If an observer wants to establish what is actually happening during a brain concussion, the very nature of the brain, its elastic characteristics must be presented.
“Brain tissue springs back to its original position after transient deformation, indicating that many cellular processes are elastic and under resting tension.” NATURE /Vol 385/ 23 January 1997 A tension-based theory of morphogenesis and compact wiring in the central nervous system by David C Van Essen
The macroscopic mechanical properties of the brain in vivo are consistent with axon-like neurites extended by neurons in vitro. The macroscopic mechanical properties of neural tissue depend on the active and passive mechanical characteristics of its microscopic constituents. In other words, macro shape depends on a scalable relationship to microscopic constituents of shape assembly. The most relevant neural structures in the central nervous system are axons, dendrites and glial processes.
In vitro, if neurites are grown on a sticky type surface they generate substantial mechanical tension (stress). If a neurite is transiently stretched, its length increases in proportion to applied tension, indicating simple elastic behavior. When resting tension is completely released, a neurite initially shortens only slightly, if further pushed, it becomes wavy like a compressed rubber band. Under sustained stretching neurites behave viscoelastically by relaxing passively to a lower level in a short time frame. Active elongation occurs when sustained tension is maintained above a threshold level, active retraction occurs when tension is fully released. Collectively these mechanical in vitro properties allow neurites to adjust their length by a negative feedback mechanism that tends to maintain a steady tension.
What are the patterns of brain tissue growth?
The prospect of a major role for tension arises because of two basic anatomical characteristics of central nervous system tissue.
Like wood which has a preferred grain, brain has preferred directional anistropies in terms of the orientation of axons, dendrites and glial processes. If these processes are under tension, their springiness will make the tissue elastic, but the elasticity will not be uniform in all directions. Instead the mechanical compliance should be lowest (resistance to stretching highest) along the axis of preferred orientation.
Does the reader recall a previous post concerning diffusion tensor imaging from a special technique with Magnetic Resonance Imaging that measures the directionally dependent property from the random motion (Brownian motion) of water molecules in the brain? Water molecules located in fiber tracts are restricted in their movement similar to being caught in a dense crowd, (they move more in the direction parallel to the tract then in the other two dimensions perpendicular to it). Water molecules dispersed in the rest of the brain have less restricted movement and move in any direction, uniform motion in all directions, isotropy.This difference in fractional anistropy is exploited to create a map of the fiber tracts in the brain of the individual. Directional cortical tract functional alterations can now be tracked following minor traumatic brain injury using MRI diffusion tensor imaging.
Tension based cellular morphogenesis
‘Cortical flows of actomyosin are central to many processes in cellular and developmental biology. In the one-cell Caenorhabditis elegans embryo, anteroposterior polarity, a prerequisite for asymmetric cell division, is established by large-scale flow of the actomyosin cortex that segregates cortical polarity proteins between the anterior and posterior domains. The underlying forces and physical principles behind long-range flow remain unclear. Mayer et al. have devised a novel method to measure cortical tension (total mechanical tension) and find that cortical flows are driven by contractility of the actomyosin network. The direction of flow depends on anisotropies in the cortical tension, and long-range cortical flow occurs only if the cortex is sufficiently viscous.’ Nature | Letter /Vol 467/ Issue 7315/617-621 (30 September 2010) Anisotropies in cortical tension reveal the physical basis of polarizing cortical flows Mirjam Mayer, Martin Depken, Justin S. Bois, Frank Jülicher & Stephan W. Grill
Anistropies in cortical tension drive cortical flow migration which contradicts previous mechanism proposals. Balanced tension is the critical criteria toward maintaining morphogenesis within the growing brain.
To cope with embryonic growth and shape changes, axons must therefore adjust their length continually as the cortex folds, some growing faster some actually shortening. Tension, working through passive viscoelasticity plus active growth and retraction is a suitable feedback signal for regulating axonal length. Tumors growing inside the brain cause gradual tissue deformation without axons breaking. CNS synapses represent strong adhesiveness between pre and post-synaptic partners are unlikely to burst from the observation if brain tissue is mechanically homogenized yields a large pool of synatosomes which are sub cellular components with both pre and post-synaptic membranes that remain tightly opposed.
Morphogenetic shape changes involve interactions among three factors: the local forces that cells generate as they grow and migrate; the mechanical properties of the immediately surrounding tissue; any extrinsic forces arising from tension or pressure generated at a distance.
The important balanced tension along axons in the white matter appears to be the driving force for cortical folding. The cortical sheet is physically tethered from only one side, initially by radial glial processes, followed by connections between cortex and various sub-cortical nuclei. The balanced cohesive tension would be inwardly directed processes against intra-ventricular hydrostatic CSF pressure ensuring the cortical mantle remains tightly wrapped around the sub-cortical interior. Specific cortico-cortical projections are established early, since neurons that send axons across the corpus callosum even while they are migrating toward the cortex. Tension along obliquely oriented axonal trajectories between nearby cortical areas would generate tangential force components that tend to induce folds at specific locations in relation to real adjacent boundaries. When we measure internal CSF pressure are we pausing to consider the tissue tension pressure necessary to balance it from a tensegrity point of view?
The brain cortex can fold in either of two ways. In an outward fold, the crease is directed away from the interior, forming the crown of a gyrus and reducing the distance within white matter between opposite banks of the fold. tension would pull strongly interconnected regions towards one another, forming an outward fold along the common border. In an inward fold, the crease is directed towards the interior, forming the fundus of a sulcus. Geometrical constraints require an inward fold between each pair of outward folds. tension induced folding should contribute to compact wiring for the cortex as a whole.
‘When a paperback book is folded, adjacent pages slid relative to one another. When the cortex folds, sliding between layers should also occur, but to a lesser degree because of stretching and shearing forces, which alter cellular morphology and the thickness of different layers. Along inward folds, cells in deep layers should be stretched radially, making these layers thicker. It is suggested that differential growth of the cortical layers is a consequence of the forces that have induced the folding.’
The brain is built under tension and pressure. According to the perspective of D’Arcy Thompson, he discussed how tension balanced with pressure can interact with structural anistropies and asymmetries to determine the shape of biological structures applied to peripheral body parts and plants. Perhaps with the extension of Thompson’s original 1917 observations, a 2011 compatible tension-based morphogenesis can now be advocated for central nervous system tissue structure and development as outlined by Van Essen, in the tensegrity frame of view. The tensegrity of the brain creates a shape tension/strain that will self rotate within the skull during a brain concussion. Unlike the springing back to shape utilizing the inherent brain elasticity, the shape rotation distortion onto medullary vagal connections will reveal itself as a valid diagnostic with altered cardiac pacing anomalies and central bone metabolic changes.