Reticular formation

Formatio reticularis

Also known as: reticular activating system, RAS, ARAS

A diffuse network of interconnected neurons running through the core of the brainstem that controls arousal and consciousness, muscle tone and posture, autonomic and respiratory rhythms, and pain transmission.

Overview

The reticular formation fills the central tegmentum of the medulla, pons and midbrain, between the named nuclei and tracts. It continues below into the intermediate grey of the spinal cord and above into the intralaminar thalamic nuclei. Its neurons have long, branching dendrites that sample many ascending sensory pathways, and their axons project both up and down. It forms three longitudinal columns:
• the median column: the raphe nuclei, which make serotonin;
• the medial (magnocellular) column: large efferent neurons that give rise to the reticulospinal tracts;
• the lateral (parvocellular) column: mainly afferent and associative neurons, including pattern generators for chewing, swallowing and breathing.
Neurochemically defined groups embedded in or beside it are the locus coeruleus (noradrenaline), the cholinergic pedunculopontine and laterodorsal tegmental nuclei, and the dopaminergic VTA.

The ascending reticular activating system (ARAS) projects to the thalamus (intralaminar and reticular nuclei), the hypothalamus and basal forebrain, and directly to the cortex. It maintains wakefulness and attention, and its activity is modulated across the sleep–wake cycle. Descending reticulospinal fibres regulate antigravity muscle tone, posture and locomotion, and adjust spinal reflex excitability. The reticular formation also hosts the medullary cardiovascular and respiratory centres, the gaze centres (the PPRF for horizontal gaze and the rostral interstitial nucleus of the MLF for vertical gaze), and the PAG–raphe pathway that inhibits pain in the dorsal horn.

Location

Central core (tegmentum) of the medulla, pons and midbrain.

Parts & subdivisions

Raphe nuclei

Midline serotonergic neurons; rostral groups → forebrain (mood, sleep); caudal groups, including nucleus raphe magnus → spinal cord (pain inhibition, motor excitability).

Medial (magnocellular) reticular nuclei

Origin of the pontine (medial) and medullary (lateral) reticulospinal tracts: facilitate or inhibit extensor tone and stretch reflexes.

Lateral (parvocellular) reticular nuclei

Receive collaterals of ascending sensory tracts; central pattern generators; autonomic and respiratory control.

Ascending reticular activating system

Cholinergic (PPT/LDT), noradrenergic (locus coeruleus), serotonergic (raphe), dopaminergic and histaminergic projections to the thalamus and cortex that sustain arousal.

Function

Maintenance of consciousness, arousal and attention (ARAS)

Regulation of the sleep–wake cycle and REM sleep

Control of muscle tone, posture and locomotion (reticulospinal tracts)

Cardiovascular and respiratory centres

Coordination of gaze (PPRF, riMLF)

Descending modulation of pain

Habituation: filtering of repetitive, unimportant stimuli

Clinical significance

Coma

Coma results from damage to the upper brainstem ARAS (pontomesencephalic tegmentum), from bilateral thalamic damage, or from diffuse bilateral cortical dysfunction (metabolic, toxic, hypoxic). A unilateral hemispheric lesion does not cause coma unless it compresses the brainstem by herniation. The level of consciousness is graded with the Glasgow Coma Scale.

Decorticate vs decerebrate posturing

• Decorticate posturing (lesion above the red nucleus): flexed arms and extended legs, because the rubrospinal tract still drives upper-limb flexor tone.
• Decerebrate posturing (lesion between the red and vestibular nuclei, at midbrain or upper pontine level): extension of all limbs, because unopposed reticulospinal and vestibulospinal extensor drive takes over. It carries a worse prognosis.