Cerebellum Subdivisions and Afferent Pathways

Knowledge of cerebellar connections with extracerebellar structures is critical to understanding the diverse roles of the cerebellum and the consequences of cerebellar injury. Afferents to cerebellum are conveyed predominantly by mossy fibers and climbing fibers that are organized in a fundamentally different manner (see Plates 8-6 and 8-7).

MOSSY FIBER PATHWAYS

Spinocerebellar Pathways. Sensory afferents from the spinal cord terminate in a somatotopic fashion in the primary sensorimotor representation in lobules III through V and the secondary sensorimotor representation in lobule VIII, with collateral inputs to the deep cerebellar nuclei (DCN). The trunk and lower limbs are subserved by the dorsal and ventral spinocerebellar tracts, and the head, neck, and upper extremities by the cuneocerebellar, rostral spinocerebellar, and central cervical tracts. These are all uncrossed, ascending in the ipsilateral spinal cord, except for the ventral (anterior) spinocerebellar tract (VSCT), which decussates and ascends on the contralateral side.

The dorsal (posterior) spinocerebellar tract (DSCT) and the cuneocerebellar tract (CCT) convey analogous proprioceptive and exteroceptive information from the hindlimb and forelimb, respectively. Both enter cerebellum via the inferior cerebellar peduncle (ICP). Proprioceptive information comes from (1) muscle spindle afferents that signal muscle length (groups Ia and II fibers) and (2) Golgi tendon organs that signal muscle tension (group Ib fibers). DSCT or CCT neurons convey information regarding closely related muscles; some relay information from joint receptors. Exteroceptive signals provide the cerebellum with cutaneous afferents originating from touch and hair-movement receptors in small areas of skin. The DSCT in the posterolateral funiculus conveys proprioceptive and exteroceptive afferents from the trunk and legs, arising in Clarke’s column in lamina VII of the dorsal horn at spinal segments C8 to L3. It terminates in hindlimb projection areas in the intermediate part of the ipsilateral anterior lobe and lobule VIII. The CCT ascends from the medulla, conveying proprioceptive afferents from the arms, originating in the external cuneate nucleus, and exteroceptive fibers from the main cuneate nucleus.

The ventral (anterior) spinocerebellar tract and rostral spinocerebellar tract (RSCT) convey information to cerebellum regarding complex motor repertoires. Afferents arise from (1) interneurons within spinal motor centers controlling the hindlimbs (VSCT) and forelimbs (RSCT), (2) group I muscle afferents from Golgi tendon organs in groups of muscles involved in synchronized movements, and (3) multisynaptic spinal pathways activated by cutaneous and high-threshold muscle afferents. The VSCT originates from spinal border cells, mostly in Rexed lamina VII at the posterolateral aspect of the anterior horn of the lumbosacral spinal cord. It decussates close to the cell bodies, ascends in the contralateral anterolateral funiculus, and enters the cerebellum through the superior cerebellar peduncle (SCP). Most of its fibers cross to the other side, hence the double crossing. It terminates in longitudinal zones in the hindlimb representations in the anterior lobe and, to a lesser extent, in lobule VIII. The RSCT originates from neurons at the base of the posterior spinal horn in Rexed lamina VII at spinal cord levels C4 to C8, ascends in the ipsilateral posterolateral funiculus, and enters the cerebellum via both the ICP and SCP, terminating bilaterally in primary and secondary forelimb sensorimotor representations. The corticospinal system facilitates excitatory or inhibitory effects of cutaneous and muscle afferent fibers in the VSCT and RSCT, whereas the reticulospinal system inhibits them. The rubrospinal and propriospinal pathways produce excitation independent of spinal afferents.

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