Red Nucleus

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.

Gross Anatomy

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.

Red nucleus - caudal view

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.

Location

Red nucleus - lateral-left view

Histology

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

Internal capsule - cross-sectional view

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.

Central tegmental tract - cross-sectional view

Blood Supply

Posterior cerebral artery - caudal view

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.