The cerebellum is a large structure at the back of the brain, immediately behind the brainstem and below the occipital cortex. The cerebellum receives direct inputs from the sensory fibers in the spinal cord as well as indirect inputs from the motor cortex, possibly as a copy of the commands being sent downward to the spinal cord.
In this manner, the cerebellum monitors input and output of the motor system, and it sends its own outputs via the VL (ventral lateral) and VPL (ventral postero-lateral) regions of the thalamus to the motor cortex. The outputs of the cerebellum are sent in order to modify the motor commands. It also sends outputs to motor regions of the brainstem and spinal cord.
The presumed function of the cerebellum is to process information from multiple joints and limbs and to use the combination of this information to compensate for interaction forces and other mechanical forces. In this way, the cerebellum helps to "fine-tune" the motor commands. For example, the cerebellum is able to measure the forces that occur at the elbow in response to movement at the shoulder. In theory, the cerebellum could send commands to the motor cortex to compensate for this interaction. The structure of the neuronal networks within the cerebellum seems well suited to rapid learning, allowing for compensation in response to changing tasks or disturbances in the environment.
Inside the Cerebellum
Within the cerebellum, information arrives on the "mossy fibers," which then connect to the granule cells, which then send to "parallel" fibers on the cerebellum's surface. The parallel fibers run in large bundles on the surface. These bundles come into contact with the dendrites that provide inputs to the large purkinje cells. The purkinje cell dendrites are arranged in a flat lattice at right angles to the parallel fibers, and contact many thousands of these fibers. Each purkinje cell responds to a particular pattern on the parallel fibers. When these cells recognize this pattern, they send a signal down, inhibiting one of the deep nuclei within the cerebellum. This arrangement allows the cerebellum to recognize certain specific patterns and perform specific computations. These computations integrate information from tens of thousands of inputs as they arrive on the parallel fibers of the granule cells and are collected by the purkinje cell dendrites.
There are three deep cerebellar nuclei in humans: the dentate, the interpositus, and the fastigial nuclei. There is much debate concerning the exact function of the outputs of each nucleus. The dentate nucleus exchanges signals primarily with cortical areas and seems to be involved with the control and coordination of multi-joint reaching movements and other purposeful high-level functions. The interpositus nucleus receives signals from motor cortical areas and the spinal cord. This nucleus seems to be involved with correction of tremor and adjustment of tone in the muscles and trunk. The fastigial nucleus receives signals from vestibular systems and sends its outputs to...
There is an additional input to the cerebellum that comes from the inferior olive. The olive sends "climbing" fibers, which contact the purkinje cells (among others), and trigger powerful complex spikes. It is believed that when a climbing fiber fires, it tells the purkinje cell to decrease its response to the particular pattern of parallel fiber inputs that are occurring at that time. In this way, the climbing fibers may be a "teaching" signal that modifies the response of the cerebellum. Some authors believe that the inferior olive compares the desired motor commands to the actual results of the commands, and that it sends the teaching signal to correct any errors that it detects. Therefore, the cerebellum is important in learning new motor skills and adapting to changes in the environment or control tasks.
Damage to the Cerebellum
If the cerebellum is damaged, it leads to a lack of coordination between the movement and forces at different joints and between different body parts. It may no longer be possible to compensate for interaction forces; therefore, that movement of an arm or turning the head may make the child fall over, or moving the shoulder may make the elbow move in an unpredictable manner. This leads to a lack of smooth movement, inaccurate movements, and ultimately ataxia.
Damage to the cerebellar areas controlling gait and balance (most of which are in an area called the "vermis" that lies in the middle between the two outer halves of the cerebellum) leads to an unsteady and ataxic gait. Damage to areas controlling the eyes, most of which lie near the base of the cerebellum in the "flocculo-nodular lobe," leads to inaccurate eye movements or the random jerky movements called opsoclonus. Damage to the cerebellum maybe a direct cause of tremor, or tremor may be the result of the child attempting to overcompensate for inaccurate movements. Tremor may also occur if there is injury to structures in the Guillain-Mollaret triangle. This triangle consists of the dentate or interpositus nuclei, fibers traversing the red nucleus, and the inferior olive. Tremor from injury to these structures is called a "midbrain tremor" (or "rubral" tremor). This form of tremor may be severe and resistant to treatment.
Some types of cerebellar tremor improve following a surgical procedure to place a lesion (or deep-brain stimulation electrode) in the Vim nucleus of the thalamus, thereby blocking the output from the cerebellum to the cortex and possibly interrupting the tremor signals.