Ascending reticular activating system

Reticular formation
Axial section of the pons, at its upper part. (Formatio reticularis labeled at left.)
Section of the medulla oblongata at about the middle of the olive. (Formatio reticularis grisea and formatio reticularis alba labeled at left.)
Details
Identifiers
Latin	formatio reticularis
MeSH	D012154
NeuroNames	1223
NeuroLex ID	nlx_143558
TA	A14.1.00.021 A14.1.05.403 A14.1.06.327
FMA	77719
Anatomical terms of neuroanatomy []

The reticular formation is a set of interconnected nuclei that are located throughout the brainstem. The reticular formation is not anatomically well defined because it includes neurons located in diverse parts of the brain. The neurons of the reticular formation make up a complex set of networks in the core of the brainstem that stretches from the upper part of the midbrain to the lower part of the medulla oblongata. The reticular formation includes ascending pathways to the cortex in the ascending reticular activating system (ARAS) and descending pathways to the spinal cord via the reticulospinal tracts of the descending reticular formation.

Neurons of the reticular formation, particularly those of the ascending reticular activating system, play a crucial role in maintaining behavioral arousal and consciousness. The functions of the reticular formation are modulatory and premotor. The modulatory functions are primarily found in the rostral sector of the reticular formation and the premotor functions are localized in the neurons in more caudal regions.

The reticular formation is divided into three columns: raphe nuclei (median), (medial zone), and parvocellular reticular nuclei (lateral zone). The raphe nuclei are the place of synthesis of the neurotransmitter serotonin, which plays an important role in mood regulation. The gigantocellular nuclei are involved in motor coordination. The parvocellular nuclei regulate exhalation.

The reticular formation is essential for governing some of the basic functions of higher organisms and is one of the phylogenetically oldest portions of the brain.

The human reticular formation is composed of almost 100 brain nuclei and contains many projections into the forebrain, brainstem, and cerebellum, among other regions. It includes the reticular nuclei, reticulothalamic projection fibers, diffuse thalamo-cortical projections, ascending cholinergic projections, descending non-cholinergic projections, and descending reticulospinal projections. The reticular formation also contains two major neural subsystems, the ascending reticular activating system and descending reticulospinal tracts, which mediate distinct cognitive and physiological proceses. It has been functionally cleaved both sagittally and coronally.