The Cerebellum

Figure 1. Superior view of the cerebellum. Anterior and posterior lobes are visible, with the two cerebellar hemispheres linked medially by the vermis.

The cerebellum, as the name suggests, is the “little brain.” It is covered in gyri and sulci like the cerebrum, and is divided into right and left cerebellar hemispheres that are linked medially by a structure called the vermis Figure 1.  As such, the cerebellum and looks like a miniature version of the cerebrum (Figure 2). The cerebellum is largely responsible for comparing information from the cerebrum with sensory feedback from the periphery through the spinal cord. It accounts for approximately 10 percent of the mass of the brain.

The cerebellum can be separated into three anatomically distinct lobes: the anterior lobe, the posterior lobe, and the flocculonodular lobe (Figure 1, Figure 2).  The anterior lobe sits nearest to the midbrain, while the posterior lobe occupies most of the posterior and inferior areas of the cerebellum.  Both the anterior and posterior lobes play a role in coordinating movements of the trunk of the body and those of the arms and legs using signals from proprioceptors in these areas.  The flocculonodular node is located anterior to the posterior lobe, and plays an important role in coordinating the movements of the head and eyes, and processes signals from the inner ear so that the body can maintain balance.

Figure 2. The Cerebellum. The cerebellum is situated on the posterior surface of the brain stem. Major structures of the cerebellum include the the anterior lobe, posterior lobe, flocculonodular lobe, and arbor vitae. Descending input from the cerebellum enters through the large white matter structure of the pons. Ascending input from the periphery and spinal cord enters through the fibers of the inferior olive. Output goes to the midbrain, which sends a descending signal to the spinal cord.

The cerebellum is linked to the structures of the brainstem via three cerebellar penduncles.  Axons in the inferior cerebellar peduncle link cerebellum to the medulla, those in the middle cerebellar peduncle link the cerebellum to the pons, and fibers in the superior cerebellar peduncles link the cerebellum to the midbrain.  As a result, descending fibers from the cerebrum with branches that connect to neurons in the pons allow for communication of motor signals from the cerebrum to the cerebellum. Those neurons that project into the cerebellum, provide a copy of motor commands sent to the spinal cord. Sensory information from the periphery, which enters through spinal or cranial nerves, is copied to a nucleus in the medulla known as the inferior olive. Fibers from this nucleus enter the cerebellum and are compared with the descending commands from the cerebrum. If the primary motor cortex of the frontal lobe sends a command down to the spinal cord to initiate walking, a copy of that instruction is sent to the cerebellum.

Like the cerebrum, the cerebellum possesses a superficial layer of grey matter called the cerebellar cortex that is linked to the cerebellar peduncles by a complex, branching area of deep white matter called the arbor vitae or “tree of life”.  This superficial grey matter plays a role in moderating voluntary motor functions, so that an intended motion is correctly completed.  This is achieved when sensory feedback from the muscles and joints, proprioceptive information about the movements of walking, and sensations of balance are sent to the cerebellum through the inferior olive and the cerebellum compares them. If walking is not coordinated, perhaps because the ground is uneven or a strong wind is blowing, then the cerebellum sends out a corrective command to compensate for the difference between the original command from the cerebrum and the sensory feedback. The output of the cerebellum is into the midbrain, which then sends a descending input to the spinal cord to correct the messages going to skeletal muscles.  The cerebellar cortex is also responsible for “muscle memory”, where learned voluntary muscle movements become completed involuntarily by the cerebellum.  An example of this is learning to tie two shoelaces in a bow.  At first, one has to focus closely on how to tie the shoelaces around each other when trying to make a bow, but after much practice the motion can be completed almost automatically.  This involuntary set of motor movements is now being completed by the cerebellum once the movement is learned.  This function of the cerebellar cortex can range from very simple, like the muscle memory that allows us to walk upright once we learn to do so after practicing as a toddler, to very complex like the movements needed to play a musical instrument.  Research is also beginning to show that the cerebellar cortex may play a role in understanding sequences of events that are used for language, problem solving, and planning.