Sport-related Concussion (SRC): Brains Don’t Bounce!

By Dave Norris April 13, 2022

Helmet companies want us to believe the brain bounces back and forth after impact that is what causes a SRC. And the NFL has coined the term HeadHealth having us believe the term means they are addressing the brain and SRC. Neither are the whole truth!   The brain does not bounce and HeadHealth does not always address brain health.  In order to better understand this, we need to review the structural anatomy of the brain. It is absolutely crucial to understand that the brain is separate from the skull. When the literature regarding the mechanism of injury for sport related concussion talks about the head, in most instances, they are referring to the brain and the skull as one unit. But when defined by human anatomy the upper portion of the body consisting of the skull with its coverings and contents, including the lower jaw is defined as the head.  The brain might be contained in the skull, but the brain and head are two separate entities. You can move your head, and your head moves your brain. You cannot move your brain inside your skull.

The best way to see this is through a picture of how the brain is housed inside the skull.

The theories we have been led to believe about how the brain moves in a concussion I will call the “Bouncing Brain Theory” and the “Floating Brain Theory”. From Figure 1 and 2 above you can see the different layers and spaces between the skull and the cerebrum which lies just below the pia mater. In most pictures like this the cerebrum is labeled the brain (as in Fig. 1) which can be confusing when talking about a SRC because everything below the skull is part of the brain and includes the dura mater which lies directly next to the inferior side of the skull . .  Pictures like this also make us think there is a significant distance between the skull and the cerebrum and other structures of the brain. When in reality the space between the inferior skull to the cerebrum or other parts depending on the location is only .4 and 7 mm which is extremely small.[1] Most of the cerebrospinal (CSF) fluid lies in the subarachnoid space which leads to why we are made to think that the brain bounces or floats. These two facts: 1) the brain is separate from the skull; and 2) it is packed inside very tightly should make it easy to deduce that the brain cannot bounce back and forth. But there is other anatomy of the brain which also keeps the brain from bouncing or floating.

In addition to this, CSF and the ventricles contained within the brain integrate with the CSF contained in the spaces between the linings so that the CSF flows in a system designed to protect the structures of the brain enclosed by the skull (Fig. 3).

The ventricular system consists of four ventricles within the brain which do provide some buoyancy to the brain in order support and protect structures, but not enough to make it float around inside the skull. CSF surrounding the brain combined with flowing through the four ventricles and folds protect the brain by acting as a shock absorber and supporting the brain through suspension by providing buoyancy. [2,3]

And then you incorporate the corpus callosum into the mix which is the largest commissural tract in the human brain, with 200-300 million axons connecting the two cerebral hemispheres. [4,5] The corpus callosum (Fig. 4) is a thick bundle of myelinated nerve fibers 10 cm long and 25 mm high made up of white matter. [6] White matter has a higher elastic modulus than gray matter and myelination of nerves increases this modulus of elasticity. [7]  Elastic modulus is a quantity that measures a substance’s resistance to being deformed elastically when a stress is applied to it, and white matter (corpus callosum) has an elastic modulus which on the average is 39% stiffer than gray matter (cerebrum). [8,9] So not only  is the corpus callosum’s function to connect the two cerebral hemispheres for communication but based on its histological make up it is a supportive structure of the cerebrum during excessive motion caused by forces acting on the brain from the different impacts causing SRC.

Based on the brain’s mechanical and structural anatomy designed to absorb the kinetic energy created by its  own movement in reaction to impacts causing violent movement of the head the brain is not bouncing back and forth inside the skull as depicted in the movie “Concussion”.  The brain’s motion is always and only in reaction to the movements of the head. The head initiates all movement and contact of the brain that causes a SRC. 

My next blog will explain the types of impacts and the mechanisms of injury which cause SRC. 

Helmets protect heads. Kato Collar protects brains!

  1. In an average adult human, how much distance is between the skull and the brain?. https://www.quora.com/In-an-average-adult-human-how-much-distance-is-between-the-skull-and-the-brain
  2. Human Anatomy, 8th Edition. Martini, F. H., Timmons, M. J., Tallitsch, R. B.; Pearson Education, Inc. 2015, Glenview, IL
  3. Protection of the Brain. https://courses.lumenlearning.com/boundless-ap/chapter/protection-of-the-brain/#:
  4. Ethnicity Influences Corpus Callosum Dimensions. Hilda Nouri Hosseini,1 Mohammad Reza Mohammadi,2 Mohsen Aarabi,3 Narges Mohammadi,4 and Mohammad Jafar Golalipour; https://www.hindawi.com/journals/nri/2018/8916035/
  5. Axon position within the corpus callosum determines contralateral cortical projection  Jing Zhou, Yunqing Wen, Liang She, Ya-nan Sui, Lu Liu, Linda J. Richards, and Mu-ming Poo; PNAS July 16, 2013; 110 (29) E2714-E2723; https://doi.org/10.1073/pnas.1310233110
  6. Corpus callosum. https://en.wikipedia.org/wiki/Corpus_callosum#
  7. Brain stiffness increases with myelin content. J Weickenmeier 1, R de Rooij 1, S Budday 2, P Steinmann 2, T C Ovaert 3, E Kuhl 4  2016 Sep 15;42:265-272. doi: 10.1016/j.actbio.2016.07.040. Epub 2016 Jul 27. https://pubmed.ncbi.nlm.nih.gov/27475531/
  8. Elastic modulus. https://en.wikipedia.org/wiki/Elastic_modulus
  9. Mechanical properties of gray and white matter brain tissue by indentation. Silvia Budday, Richard Nay, Rijk de Rooij, Paul Steinmann, Thomas Wyrobek, Timothy C Ovaert, Ellen Kuhl

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