The Cerebellar Peduncles

If the cerebellum is the “tree of life” due to its branched interior, then the cerebellar peduncles are the roots that anchor the cerebellum firmly to the pons, extended spinal cord, and neighboring brain regions. The three thick bundles of fibers are direct extensions of the white matter of the cerebellum. They transport information from all possible parts of the body and carry outgoing signals to other brain regions. Like powerful cables, the cerebellar peduncles connect the otherwise isolated cerebellum to the rest of the brain.

The cerebellum can be thought of as a major consumer of information, as there are significantly more fibers that bring information in than those that carry signals out of the cerebellum: there is one outgoing fiber for every 40 incoming fibers. After all, the cerebellum needs to keep track of what is happening in the body at all times. To do this, it needs a great deal of input, for example from the vestibular system, receptors of proprioception, spinal cord, and motor cortex, both about movements that are currently taking place and those that are being planned in the cerebral cortex. Once it has coordinated the various signals, the corrective output that is the result of its workrequires significantly less transmission capacity. In short, the cerebellum thrives on input – only in this way can it continuously monitor movements and adjust them with millimeter precision.

Whoever once gave the three cerebellar peduncles their names was not very imaginative: they are simply called the upper, middle, and lower peduncles, or in Latin, pedunculus cerebellaris superior, medius, and inferior.
The upper stem connects the cerebellum to the midbrain, the middle stem to the pons, and the lower stem to the extended spinal cord, the medulla oblongata. A thin layer of white matter, the velum medullare superius, stretches between the upper cerebellar peduncles. It forms the roof of the fourth ventricle and borders the tectum of mesencephalon at the rear.

The middle cerebellar peduncle is located further to the side. It is the strongest of the three fiber bundles, and, unlike the other two peduncles, contains only afferent fibers. It developed later in evolution and has pushed itself between the upper and lower cerebellar peduncles. In the process, it also pushed apart some fiber bundles that belong together functionally, namely the pathways that carry information from the spinal cord. As a result, only some of the fibers from the spinal cord to the cerebellum now run through the inferior cerebellar peduncle, while the rest of this pathway is routed through the superior cerebellar peduncle. Medical students are explicitly taught this as an exception to the rule in exam swoting books. Otherwise, the pathways can be logically assigned to the three cerebellar peduncles according to their place of origin.

Most signals reach the cerebellum via the middle cerebellar peduncle. It is the largest bundle of fibers and contains only incoming pathways. These ultimately originate in the cerebral cortex, are connected in the pons, and provide the cerebellum with information about the movements the cerebrum is currently planning.

Most of the information from the rest of the body otherwise runs through the inferior cerebellar peduncle: it informs the cerebellum, for example, about the position of a leg, the trunk, or an arm, how they are moving, and what the vestibular system is reporting. Specifically, it carries fibers from the vestibular nuclei, the brainstem, the spinal cord, and the inferior olivary body.

Finally, there is the upper cerebellar peduncle. It not only transmits parts of the spinal cord pathways, but also signals from nuclei in the midbrain, such as the nucleus ruber. It thus supplements the information from the spinal cord and brain stem, and provides the cerebellum with an even more complete picture of the body's posture and movement.
All fibers that reach the cerebellum end as climbing fibers or mossy fibers in the cerebellar cortex, where they are integrated into the dense network of circuits and further processed.

The signals processed by the cerebellum travel from its cortex to the cerebellar nuclei inside. Almost all fibers that leave the cerebellum originate here; only a few originate directly in the cerebellar cortex.

Most of the output fibers leave the cerebellum via the upper peduncle. These pathways lead to motor centers in the brain stem, to the red nucleus, and via the thalamus also to the cerebral cortex. A smaller portion of the outgoing fibers runs back to the brain stem via the inferior peduncle, including to the vestibular nuclei and the inferior olivary body. In this way, the cerebellum can also regulate its own inputs and at the same time influence the balance system.

A special feature is the mode of action of these pathways: they run ipsilaterally, i.e., they act on the same side of the body – the right hemisphere of the cerebellum controls the right side of the body, and the left hemisphere controls the left side. This distinguishes the cerebellum from the cerebrum, in which each hemisphere primarily controls the opposite side of the body, i.e., contralaterally.

First published on August 23, 2011
Last updated on September 20, 2025