Cerebellum




Keywords

cerebellar peduncle, ataxia, cerebellar tonsil, flocculonodular lobe, mossy fiber, climbing fiber, inferior olivary nucleus, Purkinje cells

 






  • Chapter Outline



  • The Cerebellum Can Be Divided Into Transverse and Longitudinal Zones, 130




    • Deep Nuclei Are Embedded in the Cerebellar White Matter, 130



    • Three Peduncles Convey the Input and Output of Each Half of the Cerebellum, 131




  • All Parts of the Cerebellum Share Common Organizational Principles, 131




    • Inputs Reach the Cerebellar Cortex as Mossy and Climbing Fibers, 131



    • Purkinje Cells of the Cerebellar Cortex Project to the Deep Nuclei, 131



    • One Side of the Cerebellum Affects the Ipsilateral Side of the Body, 132



    • Details of Connections Differ Among Zones, 132




  • Cerebellar Cortex Receives Inputs From Multiple Sources, 133




    • Vestibular Inputs Reach the Flocculus and Vermis, 133



    • The Spinal Cord Projects to the Vermis and Medial Hemisphere, 133



    • Cerebral Cortex Projects to the Cerebellum by Way of Pontine Nuclei, 133



    • Climbing Fibers Arise in the Inferior Olivary Nucleus, 133



    • Visual and Auditory Information Reaches the Cerebellum, 133




  • Each Longitudinal Zone Has a Distinctive Output, 134



  • Patterns of Connections Indicate the Functions of Longitudinal Zones, 134




    • The Lateral Hemispheres Are Involved in Planning Movements, 134



    • The Medial Hemispheres Are Involved in Adjusting Limb Movements, 134



    • The Vermis Is Involved in Postural Adjustments, 135



    • The Flocculus and Vermis Are Involved in Eye Movements, 135



    • The Cerebellum Is Involved in Motor Learning, 136



    • The Cerebellum Is Also Involved in Cognitive Functions, 136




  • Clinical Syndromes Correspond to Functional Zones, 136


The cerebellum helps coordinate movement by sampling most kinds of sensory information, comparing current movements with intended movements, and issuing planning or correcting signals. The comparisons are made in a uniform, precisely organized, cerebellar cortex, and the planning or correcting signals are issued through a set of deep cerebellar nuclei. Because its output is concerned with coordination of movement and not with perception, cerebellar lesions cause incoordination but no sensory changes.




The Cerebellum Can Be Divided Into Transverse and Longitudinal Zones




Key Concepts





  • Transverse fissures divide the cerebellum into lobes.



  • Functional connections divide the cerebellum into longitudinal zones.


In a gross anatomical sense, the primary fissure divides the bulk of the cerebellum into anterior and posterior lobes , and another deep fissure separates the flocculus and nodulus (together forming the flocculonodular lobe ) from the body of the cerebellum ( Fig. 20.1 ). Assorted exotic names are sometimes applied to various parts of the anterior and posterior lobes, but most are of limited clinical utility. One worth remembering is the tonsil , the part of the posterior lobe nearest to the flocculus. The tonsil is one of the most inferior parts of the cerebellum, and expanding masses in the posterior fossa can cause it to herniate through the foramen magnum, compressing the medulla.


FIG 20.1


A posterior view of the cerebellum (A), and two similar cartoon views showing its division transversely into lobes (B) and longitudinally into functional zones (C). The cartoon view is as though you were looking from behind at a cerebellum that had been flattened out so that all of its parts could be seen.


In terms of connections and functions, however, it is more useful to divide each half of the cerebellum into three longitudinal zones—a midline vermis , and a hemisphere with a medial and a larger lateral part. The vermis is involved in coordination of trunk movements. The medial and lateral parts of the hemisphere are both involved in ipsilateral limb movements, but in different ways.


Deep Nuclei Are Embedded in the Cerebellar White Matter


The fundamental building plan of the cerebellum as a whole involves afferents that reach the cerebellar cortex, which in turn projects to deep nuclei embedded in the cerebellar white matter. The deep nuclei then give rise to the output of the cerebellum. There are a series of three deep nuclei on each side, arranged in a medial to lateral array: the fastigial (most medial), interposed (composed of the emboliform and globose nuclei), and dentate (most lateral) nuclei .


Three Peduncles Convey the Input and Output of Each Half of the Cerebellum


Three peduncles containing the cerebellar afferents and efferents attach the cerebellum to the brainstem ( Fig. 20.2 ). The superior cerebellar peduncle is the major output route from its side of the cerebellum, carrying all the efferents from the dentate and interposed nuclei and some of the efferents from the fastigial nucleus. The middle cerebellar peduncle is the input route for information from the cerebral cortex, carrying the fibers from contralateral pontine nuclei . By elimination then, the inferior cerebellar peduncle is a complex bundle, carrying most of the remaining cerebellar afferents (including climbing fibers of the inferior olivary nucleus , as described a little later) and from the spinocerebellar tracts, as well as the remaining cerebellar efferents.




FIG 20.2


(A) Contents of the cerebellar peduncles. For simplicity, the inferior and superior peduncles are shown as being entirely afferent and efferent; each actually contains a smaller number of fibers traveling in the opposite direction. ICP, Inferior cerebellar peduncle; MCP, middle cerebellar peduncle; SCP, superior cerebellar peduncle. (B) A beautiful diffusion tensor image of the cerebellar peduncles, seen from the left side. Fibers from widespread cortical areas descend through the internal capsule (IC), funnel into the cerebral peduncle (CP), and reach pontine nuclei in the basal pons (BP); pontine nuclei then project to the contralateral half of the cerebellum through the middle cerebellar peduncle (MCP). Spinocerebellar fibers (SC) enter the cerebellum through the inferior cerebellar peduncle. Finally, cerebellar efferents leave through the superior cerebellar peduncle (SCP). Shorter fibers that interconnect different parts of the cerebellum are shown in blue.

(Modified from Catani M et al.: Altered cerebellar feedback projections in Asperger syndrome, NeuroImage 41:1184, 2008. Thanks to Dr. Marco Catani.)




All Parts of the Cerebellum Share Common Organizational Principles


All parts of the cerebellum have a cortex with the same structure and use the same basic circuitry ( Fig. 20.3 ; inputs → cortex → deep nuclei → outputs). This suggests that all parts of the cerebellum perform the same basic (still mysterious) operation, and that the functional differences among different cerebellar regions are simply reflections of different input sources and output targets.




FIG 20.3


General pattern of cerebellar inputs and outputs.


Inputs Reach the Cerebellar Cortex as Mossy and Climbing Fibers


Afferents reach the cerebellar cortex in two forms: mossy fibers and climbing fibers ( Fig. 20.4 ). Climbing fibers, all from the contralateral inferior olivary nucleus of the rostral medulla, end directly on the dendrites of Purkinje cells , which provide the output from cerebellar cortex. Mossy fibers, in contrast, come from many other places and end on the tiny granule cells of the cerebellar cortex; granule cells in turn issue parallel fibers that synapse on dendrites of Purkinje cells. Although mossy fibers arise on both sides of the CNS, those reaching one side of the cerebellum carry information related to the ipsilateral side of the body (e.g., see Figs. 20.6 and 20.7 later in this chapter).




FIG 20.4


Cerebellar cortex and deep nuclei. Excitatory connections are shown in green, inhibitory connections in red. Projections from the deep nuclei are shown in a third color because, although most are excitatory, some are inhibitory. G, Granule cells.


The majority of the Purkinje cell output is directed to the deep cerebellar nuclei, which in turn provides the majority of the output from the cerebellum. Yet, some Purkinje cell axons do not use the cerebellar nuclei and project directly to the vestibular nuclei in the brainstem.


Purkinje Cells of the Cerebellar Cortex Project to the Deep Nuclei


The flocculonodular lobe is primarily concerned with vestibular function (posture and eye movements), and most of its output is directed to the vestibular nuclei either directly or indirectly. The three longitudinal zones of the remainder of the cerebellum direct their outputs to three deep nuclei arranged in a corresponding medial-to-lateral array ( Fig. 20.5 ): vermis → fastigial nucleus, medial hemisphere → interposed (globose + emboliform) nucleus, and lateral hemisphere → dentate nucleus.


Tags:
Jun 23, 2019 | Posted by in NEUROLOGY | Comments Off on Cerebellum

Full access? Get Clinical Tree
Get Clinical Tree app for offline access