Are the Brain's Axonal Corners Particularly Vulnerable to Breakage?

Like the leg of a table breaking off where the leg meets the table top, cerebral fibers in the brain have sharp branches and turns, which might make them particularly vulnerable to inertial trauma. Dr. Van Wedeen explains with slides.

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This is a picture of the amount of axonal stretch that was going on from the twisting motion--from the left to right motion. So you have the left-to-right motion--here, and here in the splenium of the corpus collosum and in the genu of the corpus collosum. You have--the red is the amount of stretch calculated for the axons at that point. Where did this come from? We actually have practiced exactly the same thing in the heart. The way this whole diffusion business got started was we were taking pictures of the deformation of the heart when it beat, and we wanted to know how much the fibers shorten. And to calculate that, all you needed was the fiber direction. So we were doing this in the heart to calculate fiber shortening. Exactly the same story in the brain, except now it's passive elongation of axons due to inertial deformation. How will the grid structure affect this understanding of axonal injury? Well, this is the news. This is a picture taken from Wendell Kreig, 1953. The point is, the red is the corticospinal tract, and look at the way--everybody draws the corticospinal tract as a continuous arc going from the top to the bottom, and these are the transverse fibers in the collosum in green. But look what's really happening. They're all making 90 degree turns. They don't just go straight down. They follow this grid structure down to the cellular scale. Corners are mechanically fragile. They're vulnerable--you break the leg off a table and it breaks at the table. Because when you have an impedance mismatch that's where the injury occurs. Is it possible that these corners are actually mechanically vulnerable? Because you have more strain components that can break them.
Posted on BrainLine March 18, 2013.

Produced by Brian King, Vicky Youcha, and Lara Collins, BrainLine.

About the author: Van Wedeen. MD

Van Wedeen, MD is associate professor in Radiology at Harvard Medical School, assistant neuroscientist at Massachusetts General Hospital, and director of Connectomics at Martinos Center, Department of Radiology, Massachusetts General Hospital.

Van Wedeen