How can a tenth of your brain hold most of its neurons?

By being stuffed with tiny cells. The cerebellum takes up only about 10% of the brain's total volume, yet because of its huge number of tiny granule cells it contains more neurons than the rest of the brain combined, about 3.6 times as many as the neocortex, a ratio that holds across many mammal species. Its surface hides another surprise: the finely spaced parallel grooves conceal a thin, continuous layer of tissue folded tightly like an accordion.

The cerebellum, Latin for "little brain," is found in the hindbrain of every vertebrate, and in some animals, like mormyrid fishes, it can be as large as the cerebrum or larger. In humans, it does not start movements, but it takes in signals from the spinal cord and other brain regions and uses them to fine-tune coordination, precision and timing; damage to it causes problems with fine movement, balance, posture and motor learning. That link was uncovered gradually: in 1809, Luigi Rolando showed that damaging it causes motor disturbances, and later Jean Pierre Flourens found that animals with cerebellar damage could still move, but clumsily, with strange movements and an awkward gait, and that they could recover nearly completely unless the damage was very extensive.

At the microscopic level, cerebellar neurons called Purkinje cells receive two very different kinds of input: thousands of weak inputs from granule cell fibers, and one single, extremely strong input from a "climbing fiber." The influential Marr-Albus theory treats that climbing fiber as a teaching signal that brings about a long-lasting change in the strength of the weaker inputs.