Reticular formation

Lateral group of nuclei

The lateral group of nuclei, as the name suggests, occupies the lateral region of the brainstem. When superimposed on the posterior surface of the brainstem, the lateral group extends caudally from the level of the inferior colliculus (lower two bodies of the corpora quadrigemina) to the spinal cord.

The following nuclei are a part of the lateral group of the reticular formation:

• There are three groups of cells found throughout the lateral group of reticular nuclei. Two of these cell groups are catecholamine based and have been classified as noradrenergic cells A1-A7 (excluding A3 in primates) and adrenergic cells C1 and C2. The third are cholinergic cells Ch5 and Ch6.
• The lateral pontine tegmentum is the most rostral aspect of the lateral group of nuclei. It is subdivided into the medial and lateral parabrachial nuclei and the Kölliker-Fuse nucleus. The parabrachial nuclei are found in the inferior aspect of the midbrain, adjacent to the superior cerebellar peduncle and cranial to another reticular nucleus known as the parvocellular area (discussed later). The Kölliker-Fuse nucleus is a ventral continuation of the lateral pontine tegmentum. Sensory fibers associated with these nuclei are associated with the nucleus tractus solitarius and the island of Reil (insula). The medial parabrachial and Kölliker-Fuse nuclei also contain A4, A5 and A7 cells. The area has the responsibility of regulating cardiovascular, digestive and respiratory functions.
• There is another group of reticular nuclei found in the superficial ventrolateral reticular area. These cell bodies, known as the lateral paragigantocellular nucleus, can be found at the level of the facial nucleus, which is located deep to the vestibule of the fourth ventricle (inferolateral to the facial colliculus). They extend as far as the level of the spinomedullary junction, where the medulla oblongata transitions into the spinal cord. It is further subdivided into a caudal portion, called the nucleus retroambiguus, and a cranial segment that integrates with the lateral paragigantocellular nucleus. Catecholaminergic cell types A1, A5 and C1 are also found in this region. It is said to be involved in regulating cardiopulmonary activities and nociceptive response.
• The parvocellular reticular area, also known as the area reticularis parvocellularis, is found deep to the area of the cuneate tubercle, in the caudal aspect of the medulla oblongata. More accurately, the nucleus lies medial to the spinal sensory nucleus of the trigeminal (CN V) nerve. This reticular area contains other nuclei, such as the central nucleus of the medulla oblongata (nucleus reticularis dorsalis), the parvocellular nucleus and the nucleus reticularis ventralis. These nuclei are believed to influence the reflex activity of glossopharyngeal (CN IX) , vagus (CN X), spinal accessory (CN XI) and hypoglossal (XII) nerves.

Medial group of nuclei

Like the lateral group of reticular nuclei, the medial group of nuclei begin rostrally in the mesencephalic midbrain, deep to the level of the superior colliculus. They then extend inferiorly, below the level of the striae medullaris of the fourth ventricle (inferior to the pontomedullary junction). The nuclei contain a mixture of both medium and large neurons; however, the medium sized neurons are of a greater preponderance in the region. The nuclei of the medial reticular group include:

• The nucleus reticularis ventralis (also known as the ventral subnucleus of the medulla oblongata) is the caudal representation of the reticular formation in the medulla oblongata. It continues rostrally as the gigantocellular nucleus (magnocellularis). This nucleus is posterior to the inferior olivary complex, ventrolaterally related to the nuclei of CN XII, and anterior to the CN X nuclei. It is divided into pars alpha and ventral gigantocellular nuclei as the nucleus is traced rostrally. The pars alpha component, which is found lateral to the nucleus raphe magnus (discussed below), contains serotoninergic B3 cells.
• The medial group of reticular cells then transitions from the magnocellularis to the caudal and oral pontine reticular nuclei. The key difference between these groups is that the oral pontine reticular nucleus has small and large cells, but no giant cells; while the caudal pontine reticular nucleus has small and large cells, as well as giant cells. Both the caudal and oral groups of nuclei occupy the space around the motor nucleus of CN V (lateral to the median eminence of the fourth ventricle).
• The cuneiform and subcuneiform nuclei are the mesencephalic midbrain representations of the reticular formation. The latter is situated ventral and lateral to the former; both of which are in the region deep to the corpora quadrigemina. The cuneiform nucleus is made up chiefly of small cells mixed with medium and large cells, while the subcuneiform has a similar composition; however the cells are less tightly packed together when compared to those in the cuneiform nucleus.

Median group of nuclei

The dorsal median sulcus that traverses the dorsal spinal cord and continues cranially to divide the brainstem into symmetrical halves also serves as a landmark for the location of the median group of nuclei. These nuclei are also known as the raphe nuclei, as they are found deep to the level of the midline raphe (or the paramedian zone) from the level of the superior colliculus to the superior two-thirds of the medulla oblongata. The raphe nuclei are divided into nine groups of serotoninergic cell clusters B1-B9 (except B4 cells in primates) that appear almost continuous along the tegmentum.

Included in the group of raphe nuclei are:

• The dorsal raphe (tegmental) nucleus can be found throughout the mesencephalic midbrain. It is the most rostral of the raphe nuclei and contains chiefly B7 cells.
• Inferior to the dorsal raphe nucleus is the superior central nucleus. This nucleus is populated by B6 and B8 cell types.
• The pontine raphe nucleus is located between the superior central nucleus and the nucleus of raphe magnus. The nucleus is made up of B5 cells.
• The nucleus raphes magnus is a B3 filled nucleus that is found inferior to the pontine raphe nucleus, the nuclei raphe obscuris and pallidus.
• The nucleus raphes obscurus and nucleus raphes pallidus reside in the upper two-thirds of the medulla. It crosses the pontomedullary junction and enters the area deep to the obex, and hypoglossal and vagal trigones. Raphe obscuris is mainly populated by B2 cells, while raphe pallidus has mostly B1 cells.