The anatomical structure and composition of the red nucleus has long been the source of debate among anatomist. This midbrain structure plays an important role in locomotion and several theories suggest that it has evolved dramatically with the advent of bipedalism. Since there is still quite a bit of discord in the scientific community regarding this structure, this article will focus primarily on those characteristics (anatomical and functional) that have been agreed upon within the arena. Special emphasis will be placed on the efferent and afferent pathways, gross anatomy and clinical points associated with the nucleus. Of note, the prefix rubro- and suffix –rubral are used to describe tracts associated with the red nucleus.
The red nuclei are paired oval shaped, midline structures that appear bright red in the freshly dissected specimen. This unique appearance has been attributed to the high vascularity of the structure in addition to the high level of iron pigments within the cytoplasm of its constituent neurons. The nuclei blend rostrally with the nearby reticular formation and interstitial nucleus.
At about 5mm in diameter, it spans the region from the inferior extent of the superior colliculus to the subthalamic part of the diencephalon in the midbrain. Each consists of caudal magnocellular and rostral parvocellular parts that give rise to specific efferent tracts. Each red nucleus is pierced by (but does not communicate with) the oculomotor nerve (CN II) before the nerve leaves the midbrain and passes through the interpeduncular fossa. Fibers of the superior cerebellar peduncle and the retroflex fasciculus also cross this structure. This gives the red nucleus a perforated appearance when stained with Weigert stains.
Anterior to the red nucleus is the anterior tegmental decussation, interpeduncular nuclei and the medial third of the pars reticulata of the substantia nigra. Laterally, the cerebellothalamic fibers and the medial lemniscus tract can be seen. The central tegmental tract (which is a paired structure) is located posterior to each red nucleus. Other posteriorly related structures include the medial longitudinal fasciculi, oculomotor nuclei, mesencephalic nuclei and tracts, trigeminothalamic (anterior and posterior tracts) and the periaqueductal grey region.
As previously stated, the nuclei are midline structures found within the tegmentum of the midbrain at the level of the superior colliculi. The tegmentum is the region of the midbrain anterior to the cerebral aqueduct and posterior to the crus cerebri. The transition point from the tegmentum to the crus cerebri is demarcated by a hyper-pigmented band known as the substantia nigra. The medial third of the pars reticulata of the substantia nigra is proximal to the red nucleus (while the pars compacta is distal to the same).
There are numerous multipolar neurons containing iron pigments throughout the substance of the red nucleus. The smaller multipolar neurons are found throughout the entire red nucleus, but the larger multipolar neurons are only observed in the caudal end. The nuclei are encapsulated by a fibrous layer that circumscribes portions of the corticobulbar (the portion extending through the lenticular fasciculus or Forel’s field H2) and superior cerebellar (from the dentate and interposed nuclei) projections entering the nucleus.
Rubral Afferent Tracts
The ipsilateral motor regions of the cerebrum give rise to corticorubral fibers that converge from its cortical origins to travel through the posterior limb of the internal capsule before terminating in the red nucleus. This particular tract travels parallel to the path taken by the corticospinal tract and has significant overlapping (albeit, non-communicating) axons around the telencephalodiencephalic junction. While there is still debate surrounding the specific origins of these fibers, scientists have so far agreed that the tract commences in the rostral prefrontal (Brodmann areas 9 and 10), premotor (Brodmann areas 6 and 8) and sensorimotor cortices.
Cerebellorubral tracts originate in the contralateral cerebellar nuclei and leave the cerebellum via the superior cerebellar peduncles. After they decussate at the level of the inferior colliculi, the fibers travel from inferomedial to superolateral in order to pierce the tail of the red nucleus. Other fibers of this tract also circumscribe the red nucleus to give the appearance of a capsule.
Rubral Efferent Tracts
The efferent fibres of the red nucleus leave from the structure’s dorsomedial region. These tracts are derived from the magnocellular and parvocellular regions. The magnocellular component gives rise to fibers of the rubrospinal tract. In humans, the magnocellular part of the red nucleus is smaller than in other mammals; the size of the rubrospinal tract is similarly smaller than in other mammals. The fibers decussate in the ventral tegmental decussation and travel in the anterior spinal cord juxtaposed to the lateral corticospinal tract. They subsequently terminate in laminae V (nucleus proprius), VI (controls flexion withdrawal from painful stimuli) and VII (dorsal nucleus of Clarke) of the spinal cord, but do not extend beyond the cervical enlargement. Additionally, unlike the reticulospinal tract, the rubrospinal tract is devoid of collateral fibers that innervate the spinal cord at multiple levels. While some authors believe that this tract is vestigial in humans, others are of the belief that the rubrospinal tract is primarily associated with exciting the proximal limb motor neurons. However, it is believed that the pyramidal tract (consisting of the corticospinal and corticobulbar tracts) have taken over its function and enabled the advent of bipedalism.
The parvocellular region derives the rubroolivary tract. It is considered to be the more developed of the two efferent pathways of the red nucleus. It travels ipsilaterally in the central tegmental tract in order to gain access to the inferior olivary nucleus. The inferior olivary nucleus then projects via the contralateral inferior cerebellar peduncle to the cerebellum. Other fibers also terminate in the reticular nuclei of the brainstem. It is related to the medial longitudinal fasciculus and both red nuclei laterally and dorsolaterally, respectively, at its proximal end.
The posterior cerebral artery is a product of the bifurcation of the basilar artery; the latter of the two being formed from the union of the vertebral arteries. In addition to contributing to the posterior limbs of the circle of Willis, the posterior cerebral artery also gives off thalamo-perforating and posteromedial central (perforating) arteries that supply the red nucleus, along with other midbrain and diencephalic structures.
The posterior communicating artery (an anastomotic branch between the middle cerebral and posterior cerebral arteries) also gives off premammillary arteries to the midbrain. The perforating fibers of both the posterior cerebral and posterior communicating arteries enter the midbrain through the posterior perforated substance in the interpeduncular fossa.