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Tuesday, May 6, 2008

The Cerebellum

I'm studying for my Motor Control Systems course tonight, and thought I might make a post about one of the regions of the brain that the course focuses on: the cerebellum. The cerebellum is a bit of a conundrum for me in my understanding of the brain, which makes it fascinating. The cerebellum is the "little brain" that sits at the back of a person's head and looks a bit like the head of a piece of cauliflower in anatomical drawings. While volumetrically much smaller than the rest of your brain, the vast majority of a person's neurons are in the cerebellum. Oddly enough, despite the concentration of neurons, it is actually possible to survive without a cerebellum. Quality of life will be dramatically reduced, as cerebellar damage results in a condition known as dysmetria, which basically means that movements are not properly timed. For example, if one were trying to lightly toss a ball while suffering from dysmetria, one may just as well hurl the ball with great force or even release it at a random moment during the arm swing. One of the other oddities about the cerebellum is that, despite it being virtually entirely involved in motor control and that entire post I previously made about the brain being primarily set up with contralateral control, most cerebellar functions are ipsilateral in nature.

However, what puzzles me the most about the cerebellum is that it is made up in a large proportion by constantly active neurons. Basically, the way in which it functions to deliver precise motor timing is that a set of nuclei project to several different motor areas (interestingly, the nuclei clearly follow an evolutionary pattern in which the most medial, and therefore oldest, nuclei project to the older motor areas of the brain. The most medial, the fastigial nucleus, projects to two of the spinal tracts, the next in line, the interposed nucleus, projects to the red nucleus and the ventralis intermediate nucleus of the thalamus, and finally the dentate nucleus projects to both the ventralis intermediate and the ventrooralis nuclei of the thalamus). The output from the cerebellum to these motor areas triggers activity in the corresponding motor areas (the target motor areas are where the movement patterns are actually stored, which is why a person can still survive without the cerebellum. Only the precise timing of those movements is lost with destruction of the cerebellum). However, what is strange about the architecture of the cerebellum is that the neurons of the cerebellar nuclei are tonically active, which means that they are constantly firing. The only thing that prevents their continuous activity is the set of cerebellar cells, the Purkinje cells located in the cerebellar cortex, which inhibit the cerebellar nuclei and are also tonically active. It all gets extremely confusing (to me, at least) at this point, but it basically boils down to a chain of constantly active excitation and inhibition (though to varying degrees, which is where the computational part of the cerebellum must come in), which is extremely metabolically expensive. That, to me, is where the biggest mystery of the cerebellum lies. What makes such a ridiculously expensive system of tonic activity computationally worth the metabolic cost it incurs? Perhaps someday I will know the answer to that question, but for now the cerebellum remains an intriguing oddity in my mind.