The Lies You Learn in College: Motor Cortex Edition

What is wrong with this picture?

It's wrong for one. What this picture represents is what is called the "motor homunculus." The motor homunculus is the idea that on a particular gyrus of the brain -- the precentral gyrus -- all muscle are represented by discrete pools of neurons. Activation of these discrete pools of neurons leads to activation of the muscles.

This model was generated when a neurologist named Wilder Penfield, while performing brain surgery, used an electrode to zap different regions of the brain. When he would zap this gyrus at different points, it would result in discrete muscle movements.

However, Rathelot and Strick show that the representation is not half as discrete as we thought. In order to do this they inject rabies virus into particular muscles in the macaque hand. Rabies virus enters the axons that innervate those muscles and travels back up into the brain to label the neurons associated with those axons. It is what we call a retrograde tracer.

What they found when they did this is that the parts of the motor cortex that were labelled for each muscle did not fall into discrete groups -- though it did fall in the same general area. Representations for different muscles were not nearly so segregated as the classical model would hold. Also, oddly enough, some of the axons innervating the muscle originated in what was supposed to be the sensory cortex (postcentral gyrus).

I was so lied to; this brings all the other things I learned into question.

Hat-tip: Faculty of 1000.