We have two eyes, which is handy for wide-angle vision, two hands for extra strength to lift heavy loads, and two legs for balance and controlled motion. So why not have two brains instead of one? It turns out that in some sense; we do have a second brain. It is called the cerebellum, also nicknamed “the little brain.”

The cerebellum is a four-inch lobe lying at the back of the brain. Despite its small size, it houses an astonishing three-quarters of the brain’s nerve cells or neurons, around 50 billion neurons to be more precise.

For a long time, it was thought that the cerebellum performs only one function – to coordinate movement, a seemingly humble task compared to the other two major parts of the brain – the cerebral cortex or cerebrum, which is responsible for reasoning, thought, learning, decision-making, and language, and the limbic system, which is responsible for memory, emotions and stimulation or arousal. But recent neuroscience research has unveiled startling evidence that the cerebellum deserves to be given a lot more credit for what it does. The gist of the latest research findings is that in addition to controlling movement, the cerebellum also regulates complex behaviors, social interactions, working memory, learning, emotions and more.  In other words, that nondescript four-inch lobe at the back of our brains is in fact a busy hub of sensory and emotional processing.

One of the first clues that the cerebellum is like a second brain comes from the case of a young ice-skater who, in 1991, experienced a fall while ice skating. A CT scan revealed a tumor in her cerebellum. After it was removed, she was a completely different person. The bright college student had lost her ability to write with proficiency, do mental arithmetic, name common objects or copy a simple diagram. Her mood also changed; she became dour, hid under covers and behaved inappropriately such as undressing in the corridors and speaking in baby talk. Her social interactions, including her ability to recognize familiar faces, were also impaired. This and other similar cases led neuroscientists to suspect that higher order cognitive functions as well as emotional regulations did not simply reside in the cerebral cortex and the limbic system; the cerebellum, too, play a significant role in modulating these functions. This was a stunning revelation, upending the “standard model” of the brain as essentially an organ whose key functions are shared between the cerebral cortex and the limbic system.

In order to do what it does, the cerebellum must possess a special structure, and it does.  The vast majority of neurons in the cerebellum are relatively large nerve cells known as Purkinje cells. Each Purkinje cell is shaped like a coral fan but flattened and almost two-dimensional. The fan’s “blades” are the neuron’s dendrites, the finger-like structures that radiate out of a neuron) whose role is to receive incoming signals from elsewhere in the brain (see figure below).

Purkinje cells are stacked atop each other in a tight bundle. Running perpendicularly through them are thousands of axons which are the brain’s transmission cables for electrical impulses. Each axon connects with the dendrites of tens of thousands of Purkinje cells as shown below.

It is this unique neural circuitry that endows the cerebellum’s 50 billion neurons with the ability to crunch an enormous amount of information coming from other parts of the brain, information that it processes to regulate body movements and emotions. The fluid movement of a ballet dancer would not be possible without the astonishing ability of her cerebellum to control muscular movements as she leaps across the stage, while rapidly processing spatial information, tracking the changing position of her limbs and maintaining balance and poise, all at the same time, and to do so in the right context, with the appropriate emotion and motivation. Now we know why the cerebellum has so many neurons. 

Note: This post is adapted from a Quanta Magazine article dated January 23, 2024 entitled, “The brain region that controls movement also guides feelings” by R. Douglas Fields.