Myoclonic triangle

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The myoclonic triangle (also known by its eponym Triangle of Guillain-Mollaret) is an important feedback circuit of the brainstem and deep cerebellar nuclei which is responsible for modulating spinal cord motor activity.

The circuit is composed thusly:

1. Fibers project from the red nucleus via the central tegmental tract to the ipsilateral inferior olivary nucleus.
2. The inferior olivary nucleus sends its afferents via climbing fibers in the inferior cerebellar peduncle to Purkinje cells of the contralateral cerebellar cortex.
3. The Purkinje cells send their afferents to the ipsilateral dentate nucleus.
4. The dentate nucleus afferents travel via the superior cerebellar peduncle to the contralateral red nucleus, thus completing the cycle.

Of note, this circuit contains a double decussation, implying that a lesion in this tract will cause ipsilateral symptoms.

The descending rubrospinal tract and reticulospinal tract originate in the red nucleus and reticular formation (which is closely associated with the central tegmental tract) respectively, thereby providing the mechanism by which this circuit exerts its effects on spinal cord motor activity.

Lesions involving this circuit may produce palatal myoclonus, one of the few involuntary movements that do not disappear during sleep. Palatal myoclonus may be seen as a component of the lateral medullary syndrome (a.k.a. Wallenberg Syndrome), if the infarction extends to involve the central tegmental tract.

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