Cerebellar Peduncles

The inferior cerebellar peduncle (ICP) has two components. The larger is the restiform body, a purely afferent system, whereas the smaller juxtarestiform body carries both afferent and efferent fibers.

The restiform body (or ICP proper) is located in the dorsolateral medulla, lateral to the vestibular nuclei. Entering the cerebellum, it is situated medial to the middle cerebellar peduncle, conveying uncrossed mossy fiber afferents to cerebellum from the ipsilateral spinal cord and brainstem, and crossed climbing fiber inputs from the contralateral inferior olivary nucleus. Spinal cord inputs in the ICP are from the dorsal (posterior) spinocerebellar tract (DSCT), conveying information from the trunk and lower limbs. The rostral spinocerebellar tract carries information from the upper limbs and the central cervical tract arising from upper cervical segments. From the brainstem, the ICP conveys the cuneocerebellar tract arising in the external cuneate nucleus (also known as the lateral or accessory cuneate nucleus), which conveys information from the upper limb and the reticular formation (reticulocerebellar fibers), the trigeminal principal sensory nucleus (trigeminocerebellar fibers), and the midline raphe. Climbing fibers arise in the inferior olive, cross in the medulla, and course within the ICP to reach the contralateral cerebellar hemisphere.

The juxtarestiform body is a small aggregation of fibers situated medial to the restiform body that enters the cerebellum passing through the vestibular nuclei. It conveys afferent fibers to vermal lobule IX (uvula) and lobule X (the flocculonodular lobe). Primary vestibular afferents arise from the vestibular sense organs (the saccule and utricle) and terminate ipsilaterally; secondary vestibular fibers from the vestibular nuclei terminate bilaterally. Efferent fibers in the juxtarestiform body arise from the cerebellar cortex and fastigial nucleus. Cerebellar cortical axons in the juxtarestiform body emanating from Purkinje cells in the vestibulocerebellum (part of lobule IX, and lobule X) terminate in Deiters lateral vestibular nucleus, and, together with efferents from the anterior vermis, are the only instance of projections from cerebellar cortex bypassing the deep cerebellar nuclei to terminate on a target outside the cerebellum. Juxtarestiform body fibers arising from the fastigial nuclei lead to the vestibular and the reticular nuclei. Axons from the rostral half of the fastigial nucleus course to the ipsilateral brainstem in the fastigiobulbar tract. Axons from the caudal half of the fastigial nucleus cross to the contralateral cerebellum in the uncinate bundle, that is, the hook bundle of Russell, before traveling to the brainstem in the contralateral juxtarestiform body (see Plate 8-10).

The middle cerebellar peduncle (MCP) is a massive tract situated at the lateral aspect of the basis pontis. Axons leave the pontine nuclei, cross to the opposite side of the pons, and course in the contralateral MCP to the cerebellum. The pontine nuclei are an obligatory intermediate link between the ipsilateral corticopontine input via the cerebral peduncle and the contralateral pontocerebellar projections by way of the MCP. A minor projection from cerebellar nuclei back to the pons is also present.

The superior cerebellar peduncle (SCP) transmits efferents from and afferents to the cerebellum. It lies within the posterolateral wall of the fourth ventricle, ascends as the brachium conjunctivum to the midbrain, where it decussates and continues rostrally, carrying ascending projections from the cerebellum to the reticular nuclei in the pons and midbrain, red nucleus, hypothalamic area, and thalamus. The hilus of the dentate nucleus is continuous with the SCP, but there are also axons in the SCP arising from the fastigial, globose, and emboliform nuclei. A descending branch of the SCP leaves the larger ascending component in the rostral pons, descends in the pontomedullary tegmentum, and crosses obliquely to the opposite side of the ventral medulla to terminate in the inferior olive (the cerebello-olivary projection). Afferents to the cerebellum coursing in the SCP arise in the spinal cord and brainstem. These include crossed ventral (anterior) spinocerebellar tract fibers conveying information concerning the contralateral trunk and lower limbs, and both crossed and uncrossed fibers in the central cervical tract. Ipsilateral afferents include tectocerebellar projections from the superior and inferior colliculi in the midbrain, trigeminocerebellar fibers from the trigeminal mesencephalic nucleus, and coeruleocerebellar projections from the locus coeruleus in the pons.

The three peduncles are differentially affected by ischemic, compressive, demyelinating, neurodegenerative, and other disorders. Clinically, peduncle lesions manifestations are heterogeneous, reflecting the wide range of functions subserved by the information they convey between the cerebellum and the remainder of the neuraxis.

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