Concussion and traumatic brain injury

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Introduction0:00–0:37

Concussion, also called a mild traumatic brain injury, usually starts with a hit to the head. But what makes it a concussion is that the hit results in diffuse brain injury--meaning a large part of the brain is affected rather than a small specific area.
Also, concussions don’t cause obvious brain trauma that can be seen on imaging--like bleeding. Most concussions are the result of injuries from things like car crashes, falling down the stairs, recreational activities - like getting hit in boxing or getting tackled in a football game, or even violence in the home.

Physiology0:37–2:29

Now, the brain is made up of neurons--the functional cells of the nervous system. Neurons are made up of three main parts.
The dendrites, which are little branches off of the neuron that receive signals from other neurons, the soma, or cell body, which has all of the neuron’s main organelles like the nucleus, and the axon which is intermittently wrapped in fatty myelin.
When an electrical impulse called an action potential flows through a neuron, it causes the release of stored neurotransmitters into the gap between two neurons called the synapse.
This first neuron is called the presynaptic neuron. And the next neuron, called the postsynaptic neuron, and it has receptors for the neurotransmitters on its dendrites which trigger the opening of ion channels.
When the neurotransmitter glutamate binds to a postsynaptic neuron, it causes ion channels to open, and positively charged ions like sodium, potassium, and calcium enter the cell.
This is called an excitatory postsynaptic potential, or EPSP, because more positive charge inside the cell causes a depolarization to happen.
If the overall charge of the cell increases enough, it triggers an action potential, which is an electrical signal that races down the axon at speeds of up to 100 meters per second, triggering the release of more neurotransmitter at the next synapse.
In contrast to glutamate, neurotransmitters like GABA, or gamma-aminobutyric acid, binds to postsynaptic neurons and open ion channels that lets in negatively charged chloride ions, creating an inhibitory postsynaptic potential, or IPSP, which make the cell potential more negative by hyperpolarizing it.
So ultimately it’s a bit of a tug-of-war between stimulation from EPSPs and inhibition from IPSPs that ultimately decides if a neuron fires an action potential.

Causes2:29–3:56

Now, keep that idea in mind as we shift gears a bit. A key concept with concussions is to understand the mechanisms of injury.
With concussions, there are two main types of injuries--coup-contrecoup injuries and torque injuries. Lets use boxing as an example.
In coup-contrecoup injuries, imagine a boxer punched in the middle of his forehead. The initial blow causes a contusion, or bruise, at the point of impact.
This is called a coup injury. Further this causes both his skull and brain to move backwards.
As this happens, the momentum of the brain causes it to strike the inside of the back of the skull resulting in a contusion on the brain on the opposite side of the initial blow.
This is the contrecoup injury. A torque injury on the other hand is a bit different, and using our boxing example, let's say the boxer is now struck with a right hook--causing his head and neck to twist.
The brain suspended in the skull twists against the thinner stalk of the brain stem and stretching the structures within.
The area most affected is called the reticular formation, which is a network of nerves throughout the brainstem that controls the level of consciousness - which is how awake we are.
So when the boxer gets punched, the reticular formation gets stretched and damaged, potentially causing the person to pass out.
So compared to coup-contrecoup injuries, torque injuries usually result in more severe concussions, as well as the loss of consciousness.

Pathology3:56–5:54

With either type of injury, damage occurs on a microscopic scale at the level of the neuron, rather than causing visible damage to a specific part of the brain.
Both coup-contre-coup and torque injuries damage the long axons of neurons, and that allows potassium ions to leak out while sodium and calcium leak into the neuron, causing it to depolarize and fire an action potential.
Now, when millions of neurons get depolarized from this sort of damage, excessive glutamate gets released, generating a ton of EPSPs, causing even more action potentials to be generated!
So in short, the injuries cause a huge increase in the brain’s metabolic activity - and that increases the brain’s demand for blood.
Now at that very crucial time when the brain needs more blood, for some unknown reason, blood flow transiently decreases right after a concussion - so there’s actually a decreased supply of blood.
An increase in demand and a decrease in supply is a bad combination, and it leads to neurons being starved of energy, and it can remain that way for a few hours up to a day or two.
Because a concussed brain is in a state of glucose starvation, subsequent head trauma can cause secondary impact syndrome.
Secondary impact syndrome is a rare disorder that results in rapid cerebral edema, or brain swelling. But the cause is not well understood.
Another consequence of repeated concussions is chronic traumatic encephalopathy, which is when certain proteins become dysfunctional and accumulate in neurons over many years.
These proteins include tau proteins which also cause alzheimer's disease. Build up of dysfunctional proteins can cause neurons to die, and if that happens on a large scale, the brain can atrophy or shrink while the ventricles, or fluid-filled cavities in the brain get larger.

Symptoms5:54–6:12

The main symptoms of an acute concussion are confusion, memory loss, and sometimes loss of consciousness, and usually these symptoms improve within 1 or 2 days.
Symptoms that may appear a bit later include headache, dizziness, and nausea or vomiting. The diagnosis of a concussion is mainly based on the description of the injury and the symptoms.

Diagnosis6:12–6:25

Usually a head CT scan is normal, and there are no focal neurologic deficits. The treatment for a concussion is rest and making sure that they have completely recovered from the concussion before returning to an activity.

Treatment6:25–6:33

Review6:33–7:09

All right, as a quick recap, a concussion is usually the result of either a coup-contrecoup or torque injury, and typically torque injuries are more severe and more often result in a loss of consciousness.
After an injury, there are more EPSPs in the concussed brain creating a larger demand for blood, with a subsequent decrease in blood supply, and that results in the neurons getting starved for energy and getting damaged.
The main symptoms are confusion, memory loss, and loss of consciousness, followed by a headache, dizziness, and nausea or vomiting.