SUPERIOR CEREBELLAR PEDUNCLE (BRACHIUM CONJUNCTIVUM)

The cerebellum plays a very important role in motor coordination. We know many of its inputs (dorsal spinocerebellar, cuneocerebellar, pontocerebellar). Now we need to understand how information EXITS the cerebellum in order to influence overall motor activity.

The majority of fibers that pass OUT of the cerebellum to the brain stem do so via the SUPERIOR CEREBELLAR PEDUNCLE. This peduncle is formed by axons that arise from certain deep cerebellar nuclei. These nuclei lie deep in the cerebellum. The “most famous” of these deep cerebellar nuclei resembles the inferior olive and is called the dentate nucleus (the remaining will be discussed later). This bundle passes rostrally into the dorsal pons where it forms a compact bundle along the dorsolateral wall of the fourth ventricle. At the level of the inferior colliculus all fibers of the superior cerebellar peduncle decussate.
The majority of these crossed fibers ascend and either terminate in the red nucleus or continue rostrally to end within the MOTOR NUCLEI OF THE THALAMUS, the VENTRAL LATERAL and VENTRAL ANTERIOR nuclei. These thalamic nuclei project to motor-related cortical areas.





Further details of the pathways connecting the cerebellar cortex with the brain stem and thalamus will be discussed during the lectures on the cerebellum.

You know that lesions of the cerebellar hemisphere result in IPSILATERAL incoordination/ataxia. For instance, a lesion in the RIGHT cerebellar hemisphere will result in motor incoordination/ataxia of the RIGHT arm and leg. A lesion of the RIGHT superior cerebellar peduncle CAUDAL TO (before) the decussation will also result in motor incoordination/ataxia on the RIGHT side. In contrast, a lesion ROSTRAL to the decussation of the superior cerebellar peduncle will result in CONTRALATERAL motor deficits. PLEASE UNDERSTAND THIS!

There would NOT be a Romberg sign. You cannot stand with your heels together to begin with!! The superior cerebellar peduncle is a cerebellar efferent!!

REMEMBER:

1) The cells of origin of fibers of the superior cerebellar peduncle lie in the deep cerebellar nuclei. These nuclei lie deep in the white matter of the cerebellum, under what is called the cortex of the cerebellum.
2) The superior cerebellar peduncle is the principal efferent bundle of the cerebellum, unlike the other two peduncles (inferior or restiform body; middle or brachium pontis) that are conveying information to the cerebellum.
3) The superior cerebellar peduncle crosses at caudal midbrain (inferior colliculus) levels, after which most of the fibers ascend to the red nucleus (rostral midbrain) and dorsal thalamus (ventral lateral and ventral anterior nuclei).
4) The fibers cross (in the caudal midbrain) at the decussation of the superior cerebellar peduncle (brachium conjunctivum).
5) LESIONS OF THE SUPERIOR CEREBELLAR PEDUNCLE BEFORE (CAUDAL) THE DECUSSATION RESULT IN IPSILATERAL DEFICITS IN MOTOR COORDINATION, WHILE INTERRUPTION OF THE FIBERS AFTER (ROSTRAL) TO THE DECUSSATION RESULTS IN CONTRALATERAL DEFICITS.