Diseases of the nervous system (1908) (14779159452)

The myoclonic triangle (also known by its eponym Triangle of Guillain-Mollaret or dentato-rubro-olivary pathway) is an important feedback circuit of the

brainstem The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is cont ...

and deep

cerebellar The cerebellum (Latin for "little brain") is a major feature of the hindbrain of all vertebrates. Although usually smaller than the cerebrum, in some animals such as the mormyrid fishes it may be as large as or even larger. In humans, the cerebel ...

nuclei which is responsible for modulating spinal cord motor activity. The circuit is thus composed: # Fibers of the rubro-olivary tract project from the parvocellular red nucleus via the

central tegmental tract The central tegmental tractKamali A, Kramer LA, Butler IJ, Hasan KM. Diffusion tensor tractography of the somatosensory system in the human brainstem: initial findings using high isotropic spatial resolution at 3.0 T. Eur Radiol. 2009 19:1480-8. do ...

to the ipsilateral

inferior olivary nucleus The inferior olivary nucleus (ION), is a structure found in the medulla oblongata underneath the superior olivary nucleus.Gado, Thomas A. Woolsey; Joseph Hanaway; Mokhtar H. (2003). The brain atlas a visual guide to the human central nervous syste ...

. # The inferior olivary nucleus sends its afferents via

climbing fibers Climbing fibers are the name given to a series of neuronal projections from the inferior olivary nucleus located in the medulla oblongata. These axons pass through the pons and enter the cerebellum via the inferior cerebellar peduncle wh ...

in the

inferior cerebellar peduncle The upper part of the posterior district of the medulla oblongata is occupied by the inferior cerebellar peduncle, a thick rope-like strand situated between the lower part of the fourth ventricle and the roots of the glossopharyngeal and vagus ner ...

Purkinje cells Purkinje cells, or Purkinje neurons, are a class of GABAergic inhibitory neurons located in the cerebellum. They are named after their discoverer, Czech anatomist Jan Evangelista Purkyně, who characterized the cells in 1839. Structure The ...

of the contralateral cerebellar cortex. # The Purkinje cells send their afferents to the ipsilateral

dentate nucleus The dentate nucleus is a cluster of neurons, or nerve cells, in the central nervous system that has a dentate – tooth-like or serrated – edge. It is located within the deep white matter of each cerebellar hemisphere, and it is the largest sin ...

. # Dentatorubral tract fibers: the

afferents travel via the

superior cerebellar peduncle In the human brain, the superior cerebellar peduncle (brachium conjunctivum) is a paired structure of white matter that connects the cerebellum to the midbrain. It consists mainly of efferent fibers, the cerebellothalamic tract that runs from a cer ...

to the contralateral

red nucleus The red nucleus or nucleus ruber is a structure in the rostral midbrain involved in motor coordination. The red nucleus is pale pink, which is believed to be due to the presence of iron in at least two different forms: hemoglobin and ferritin. ...

, thus completing the circuit. Of note, this circuit contains a double

decussation Decussation is used in biological contexts to describe a crossing (due to the shape of the Roman numeral for ten, an uppercase 'X' (), ). In Latin anatomical terms, the form is used, e.g. . Similarly, the anatomical term chiasma is named aft ...

, implying that a lesion in this tract will cause ipsilateral symptoms. The descending

rubrospinal tract The rubrospinal tract is a part of the nervous system. It is a part of the lateral indirect extra-pyramidal tract. Structure In the midbrain, it originates in the magnocellular red nucleus, crosses to the other side of the midbrain, and desce ...

and

reticulospinal tract The reticular formation is a set of interconnected nuclei that are located throughout the brainstem. It is not anatomically well defined, because it includes neurons located in different parts of the brain. The neurons of the reticular formation ...

originate in the

and

reticular formation The reticular formation is a set of interconnected nuclei that are located throughout the brainstem. It is not anatomically well defined, because it includes neurons located in different parts of the brain. The neurons of the reticular formation ...

(which is closely associated with the

) respectively, thereby providing the mechanism by which this circuit exerts its effects on spinal cord motor activity.

Pathologies

Hypertrophic olivary degeneration

HOD is caused by lesions in the dentatorubral or central tegmental tracts. Lesions of the superior cerebellar peduncle can also result in contralateral HOD, whereas primary lesions of the central tegmental tract cause ipsilateral HOD. Lesions involving this circuit may produce

palatal myoclonus Palatal myoclonus is a rare condition in which there are rhythmic jerky movements or a rapid spasm of the palatal (roof of the mouth) muscles. Chronic clonus is often due to lesions of the central tegmental tract (which connects the red nucleus to ...

, 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 Lateral medullary syndrome is a neurological disorder causing a range of symptoms due to ischemia in the lateral part of the medulla oblongata in the brainstem. The ischemia is a result of a blockage most commonly in the vertebral artery or the p ...

(a.k.a. Wallenberg Syndrome), if the infarction extends to involve the central tegmental tract.

Holmes tremor

Descriptions of Holmes tremor associated with HOD are scarce. It is most likely that disruption of the disynaptic dentate-rubro-olivary tract degeneration is associated with tremor and disruption of the monosynaptic dentate-olivary tract is associated with HOD. The convergence of both components makes the combination of Holmes tremor and HOD after upper brainstem damage plausible and even likely.{{Cite journal, last1=Raina, first1=Gabriela B., last2=Cersosimo, first2=Maria G., last3=Folgar, first3=Silvia S., last4=Giugni, first4=Juan C., last5=Calandra, first5=Cristian, last6=Paviolo, first6=Juan P., last7=Tkachuk, first7=Veronica A., last8=Zuñiga Ramirez, first8=Carlos, last9=Tschopp, first9=Andrea L., date=2016-03-08, title=Holmes tremor, journal=Neurology, volume=86, issue=10, pages=931–938, doi=10.1212/WNL.0000000000002440, issn=0028-3878, pmc=4782118, pmid=26865524

References

Cerebellar connections Brainstem Central nervous system pathways