10 Random Facts About the Cerebellum

The cerebellum is the structure I’ve spent most of my PhD staring at. Here are ten interesting facts about it.

1. Cerebellum is Latin for “little brain.”

It sits at the base of the skull, tucked beneath the cortex, and looks like its own separate organ. The name has stuck for about 500 years.

2. It’s small, but it has more neurons than the rest of the brain combined.

The cerebellum is only about 10% of the brain’s total volume, but estimates suggest it contains roughly 50–80% of all neurons in the human brain.

3. More than 99% of cerebellar neurons are granule cells.

Granule cells are among the smallest neurons in the human brain, measuring only about 4 to 8 micrometers in diameter. They are packed densely into the cerebellum, while other cell types, including Purkinje cells and several classes of interneurons, make up the remaining fraction.

4. In a midline view, it looks like a tree.

Inside the cerebellum, the white matter branches outward in a pattern that looks remarkably like a tree. It even has a name: arbor vitae, Latin for “tree of life.” It’s one of those anatomical structures that is simply beautiful.

5. Only vertebrates have one.

Insects and worms don’t have a cerebellum. In vertebrates, it plays a major role in balance, coordination, and motor control. Its cellular wiring is remarkably conserved across species, but its overall shape and functional roles vary depending on how each animal moves and lives.

6. It’s evolutionarily older than the cortex.

The cerebellum is a deeply ancient part of the brain. More than 400 million years ago, early fish already had a cerebellum working alongside an ancient brainstem to help coordinate movement and basic bodily functions. Meanwhile, the six-layered neocortex (the wrinkled outer layer we associate with advanced human thinking) did not exist yet. It emerged much later in mammals, roughly 160 to 300 million years ago, eventually expanding so dramatically that, in humans, it dominates the view of the brain.

7. The cerebellum can be divided into subregions structurally or functionally.

Structurally, it has three lobes and ten lobules. Functionally, it’s often divided into three broad zones: one for balance and eye movements, one for coordinating limb and body movement, and one for the large lateral hemispheres, increasingly linked to planning, timing, and cognitive functions.

8. It was long thought to only control movement. That picture has changed.

For much of neuroscience history, the cerebellum was understood mainly as a motor structure: coordination, balance, and timing. Since the 1990s, research has increasingly explored its role in cognition, language, and emotion. We now know the cerebellum is involved in a wide range of functions, from basic motor coordination to attention, working memory, language, and psychiatric conditions including autism, schizophrenia, and ADHD.

9. Its fMRI signal is not always straightforward to interpret.

A lot of human brain research relies on fMRI, which tracks the BOLD signal (an indirect measure of blood flow and oxygen use). But the cerebellum is wired very differently from the cerebral cortex. The signal we measure may reflect inputs arriving into the cerebellum and local processing inside it, rather than the final output it sends to the rest of the brain. In other words, when the cerebellum “lights up” in a brain scan, we have to be careful about what that signal actually represents.

10. We still don’t fully understand what it’s doing. And that’s what makes it exciting.

The cerebellum has one of the cleanest circuit layouts in the brain: a three-layered cortex, a relatively consistent set of cell types, and a clear path from input to output. But even with this elegant structure, some of its most important functions are still being worked out. Its roles in thought, language, attention, and psychiatric conditions remain active areas of research. In my own PhD, I study how the cerebellum is altered in 22q11.2 copy number variants — one small part of a much larger effort to understand this remarkable structure.