Topics and Speakers John H. Martin, PhD and Pietro Mazzoni, MD, PhD
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Synopsis
The cerebellum, which means "little brain" in Latin, is a small structure located in the inferior posterior portion of the head and responsible for feed-forward control, motor learning, and some nonmotor functions. It contains nearly half the neurons in the brain and if stretched out, can range from between 1 to 10 square meters in length. In this lecture, Dr John Martin discusses its functional anatomy while Dr Pietro Mazzoni discusses the array of diseases that can afflict it and their surprising symptoms.
Dr Martin describes the cerebellum as a "motor control structure" that is "comparing what we want to do with what actually happens." The cerebellum is constantly receiving input from the sensory system, somatic sensory system, and cortical control centers. It is constantly reviewing muscle intent with the actual movement of the muscle in question. It influences these kinds of movements, not through the spinal cord, but through actions on motor pathways via projections to the brainstem motor control systems and via the thalamus to the corticospinal system.
The cerebellum is composed of three deep nuclei: the dentate nuclei (located within the lateral hemispheres), the fastigial nuclei (located in the vermis), and the interposed nuclei (located in the intermediate zone). These nuclei receive GABAergic (inhibitory) inputs or glutamatergic (excitatory). The way information reaches the deep nuclei is through its two major pathways: the mossy and climbing fibers.
Functionally speaking, each part of the cerebellum is in charge of some kind of motor movement. There are three major parts: the spinocerebellum (composed of the vermis and intermediate zone), the cerebrocerebellum, and the vestibulocerebellum. For instance, the spinocerebellum receives mechanosensory information from muscles and joints and discharges information back to the limbs via the spinal cord. Meanwhile, the cerebrocerebellum (located in the lateral hemisphere) is in charge in the planning of movements. The vestibulocerebellum oversees the whole body's coordination. These three major parts are then involved in what the cerebellum governs: feed-forward control (e.g., predicting the location of a baseball and trying to predict where to hit it), motor learning (e.g., learning to throw darts), and some nonmotor functions (e.g., tone and cadence of speaking). Dr Martin discusses a variety of examples and experiments to illustrate these functions.
The lecture then turns to Dr Mazzoni who introduces the subject of cerebellum diseases. Damage to the cerebellum does not cause weakness, like damage to the cerebrum would. Interestingly, damage to it presents itself as motor feedback deficits and disorders in posture, motor learning, movement, and equilibrium. Cerebellum diseases are either sporadic (noninheritable) or inheritable. A sporadic example would encompass how alcoholism contributes to the organ's degeneration, resulting in the unsteadiness associated with this addiction. A heritable example would be Friedreich's ataxia, where there is not only an inability to coordinate voluntary muscular controls but also polyneuropathy (a peripheral nerve disorder) and hyperreflexia (overactive reflexes). Other disorders such as dysmetria (displaying an abnormal trajectory when asked to perform a simple test like the finger-to-nose test) and dysarthria (slow speech with poor enunciation) are described in detail.
