Cerebellar Nuclei & Tracts
The series on the cerebellum continues with a discussion on its composition. This article will evaluate the functional organization of the cerebellum, its cortical structure, composite nuclei and tracts, and some clinically relevant points. When studying the tracts, it can become a bit confusing to remember these complex names. An easy way to understand them is to realize that the names of the tracts are made up from parts of the names of the nuclei or structure that they originate from and those that they will terminate in. Take for example, the dorsal spinocerebellar tract; “dorsal” gives the location, “spino” gives its structure of origin, and “cerebellar” tells where it is going. Putting that all together, this tract is in the dorsal part of the spinal column (between the posterior medial sulcus and the dorsal horn of the grey matter) and it is heading to the cerebellum.
Functional Longitudinal Zones
If the cerebellum is cut along the horizontal fissure and laid flat (with the external surfaces facing up), then it can be better divided into its four functional components. The vermal zone is found along the length of the vermis. This region is responsible for the maintaining balance. Resultantly, information regarding muscle tone of the hips, thoracoabdominal muscles, shoulders and neck would be transmitted to this region from the spinocerebellar tracts. Any deviation from normal posture would subsequently be mitigated by efferent fibers of the fastigial nucleus.
Just lateral to the vermal zone is a region known as the paravermal (intermediate) zone. This portion of the cerebellum is involved in skilled, volitional movements. It also receives inputs by way of the spinocerebellar tracts from the hands and feet as well as other distal muscle groups. Its regulatory activity is achieved by the interposed (globose & emboliform) nuclei.
Lateral to the intermediate zones are the larger lateral zones. They occupy the bulk of the cerebellar hemispheres and are intricately involved in regulating motor activity of the whole body. The corticopontocerebellar tracts are responsible for bring afferent impulses to the area, while the dentate nucleus regulates changes in motor planning.
Finally, the flocculonodular lobe coordinates both balance and eye movements. The labyrinths of the internal ear transmit their impulses via CN VIII (vestibulocochlear) to both the flocculonodular lobe and the vestibular nuclei of the medulla oblongata. The flocculonodular lobe also receives afferent fibers from the superior colliculus (paired, bilateral bodies posteroinferior to the pineal gland) and the striate cortex (part of the primary visual cortex; Brodmann 17 & 18) and then projects to the vestibular nuclei. The vestibular nuclei then coordinate the actions of the eyes by sending impulses to the medial longitudinal fasciculi (posterior to the periaqueductal grey matter in the superior medulla) which innervate CN III (oculomotor), IV (trochlear) and VI (abducent). It also modifies the position of the head by stimulating antigravity and neck muscles via the vestibulospinal tracts.
Cerebellar Cortex: Layers & Fibers
The cerebellar cortex is made up of grey matter that surrounds the white matter at its core. The cortex can be divided into three layers: an outer molecular layer, a middle Purkinje cell layer and an inner granular layer. The molecular layer is just deep to the pia mater and contains very few cells. The dendritic trees of the Purkinje cells extend into this area and interact with the axons of granule cells. Additionally, there are outer stellate and inner basket (stellate) cells present in this layer. These cells have an inhibitory effect on Purkinje cells, but are excited by parallel fibers of granule cells.
Deep to the molecular layer is the Purkinje cell layer. It is the sole efferent pathway from the cerebellum and provides inhibitory impulses to both the vestibular and cerebellar nuclei. The Purkinje cells are excited by climbing fibers of the inferior olivary nucleus and parallel fibers of the granule cells.
The deepest layer of the cortex is the granule cell layer, just superficial to the white matter. It houses the granule and Golgi cells. Granule cells are inhibited by Golgi cells, but are excited by mossy fibers (afferent branches of the pontocerebellar and spinocerebellar tracts).
A horizontal section through the cerebellum at the level of the superior cerebellar peduncle will reveal several important nuclei that are involved in coordination and balance. The nuclei are paired and found in both cerebellar hemispheres. The fastigial nucleus is the most central pair of cerebellar nuclei and is associated with the vermis. They are found along the midline of the cerebellum just posterior to the roof of the 4th ventricle. After receiving spinocerebellar and labyrinthine afferents from the vermis, the fastigial nuclei project to the spinal cord (via the vestibular nuclei) and the ventral lateral thalamic nucleus. This pathway eventually goes to the precentral gyrus, which sends its efferent fibers to the proximal and trunk muscles for maintenance of balance.
Lateral to the fastigial nucleus are the globose and emboliform nuclei. The globose nuclei are closer to the fastigial nucleus, while the emboliform nucleus is more lateral, and adjacent to the dentate nucleus. Together, the globose and emboliform nuclei are referred to as the interposed nucleus. By way of the interpositorubrothalamic tract (rubro = of or relating to the red nucleus), proprioceptive impulses from the paravermis are conveyed to the ventral lateral thalamic nucleus and the red nucleus. The former then relays information to the precentral gyrus (Brodmann 4), which sends regulatory signals down the lateral corticospinal tract to the distal muscles; while the latter arbitrates the management of the distal flexor muscles via the rubrospinal tract. It should be noted that although the interpositorubrothalamic tract are involved in the regulation of skilled muscle tone and flexor motor activity of the ipsilateral (same) side, the fibers decussate twice. Here is how it happens:
- The RIGHT interpositothalamic tract crosses in the decussation of the superior cerebellar peduncles to the LEFT thalamus which then sends fibers to the LEFT precentral gyrus. The LEFT precentral gyrus forms the lateral corticospinal tract which crosses to the RIGHT side at the decussation of the pyramids.
- Similarly, the RIGHT interpositorubral (rubral = red nucleus) tract crosses in the decussation of the superior cerebellar peduncles to synapse on cells of the LEFT red nucleus. The resulting rubrospinal tract coming from the red nucleus then decussates shortly after it forms to run on the RIGHT SIDE.
These decussations also apply to the opposite side.
The dentate nucleus is the largest of the cerebellar nuclei and is situated in the lateral hemispheres. It resembles a crushed paper bag with its open end facing anteromedially. Unlike the other nuclei, the dentate nucleus partially encloses bundles of white matter that forms the dentatorubrothalamic and the dentatoolivary tracts. The dentate nucleus is extremely important for the regulation of many aspects of voluntary motor activity, viz. its timing, planning and inception. Firstly, corticopontine fibers arise from the cerebral cortex and travel to the pontine nucleus via the corona radiata and internal capsule. The neocerebellum (or lateral zone) then gains information from the decussating transverse pontine fibers, which enter the cerebellum through the middle cerebellar peduncles. Thus, this tract is known as the corticopontocerebellar tract. Eventually, the axons of Purkinje cells then take these impulses from the cerebellar cortex to the dentate nucleus. The dentate nucleus then sends impulses to two main destinations, the thalamus, and the red nucleus.
- In the thalamus, the dentatothalamic fibers synapse in the contralateral ventral lateral nucleus. From here, information is sent to the premotor and primary motor cortices (Brodmann 6 & 4, respectively). These centres then form the lateral corticospinal tract (skilled, collaborative, voluntary motor activity), and corticobulbar tract (regulation of the motor components of CN V, VII, X, XI and XII). The former acts ipsilaterally, while the latter acts bilaterally on all the cranial nerves except for the lower motor nuclei of CN VII and CN XII (unilateral action on both).
- Dentatorubral fibers decussate and synapse on the contralateral red nucleus after leaving through the superior cerebellar peduncle. The red nucleus sends its fibers to the ipsilateral inferior olivary nucleus of the medulla oblongata, whose projections subsequently crosses the midline to enter the cerebellar cortex via the inferior cerebellar peduncle. These fibers then become climbing fibers that synapse on Purkinje cells in the Purkinje layer of the cerebellar cortex.
There is also a regulatory reflex arc between the dentate nucleus and the contralateral inferior olivary nucleus. In this system, decussating fibers from the dentate nucleus descend after leaving the superior cerebellar peduncle and synapse directly on the inferior olivary nucleus. Fibers from this contralateral inferior olivary nucleus then cross the midline to enter the inferior cerebellar peduncle and synapse on the dentate nucleus.