Neurobiology of Concussion - Part 2

 

This is Part 2 in a series.
  • Part 1 - Quick anatomy lessons

  • Part 2 - How the brain is organized

  • Part 3 - The effects of a concussion injury

  • Part 4 - Symptoms explained

 

How does this information help me?

As you read through this article you may be thinking:  “this is all fine and good, but how does it help me?”.  We believe knowledge is power and having better understanding about what’s happening inside our head can be empowering and benefit your recovery.  You can dive as deeply as you want into the information in these articles.  It is here to explore at your leisure.

What part of a brain is affected by concussion?

In simplistic terms, during a concussion the connections and networks throughout large areas your brain can get disrupted which means they don’t work as well (or have to work much harder).  We don’t normally see focal damage, which is injury to a specific part of the brain that can occur in more severe traumatic brain injuries and stroke.  Focal injuries are the types that usually show up well on scans like CT and MRI. Occasionally contusions (or brain bruises) do occur in concussion which can be seen on CT and MRI - but contusions don’t happen in all concussions.  In a concussion injury what actually happens is a diffuse injury which means there are tiny changes distributed throughout the brain.   Diffuse injuries in concussion are often too small to see on traditional diagnostics but they can have major effects on how your brain works (diffuse injuries also happen in more severe TBI).  

What do we really know about brains?

Depending on who you ask, some might say we know a lot about brains, and other people might say we know very little.  It’s safe to say however that there are many big questions still unanswered like:

  • What actually is consciousness?  How is our experience put together?
  • How does thinking and memory really work?
  • What’s happening when we sleep? Why do we even sleep anyway?

While answers to those big, philosophically charged mysteries are still elusive, there are a few things that we can be fairly confident about:

  • Brains (along with the rest of your nervous system) are built mostly of neurons and glial cells along with all the rest of the stuff that make up the rest of the body (connective tissue, blood vessels,  certain immune cells, etc...)
  • Brains sit inside skulls  but are actually nestled inside of a few  layers of membranes called meninges which also hold the cerebrospinal fluid.   There’s actually very little space and room to move between the brain and the skull (which is a good thing - it’s tucked in nice and cozy).
Less like this...

Less like this...

More like this!

More like this!

Brain parts

  • Brains have many distinct parts (depending on how you classify them).  Brains can be broken down into areas based the physical parts and/or on their major functions:
    • Brain stem - the top of the spinal cord as it “enters” the rest of the brain.  10 of your 12 cranial nerves live here.  The brain stem carries the pathways to and from the rest of your body and controls things like breathing, heart rate, sleep, consciousness and more (kind of important stuff).  The brain stem is pretty tough and resilient but mild injury might occur to it during some concussion events.
    • Thalamus & Hypothalamus - usually considered part of the cerebrum but mentioned here for emphasis.  The thalamus is a major hub in the brain that relays information (from all senses but smell) with the cerebral cortex. The hypothalamus sits right below the thalamus and is important because it links your nervous system with the endocrine system.   Connections in these areas might be disrupted in some concussion injuries.
    • Cerebellum - sits behind the brain stem and is involved in motor coordination and plays a roles in attention and motor learning.  The cerebellum might also be injured in some concussions.
    • Cerebrum - the uppermost part of the brain which actually in itself contains many structures including the cerebral cortex and limbic system.  This is primarily where thought and the experience of consciousness probably happen, along with voluntary action (eg. motor control), processing of sensory information, memory, learning,  language and emotion (you know...the stuff that makes us human).  The cerebral cortex usually gets all of the attention since it’s the most obvious part of the brain - but concussion injury doesn’t only affect the cerebral cortex.

 

 

Is it so simple?

Neuroscience has come a long way at describing the structure and function of parts of the brain.  There continue to be exciting new discoveries every day.  The more that’s learned about the brain, the more complex it seems to be.

Take initiatives  like the human connectome project for example: researchers are seeking to understand how each part of the brain connects and interacts with all the rest - and the results are astounding (see this cool brain-area relationship map)

White matter fiber architecture of the brain. Measured from diffusion spectral imaging (DSI). The fibers are color-coded by direction: red = left-right, green = anterior-posterior, blue = up-down. www.humanconnectomeproject.org

White matter fiber architecture of the brain. Measured from diffusion spectral imaging (DSI). The fibers are color-coded by direction: red = left-right, green = anterior-posterior, blue = up-down. www.humanconnectomeproject.org

Functional brain areas mapped in the connectome project via Washington University in St. Louis

Functional brain areas mapped in the connectome project via Washington University in St. Louis

Looking at the complexity of brain connections, we can’t be so certain that there are clear cut functions of large, ambiguously defined regions of the brain.  For most of the history of studying the brain, it was believed that functions are localized to specific and distinct regions of the brain.  You might be familiar with images like the ones below, describing primary ‘lobes’ or surface maps of the brain:

Broadmann areas - Cortical maps made almost 100 years ago but still largely relevant and used today.

Broadmann areas - Cortical maps made almost 100 years ago but still largely relevant and used today.

Oversimplified picture of large brain areas based mostly on the physical structure of the brain.

Oversimplified picture of large brain areas based mostly on the physical structure of the brain.

These views are still very popular today, and while there is no doubt that different areas of the brain specialize in certain functions - it’s way too simplistic.  

The reality is that it’s the connections that matter.  Brain functions often appear to be distributed across large, interconnected parts of the brain and it’s the relationship between parts that is so important. The functional networks of the brain are even somewhat unique to each person and can change over time (in a process of neuroplasticity, and the growth of new neurons - called neurogenesis). 

 

So, to summarize:

  1. In general, functions of the cerebrum are sometimes localized to specific regions but the overall networks in the brain might be more  important.  The brain is less like a computer and more like the internet.

  2. Concussion doesn’t usually cause damage to one specific region of the brain (But, impacts might cause specific damage to other structures like your inner ear or optic nerves).  Injury to the brain is typically diffuse, meaning subtle changes happen across large networks.  Diffuse injuries in concussion are often too small to see on traditional diagnostics but they can have major effects on how your brain works NOTE: getting checked out by a doctor after injury is critical to rule out contusion or other focal damage and ensure there are not more serious things going on.

  3. Any area of your brain can be affected by a concussion injury, even the deeper areas like the thalamus and brain stem.

  4. Mixed up networks in the brain don’t work as well, and in some cases might need to work a lot harder that normal.  This partially explains symptoms after concussion including fatigue, headache and difficulty thinking.  Biochemical issues after concussion injury also contribute to these symptoms, which we’ll discuss next time.

 

REFERENCES

  1. Humanconnectomeproject.org
  2. Humanconnectome.org
  3. Banks, S. D., Coronado, R. A., Clemons, L. R., Abraham, C. M., Pruthi, S., Conrad, B. N., ... & Archer, K. R. (2016). Thalamic Functional Connectivity in Mild Traumatic Brain Injury: Longitudinal Associations With Patient-Reported Outcomes and Neuropsychological Tests. Archives of physical medicine and rehabilitation.
  4. Bigler, E. D., & Maxwell, W. L. (2012). Neuropathology of mild traumatic brain injury: relationship to neuroimaging findings. Brain imaging and behavior, 6(2), 108-136.
  5. Churchill, N., Hutchison, M. G., Leung, G., Graham, S., & Schweizer, T. A. (2016). Changes in functional connectivity of the brain associated with a history of sport concussion: a preliminary investigation. Brain injury, 1-10.
  6. Hendelman, W. (2015). Atlas of functional neuroanatomy. CRC press.
  7. McCrory, P. (2001). The nature of concussion: a speculative hypothesis. British journal of sports medicine, 35(3), 146-147.
  8. McCrory, P., Meeuwisse, W. H., Aubry, M., Cantu, B., Dvořák, J., Echemendia, R. J., ... & Sills, A. (2013). Consensus statement on concussion in sport: the 4th International Conference on Concussion in Sport held in Zurich, November 2012. British journal of sports medicine, 47(5), 250-258.
  9. Satterthwaite, T. D., & Davatzikos, C. (2015). Towards an individualized delineation of functional neuroanatomy. Neuron, 87(3), 471-473.