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Mossy fibres and climbing fibres carry the afferent connections of the cerebellum. Mossy fibre systems terminate bilaterally in transversely oriented ‘lobular’ areas; there is considerable overlap amongst the terminations. Climbing fibres from different subnuclei of the inferior olive terminate contralaterally, on discrete longitudinal strips of Purkinje cells. This longitudinal pattern closely corresponds with the zonal arrangement in the corticonuclear projection (Fig. 20.10).


Fig. 20.10  Cerebellar corticonuclear and corticovestibular projections. The widespread projection from flocculonodular lobe to vestibular nuclei is not labelled but is indicated in green.

Spinocerebellar and trigeminocerebellar fibres

The spinal cord is connected to the cerebellum through the spinocerebellar and cuneocerebellar tracts and through indirect mossy fibre pathways relayed by the lateral reticular nucleus in the medulla oblongata. These pathways are all excitatory in nature and give collaterals to the interposed and fastigial nuclei before ending on cortical granule cells.

The posterior spinocerebellar tract takes its origin from the cells of Clarke’s column at the base of the dorsal horn in all thoracic segments of the spinal cord (Fig. 20.11). It enters the inferior cerebellar peduncle, gives collaterals to the cerebellar nuclei, and terminates, mainly ipsilaterally, in the vermis and adjoining regions of the anterior lobe and in the pyramis and adjoining lobules of the posterior lobe. Clarke’s column receives primary afferents of all kinds from the muscles and joints of the lower limbs, which reaches the nucleus via the fasciculus gracilis. It also receives collaterals from cutaneous sensory neurones. Accordingly, the tract transmits proprioceptive and exteroceptive information about the ipsilateral lower limb. Very fast conduction is required to keep the cerebellum informed about ongoing movements. The axons in the posterior spinocerebellar tract are the largest in the CNS, measuring 20 μm in diameter. The upper limb equivalent of the posterior spinocerebellar tract is the cuneocerebellar tract.


Fig. 20.11  Spinocerebellar and cuneocerebellar mossy fibre projections to the culmen and pyramis and related intermediate areas of the cerebellar cortex.

The anterior spinocerebellar tract is a composite pathway. It informs the cerebellum about the state of activity of spinal reflex arcs related to the lower limb and lower trunk. Its fibres originate in the intermediate grey matter of the lumbar and sacral segments of the spinal cord (Fig. 20.11). They cross near their origin, and ascend close to the surface as far as the lower midbrain before looping down in the superior cerebellar peduncle. Most fibres cross again in the cerebellar white matter. Thus their distributions to the cerebellar nuclei and cortex appear to be the same as those of the posterior tract.

The rostral spinocerebellar tract originates from cell groups of the intermediate zone and horn of the cervical enlargement. Although considered to be the upper limb and upper trunk counterpart of the anterior spinocerebellar tract, most of its fibres remain ipsilateral throughout their course. It enters the inferior cerebellar peduncle and terminates in the same cerebellar nuclei and folia as the cuneocerebellar tract.

The cuneocerebellar tract contains exteroceptive and proprioceptive components which originate from the cuneate and external cuneate nuclei respectively. The primary afferents travel in the fasciculus cuneatus. The tract itself is predominantly uncrossed and ends in the posterior half of the anterior lobe. Exteroceptive and proprioceptive mossy fibre components of the tract terminate differentially in the apical and basal part of the folia. The exteroceptive component overlaps the pontocerebellar mossy fibre projection in the apices of the folia of the anterior lobe.

Comparable sets of ipsilateral proprioceptive and interceptive cerebellar projections exist for the extensive territory of the trigeminal brain stem nuclei. These nuclei also project to the ipsilateral inferior olive, relaying from there to the contralateral cerebellar cortex and deep nuclei. The cortical representation of the head is directly behind the primary fissure.

Olivocerebellar fibres

Localization in the olivocerebellar system: zones and microzones

Climbing fibres originate exclusively from subnuclei of the contralateral inferior olivary complex and terminate on longitudinal strips of Purkinje cells in the cerebellar cortex; their collaterals end on the cerebellar or vestibular target nuclei of these Purkinje cells. A longitudinal zonal arrangement is therefore characteristic of the organization of the olivocerebellar projection. Moreover, the olivocerebellar projection zones correspond precisely to the corticonuclear projection zones already described (Fig. 20.10). Climbing fibres from the inferior olive are able to modify the cerebellar output in such a way that cells within each subnucleus of the inferior olivary complex monitor the output of a single cerebellar module.

The inferior olivary complex and its climbing fibres can be activated by tactile, proprioceptive, visual and vestibular stimulation, and from the sensory, motor and visual cortices and their brain stem relays. A somatotopic arrangement of body parts, matching the olivary projections on to the cerebellar cortex, has been described in animals.

Olivocerebellar climbing fibre connections

The inferior olivary complex can be subdivided into a convoluted principal olivary nucleus, and posterior and medial accessory olivary nuclei. Olivary fibres form the olivocerebellar projection to the contralateral cerebellar cortex, and give off collaterals to the lateral vestibular nucleus and to the cerebellar nuclei. Climbing fibres terminate on longitudinal strips of Purkinje cells. The zonal patterns of the olivocerebellar and Purkinje–nuclear projections correspond precisely. The accessory olivary nuclei project to the vermis and the adjacent hemispheres. The caudal halves of the posterior and medial accessory nuclei innervate the vermis. The caudal part of the posterior accessory nucleus projects to Deiters’ nucleus and to the B zone of the anterior vermis. The caudal half of the medial accessory olive gives rise to a projection to the fastigial nucleus and provides climbing fibres to the A zone. The rostral halves of the accessory olives project to the pars intermedia. Climbing fibres from the rostral dorsal accessory olive give collateral projections to the emboliform nucleus and terminate in zones C1 and C3. Zone C2 receives terminals from the rostral medial accessory olive, which provides a collateral projection to the globose nucleus. The principal nucleus projects to the contralateral hemisphere (D zone), and gives collaterals to the dentate nucleus.

The inferior olivary complex receives afferent connections from the spinal cord and from sensory relay nuclei in the brain stem, including the posterior column and sensory trigeminal nuclei. It also receives descending connections from the superior colliculus, parvicellular red nucleus and related nuclei in the midbrain and a GABAergic projection, mainly crossed, from the cerebellar nuclei and certain vestibular nuclei. The latter two connections are topically organized. Thus, the dentate nucleus projects to the principal nucleus, the emboliform nucleus to the rostral posterior accessory nucleus, and the globose nucleus to the rostral medial accessory nucleus. The fastigial nucleus is connected with the caudal medial accessory olive, but the fibres are less numerous. The caudal posterior accessory olive receives a projection from the lateral vestibular nucleus.

The posterior accessory olive and the caudal half of the medial accessory olive receive an input from the spinal cord and sensory relay nuclei. The middle region of the medial accessory olive receives a projection from the superior colliculus and projects to folium and vermis. The parvocellular red nucleus and related nuclei project to the olive through the ipsilateral descending central tegmental tract, which terminates in the rostral half of the medial accessory olive and the principal olive. The parvocellular red nucleus receives converging projections from the cerebellar nuclei and from the motor and premotor cortex. Direct pathways from the cerebral cortex to the inferior olive are sparse. The indirect pathways via the parvicellular red nucleus are much larger.

Climbing fibres that terminate in the vestibulocerebellum (flocculus and nodule) are derived from neurones of the medial accessory olive which receive a strong descending afferent connection from optokinetic centres in the midbrain. Optokinetic information is used by the flocculus in long-term adaptation of compensatory eye movements. Neighbouring neurones are under vestibular control and project to the nodule and the adjoining uvula.

Vestibulocerebellar fibres

Primary vestibulocerebellar mossy fibres are fibres of the vestibular branch of the vestibulocochlear nerve. They enter the cerebellum with the ascending branch of the vestibular nerve, and pass through the superior vestibular nucleus and the juxtarestiform body. They terminate, mainly ipsilaterally, in the granular layer of the nodule, caudal part of the uvula, ventral part of the anterior lobe and bottom of the deep fissures of the vermis (Fig. 20.9A). Secondary vestibulocerebellar mossy fibres arise from the superior vestibular nucleus and the caudal portions of the medial and inferior vestibular nuclei. They terminate bilaterally not only in the same regions that receive primary vestibulocerebellar fibres but also in the flocculus, which lacks a primary vestibulocerebellar projection (Fig. 20.9B). Some of the mossy fibres from the medial and inferior vestibular nuclei are cholinergic.

Reticulocerebellar fibres

The lateral reticular nucleus of the medulla oblongata, and the paramedian reticular and tegmental reticular nuclei of the pons, give rise to mossy fibres. The latter nuclei also supply major collateral projections to the cerebellar nuclei. Spinoreticular fibres terminate in a somatotopical pattern within the entire lateral reticular nucleus, where they overlap with collaterals from the rubrospinal and lateral vestibulospinal tracts and a projection from the cerebral cortex.

The lateral reticular nucleus projects bilaterally to the vermis and hemispheres of the cerebellum. The projection from the dorsal part of the nucleus, which receives collaterals from the rubrospinal tract in addition to spinal afferents, is centred on the ipsilateral hemisphere. The ventral part of the nucleus, which receives a strong projection from the spinal cord and a collateral projection from the lateral vestibulospinal tract, projects bilaterally, mainly to the vermis. The lateral reticular nucleus provides a strong projection to the superior fastigial nucleus, the emboliform nucleus and the medial pole of the globose nucleus.

The paramedian reticular nucleus consists of cell groups at the lateral border of the medial longitudinal fasciculus. It receives fibres from the vestibular nuclei and the interstitiospinal and tectospinal tracts (which descend in the medial longitudinal fasciculus), and from the spinal cord and the cerebral cortex. It projects to the entire cerebellum.

The tegmental reticular nucleus of the pons is located next to the midline in the caudal half of the tegmentum. It receives afferent connections from the cerebral cortex, tectum, nucleus of the optic tract and cerebellar nuclei via the crossed descending branch of the superior cerebellar peduncle. Efferents from the tegmental reticular nucleus reach the cerebellum through the middle cerebellar peduncle. Some terminate superficially in the cortex of the anterior lobe, but many more end in the simple lobule, folium, tuber, vermis and adjoining flocculus. Additional efferents terminate in the caudal fastigial nucleus, dentate nucleus and lateral parts of the globose nucleus.

Pontocerebellar fibres

The cerebral cortex is the largest single source of fibres that project to the pontine nuclei. Fibres from the pontine nuclei access the cerebellum via the middle cerebellar peduncle, which is the largest afferent system of the human cerebellum. Many corticopontine fibres are collaterals of axons that project to other targets in the brain or spinal cord, e.g. it is likely that all corticospinal fibres give off collaterals to the pontine nuclei. Although corticopontine axons arise from lamina V pyramidal cells, the projection from different areas of the cerebral cortex is highly uneven. The areas of cerebral cortex that project to the pontine nuclei are those that are particularly involved in the control of movement. For example, in the case of visual areas, the input arises from extrastriate visual areas in the parietal lobe whose cells are responsive to movement, and function as important links in the visual guidance of movement. Dorsal pontine nuclei receive collateral branches from corticotectal fibres that project to the superior and inferior colliculi from the parietal, temporal and frontal areas of the cerebral cortex, and from tectopontine relays. The onward pontocerebellar projections are to the simple lobule and to the folium and tuber of the vermis.

Fibres of the pontine reticular nuclei are distributed bilaterally, with ipsilateral predominance, to all lobules of the cerebellum other than the lingula and nodule.

More than 90% of fibres in the middle cerebellar peduncle belong to the cortico-ponto-cerebellar pathway. Corticopontine fibres travel in the cerebral peduncle. Fibres from the frontal lobe occupy the medial part of the peduncle, and fibres from the parietal, occipital and temporal lobes occupy the lateral part. They synapse on some 20 million neurones in corresponding regions of the basilar pons. The onward pontocerebellar mossy fibre projection is predominantly to the lateral regions of the posterior and anterior lobes (Fig. 20.4) but collaterals are given off to the dentate nucleus.

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