Concussion is an injury to the brain due to trauma that affects the brain's function. Trauma to the brain can result in many different ways - it doesn't always even require a a blow to the head! For instance, a car crash in which the seatbelt catches an individual's chest and causes the head to jolt forward. The most common cause of a concussion is a fall with a head strike. They are often seen secondary to sporting collisions (such as a high impact tackle in rugby). 

The brain and the skull are separated by cerebrospinal fluid. It's probably an exaggeration to say that the brain is 'floating'; but for the purpose of education, let's use the metaphor of an apple inside a big, tough water balloon.

If I strike the water balloon where the apple is resting against it, I will likely bruise the apple at the side of impact. When the brain injury is on the same side as the delivered trauma - this is called a coup injury (pronounced "coo" - like a pigeon). 

However, if I strike the water balloon and the apple is freely floating - the ripple effect of the force can send the apple hurtling to the opposite side of the balloon and bruise that side of the apple. Okay yes, the apple and water balloon metaphor is terrible (curse our lack of creativity) - but the point remains. When the opposite side of the brain is injured due to a trauma, this is termed a countrecoup injury. 

Sometimes, the mechanism of injury can make both types occur in a simultaneous accident - this is known as a coup contrecoup injury (like I said, doctors aren't the most creative) . In a car crash, imagine the skull making contact with the dashboard at a high speed - the initial impact can cause a coup injury, and the resulting force pushes the brain to the back of the skull and causes a contrecoup injury.

Head trauma leading to a concussion can cause a loss of consciousness, but mostly not. Initial symptoms can include confusion, amnesia (forgetfulness) and a headache. The headache can be accompanied by nausea, vomiting, ringing in the ears (tinnitus), and blurred vision. 
Other symptoms can even last for a few days - including sleep disturbance, a slight change in personality, concentration difficulty, ongoing memory issues and depressive mood. 

If it is indeed just a concussion, most people will fully recover within 2 weeks. If concussion is a result of a sporting injury, most leagues will require a waiting period of 2 weeks after the complete resolution of concussive symptoms and expect formal medical clearance. 

The reason for the delay, is to give the brain time to fully recover and ensure no lasting neurological deficits. Repeated trauma to the brain can absolutely be dangerous to your brain health - and so brings us to the topic of CTE.

CTE, or chronic traumatic encephalopathy, is a term made famous by the sufferers of repetitive trauma in the NFL (American Football players). NFL players weren't the first athletes to be linked to dementia-like symptoms in later life. In fact, decades earlier a term arose to describe this phenomenon in boxers - dementia pugilistica. 

CTE can only be truly confirmed in an autopsy - whereby under the microscope, we can see a build up of tau proteins - the same protein that is involved in the development of Alzheimer's disease. It is therefore no surprise, that many sports stars were incorrectly diagnosed with the Alzheimer's form of dementia when their memory began to fail them in later life - followed by changes in personality and erratic behaviour. 

The prevalence of "Alzheimer's disease" in former NFL players was something studied by Dr Bennet Omalu, a forensic pathologist, who bravely published his findings despite the manipulative legal measures the NFL brought upon him in an attempt to tarnish and suppress his research. The story was realised on the silver screen in 2015, played by Will Smith (let's not make a head trauma joke...Chris Rock). 

Consequentially, CTE is now a well recognised risk of contact sports and measures have been taken to minimise the damage taken in most sports - including the regulation of contact allowance in younger age groups. Maybe, let's permit the brain to grow before we hit it!