may penetrate the brain, tearing arteries, veins, or meninges, leading to the functional impairment of walking, communication, thinking, or feeling. Cerebral lacerations (tearing of brain tissue) or cerebral contusions (bruising of brain tissue) usually damage the cerebral cortex, resulting in permanent neurological deficits.
  
  Section::::Effects and management.:Life care plan.
   It has been postulated that the synkinesis is due to damage to cranial nerve nuclei, caused by peripheral nerve injury and the nuclear lesion releases evolutionarily older [neural] mechanisms with their tendency toward associated movements, and so primitive reflexes are not inhibited.
   BULLET::::- Basilar skull fractures, those that occur at the base of the skull, are associated with Battle's sign, a subcutaneous bleed over the mastoid, hemotympanum, and cerebrospinal fluid rhinorrhea and otorrhea.
  
  Because brain injuries can be life-threatening, even people with apparently slight injuries, with no noticeable signs or complaints, require close observation; They have a chance for severe symptoms later on. The caretakers of those patients with mild trauma who are released from the hospital are frequently advised to rouse the patient several times during the next 12 to 24 hours to assess for worsening symptoms.
  