Part 1: Image-Mapped Tutorial
Part 2: Matching Self-Test:
33a |
33b |
33c
Part 3: Multiple-Choice Self-Test
3 - Medulla and Cerebellum
The cerebellum is a phylogenetically old and structurally complex structure that plays a major role in the extrapyramidal control of movement. The cerebellum's complexity is evident when comparing the approximately 50 billion neurons that compose this structure with the approximately 22 billion neurons found in the cerebral cortex. The cerebellum projects to virtually every motor structure of the brain. Cerebellar dysfunction is characterized by awkwardness of intentional movements, much like that of a person intoxicated by alcohol. Movements become jerky and uncoordinated. The cerebellum is composed of a cortex and deep medulla that contains a number of distinct nuclei. The cerebellar cortex is divided into four main regions, the centrally located vermis, the intermediate zone on either side of the vermis, the lateral zone on the outside of the intermediate zone, and finally the flocculonodular lobe located along the caudal surface.
The vermis receives information about the spatial location of auditory and visual sensory events from the tectum, of somatosensory information from the skin, and proprioceptive information from the muscles and joints. The vermis then projects to the vestigial nucleus located within the cerebellar medulla. The vestigial nucleus in turn projects to the vestibular nuclei and motor nuclei of the reticular formation. Via the circuit formed by these projections, the cerebellum influences two of the ventromedial pathways, the vestibulospinal tract and reticulospinal tract.
The intermediate and lateral zones of the cerebellar cortex receive projections from the primary motor cortex and the association cortical areas. These cortical projections are relayed to the cerebellum via the pontine tegmental reticular nucleus. The intermediate zone of the cerebellar cortex projects to the interposed nuclei of the cerebellar medulla, which in turn project to the red nucleus. Via this projection, the cerebellum influences the movement of arms and legs. In addition, the intermediate zone projects back to the motor cortex via the ventrolateral thalamic nucleus. The lateral zone of the cerebellar cortex receives information about intended movements from the frontal association cortex and the primary motor cortex via the pontine nucleus. The lateral zone also receives information about current limb position and rate of movement. This somatosensory information is used when a movement is initiated to calculate the contribution of various muscles that will be needed to execute the movement successfully. The results of these calculations are projected to the dentate nucleus of the cerebellar medulla, which in turn projects to the primary motor cortex via the ventrolateral thalamic nucleus. The cerebellum modifies ongoing movements via the circuit formed by this pathway. The lateral zone of the cerebellar cortex also projects to the red nucleus via the dentate nucleus, and affects movements of the limbs via this pathway.
| Advanced |
Much is known about the structural and functional complexities of the cerebellum (Mauk, 2000; Parent, 1996). The sole source of cerebellar output is the deep cerebellar nuclei. There are four separate groups of cerebellar nuclei. Three of these nuclei which are arranged from medial to lateral within the cerebellum, are the fastigial, interposed, and dentate nuclei. The fourth group, the vestibular nuclei, are actually located in the ventral region of the brainstem, but are nonetheless functionally the same as the other cerebellar nuclei and included in this category. Two different inputs influence the output of these nuclei, the climbing fibers and mossy fibers. Climbing fibers make excitatory synapses with one Purkinje cell, resulting in an all-or-none response called a complex spike. Mossy fibers make excitatory synapses in the cerebellar nuclei and then branch diffusely on a large number of granule and Golgi cells located in the cerebellar cortex. Approximately 200,000 granule cells then go on to excite a single Purkinje cell. This massive input stimulates the single Purkinje cell to discharge at a very high rate. These rapidly generated spikes (80-100 action potentials per second) are called simple spikes. They are easily distinguished from the complex spike of the Purkinje cell induced by climbing fibers and are the primary output of the cerebellar cortex to the cerebellar nuclei. The final output of the deep cerebellar nuclei is an integration of the direct excitatory inputs from mossy fibers and the inhibitory effect of the cerebellar cortical excitation of inhibitory synapses in the nuclei.
Mossy fibers constitute the main input to the cerebellum. They originate in brainstem nuclei, pontine nuclei, and neurons in the spinal cord that form the spinocerebellar tracts. Mossy fibers convey information about all sense modalities (except olfaction) and information from the motor cortex. Climbing fibers, conveying information about movement errors, originate in the inferior olivary nuclei. The information conveyed by climbing fibers may induce changes in cerebellar synapses that underlie the learning of new movements and the improvement of future performance.
The functional significance of the cerebellum has traditionally been identified based on its anatomical connections and symptoms resulting from damage to its various regions. This approach to understanding cerebellar function suggests an important role in the comparison of central motor commands with ongoing information from the sensory systems. This ongoing comparison of motor and sensory information results in the adjustment of movement needed for accuracy. Three general functions of the cerebellum include the maintenance of equilibrium, the planning of limb movements, and the adjustment of ongoing movements. Equilibrium is controlled by the vestibulocerebellar nuclei, and affected by the control of trunk muscles in coordination with eye movements. Adjustments to movement that are controlled by the spinocerebellum (descending projections of the cerebellum) include the regulation of muscle tone, the compensation of force in response to variations in load placed on muscles, the smoothing out of tremors, and corrections for deviations made in ongoing movements. Sensory information is used to modify the descending projections of the cerebellum. The cerebrocerebellum (ascending projections of the cerebellum) is involved in the planning of limb movements via connections with the thalamus, primary motor cortex, and premotor cortex.
| References |
Mauk. M.D. (2000, March 12). Lecture 39 - Cerebellum. Retrieved May 3, 2000 from the World Wide Web: http://nba5.med.uth.tmc.edu/academic/neuroscience/lectures/section_3/lecture39_01.htm
Parent, A. (1996). Carpenter's human neuroanatomy (9th ed.). London: Williams & Wilkins.