The cerebellar peduncles are the paired connections between the cerebellum and the brain stem. They play a vital role in communicating information to and from the cerebellum. The cerebellum, in turn, coordinates complex movements and maintains the body’s balance and posture.
The cerebellar peduncles are three pairs of nerve tracts that connect the cerebellum to the brain stem. They all arise from the left and right sides of the brain stem at the level of the pons and enter the anterior cerebellum to form the roots of the arbor vitae, or cerebellar white matter. The cerebellar peduncles are made of white matter, which in turn consists of parallel bundles of myelinated axons. Myelin is formed by oligodendrocytes — specialized glial cells — which wrap and insulate multiple axons.
The superior cerebellar peduncles are the most superior and medial of the three peduncles. They arise in the cerebellar white matter, extend anteriorly toward the brain stem, and ascend through the brain stem posterior to the fourth ventricle. Inferior to the inferior colliculi of the midbrain, the left and right superior cerebellar peduncles meet along the midline of the brain at a point known as the decussation of the superior cerebellar peduncle. At the decussation, the nerve fibers of each peduncle cross over to the opposite side of the brain and continue through the midbrain to the red nuclei.
The middle cerebellar peduncles enter the cerebellum lateral to the superior cerebellar peduncles and are the largest and most lateral pair of cerebellar peduncles. Unlike the superior cerebellar peduncles that arise from within the cerebellum, the neurons of the middle cerebellar peduncles arise in the pontine nuclei of the contralateral side of the pons. From their origins, they cross the pons to the opposite side of the brain, pass posteriorly around the lateral edges of the fourth ventricle, and enter the cerebellum through the middle cerebellar peduncles.
The inferior cerebellar peduncles enter the cerebellum inferior to the union of the superior and middle cerebellar peduncles. Their nerve fibers arise from the spinocerebellar tracts of the spinal cord and the inferior olivary nuclei of the medulla oblongata. From their origins, these nerve fibers ascend the brain stem posterior to the fourth ventricle before turning posteriorly to enter the cerebellum.
The cerebellar peduncles form the bridge of nervous tissue that connects the cerebellum to the brain stem and spinal cord. Each of the three pairs of cerebellar peduncles provides a specific type of connection for the cerebellum and connects the cerebellum to a specific region of the brain or spinal cord. Some of these nerve signals are afferent, or directed into the cerebellum, while others are efferent, or directed out of the cerebellum. The cerebellar gray matter integrates the afferent nerve signals and provides efferent signals to the cerebrum to assist in the complex task of regulating movement.
The superior cerebellar peduncle is solely responsible for the efferent signals of the cerebellum. Its neurons arise from the gray matter of the cerebellar cortex and carry motor signals to the red nuclei and thalamus. These signals act as a feedback loop to the motor cortex of the cerebral cortex, helping it to coordinate complex movements and limit errors in movement. The cerebellum also plays a vital role in balance and posture, using these signals to help the body maintain its position.
The middle and inferior cerebellar peduncles carry afferent signals from many regions of the brain to provide sensory input to the cerebellum. Information about the types of voluntary movements that the body is trying to produce originates in the cerebral cortex and is provided by the middle cerebellar peduncle. The inferior cerebellar peduncle provides information on balance and the body’s position from the vestibule of the ear and proprioceptors throughout the body. By comparing the balance and position of the body relative to how the cerebral cortex wants to move the body, the cerebellum provides a key link in making appropriate movements.