Cerebellum

The cerebellum is a vital part of the hindbrain in all vertebrates. Its name comes from the Latin term for "little brain."

While it is often smaller than the cerebrum, it can be larger in certain animals, such as mormyrid fishes. In humans, the cerebellum is essential for motor control and cognitive functions like language and attention. It also helps regulate emotional responses, including pleasure and fear.

Although it is involved in many areas, its role in movement is its most established function.

The cerebellum does not initiate movement itself. Instead, it acts as a fine-tuning system for motor activity. It receives sensory input from the spinal cord and other brain regions. The cerebellum then integrates these signals to ensure coordination, precision, and accurate timing. If the cerebellum is damaged, humans may experience disorders in posture, equilibrium, and fine movement. It is also necessary for motor learning, specifically for adjusting to changes in sensorimotor relationships.

Anatomically, the cerebellum is attached to the bottom of the brain. It sits underneath the cerebral hemispheres and is separated from them by a layer called the cerebellar tentorium. The surface of the cerebellum features many fine, parallel grooves. These grooves hide a thin layer of tissue that is folded tightly like an accordion. This layer is known as the cerebellar cortex. Underneath this gray matter lies the white matter, which has a branched, tree-like appearance. This white matter is often called the arbor vitae, or "tree of life."

The cerebellum is divided into two hemispheres and a narrow midline zone called the vermis. Scientists use large folds to divide the structure into ten smaller lobules. Functionally, the cerebellum can be divided into three main sectors. The smallest is the flocculonodular lobe, or vestibulocerebellum, which helps with balance and spatial orientation. The middle sector is the spinocerebellum, which fine-tunes limb movements using input from the spinal cord. The largest part in humans is the cerebrocerebellum. This lateral sector receives input from the cerebral cortex and helps with planning movements and cognitive tasks.

Inside the cerebellum, the organization of neurons is incredibly dense. The cerebellum contains more neurons than the rest of the brain combined. This is due to the massive number of tiny granule cells. Despite this high cell count, the cerebellum only accounts for 10% of the total brain volume. In fact, there are about 3.6 times as many neurons in the cerebellum as there are in the neocortex. This ratio is consistently found across many different mammalian species.

Two specific types of neurons, Purkinje cells and granule cells, dominate the cerebellar circuit. The cerebellar cortex is organized into three distinct layers. The bottom layer is the granular layer, which is packed with granule cells. The middle layer is the Purkinje layer, containing the cell bodies of Purkinje cells. The top layer is the molecular layer, which contains the dendritic trees of the Purkinje cells.

Purkinje cells are highly distinctive because their dendrites branch very profusely. These cells receive two different types of input: thousands of weak signals from parallel fibers and one extremely strong signal from a climbing fiber.

Theoretical models help explain how the cerebellum learns through these inputs. Scientists David Marr and James Albus developed models based on how these cells interact. In the Marr–Albus theory, the climbing fiber acts as a "teaching signal." This signal induces long-lasting changes in the strength of the parallel fiber inputs. This process is linked to synaptic plasticity, which is how connections between neurons change. While observations of long-term depression in these fibers support these ideas, the validity of these theories remains a subject of debate.