Reticular formation


Refers to a set of interconnected nuclei that are located throughout the brainstem.

It is not anatomically well defined because it includes neurons located in diverse parts of the brain.

Its neurons 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.

It 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.

Its neurons, particularly those of the ascending reticular activating system, play a crucial role in maintaining behavioral arousal and consciousness.

It includes the reticular nuclei, reticulothalamic projection fibers, diffuse thalamo-cortical projections, ascending cholinergic projections, descending non-cholinergic projections, and descending reticulospinal projections.

The ascending reticular activating system and descending reticulospinal tracts, which mediate distinct cognitive and physiological proceses.

The functions of the reticular formation are modulatory and premotor.

The modulatory functions are primarily found in the rostral sector of the reticular formation.

The premotor functions are localized in the neurons in more caudal regions.

The reticular formation is divided into three columns:

Raphe nuclei (median)

gigantocellular reticular nuclei (medial zone), with large size cells.

parvocellular reticular nuclei (lateral zone), and have smaller size cells

The raphe nuclei are the site of synthesis of serotonin, a neurotransmitter, playing an important role in mood regulation.

The gigantocellular nuclei are involved in motor coordination.

The parvocellular nuclei regulate exhalation.

It is composed of almost 100 brain nuclei and contains many projections into the forebrain, brainstem, and cerebellum, among other regions.

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.

The medial reticular formation has long ascending and descending fibers, and is surrounded by the lateral reticular formation.

The lateral RF is close to the motor nuclei and mostly mediates their function.

The medial reticular formation and lateral reticular formation send connections through the medulla and into the cranial nerves, including the vagus nerve.

The Lateral RF has ganglions and interneurons around the cranial nerves, and mediates their characteristic reflexes and functions.

The reticular formation consists of more than 100 small neural networks manifesting:

Somatic motor control – Some motor neurons send their axons to the reticular formation nuclei, giving rise to the reticulospinal tracts of the spinal cord functioning to maintain tone, balance, and posture during body movements.

It relays eye and ear signals to the cerebellum so that the cerebellum can integrate visual, auditory, and vestibular stimuli in motor coordination.

Other motor nuclei include gaze centers, those that produce rhythmic signals of breathing with swallowing, and with defecation intensity and urination.

Iincludes the cardiac and vasomotor centers of the medulla oblongata.

The reticular formation transfers pain signals from the lower body to reach the cerebral cortex.

The reticular formation is also the origin of the descending analgesic pathways that act to block the transmission of some pain signals to the brain.

It has projections to the thalamus and cerebral cortex that allow it to exert some control over which sensory signals reach the cerebrum and come to our conscious attention.

It plays a central role in consciousness, like alertness and sleep.

Injury to the reticular formation can result in irreversible coma.

It has a role in habituation, when the brain learns to ignore repetitive, meaningless stimuli while remaining sensitive to others.

The reticular formation nuclei modulate activity of the cerebral cortex and is part of the ascending reticular activating system.

The ascending reticular activating system (ARAS), also known as the extrathalamic control modulatory system or the reticular activating system (RAS).

The ascending reticular activating system is a set of connected nuclei that is responsible for regulating wakefulness and sleep-wake transitions.

The ascending reticular activating system is mostly composed of various nuclei in the thalamus and a number of dopaminergic, noradrenergic, serotonergic, histaminergic, cholinergic, and glutamatergic brain nuclei.

The ascending reticular activating system is composed of several neuronal circuits connecting the dorsal part of the posterior midbrain and anterior pons to the cerebral cortex via distinct pathways that project through the thalamus and hypothalamus.

The ascending reticular activating system has more than 20 different nuclei on each side in the upper brainstem, the pons, medulla, and posterior hypothalamus.

The neurotransmitters of the ascending reticular activating system release include dopamine, norepinephrine, serotonin, histamine, acetylcholine, and glutamate.

The thalamic pathway consists primarily of cholinergic neurons in the pontine tegmentum.

The hypothalamic pathway is composed primarily of neurons that release monoamine neurotransmitters, namely dopamine, norepinephrine, serotonin, and histamine.

Key components of the ascending reticular activating system:

Nucleus type nuclei that mediate arousal Dopaminergic nuclei

Ventral tegmental area

Substantia nigra pars compacta

Noradrenergic nuclei

Locus coeruleus

Related noradrenergic brainstem nuclei

Serotonergic nuclei

Dorsal raphe nucleus

Median raphe nucleus

Histaminergic nuclei

Tuberomammillary nucleus

Cholinergic nuclei

Forebrain cholinergic nuclei

Pontine tegmental nuclei: laterodorsal and tegmental nucleus

Glutamatergic nuclei

Brainstem nuclei: parabrachial nucleus, precoeruleus, and pedunculopontine tegmental nucleus

Hypothalamic nuclei: supramammillary nucleus

Thalamic nuclei

Thalamic reticular nucleus

Intralaminar nucleus, including the centromedian nucleus

The ARAS consists of areas of the brain, which crucial to survival and protected during adverse periods.

It functions during inhibitory periods of hypnosis.

It sends neuromodulatory projections to the cortex, mainly connecting to the prefrontal cortex.

It has only low connectivity to the motor areas of the cortex.

The ascending reticular activating system is an significant enabling factor for the state of consciousness, contributing to wakefulness as characterised by cortical and behavioural arousal.

ARAS’s main function is to modify and potentiate thalamic and cortical function.

The brain’s electrical activity varies during periods of wakefulness and sleep.

Low voltage fast burst brain waves are associated with wakefulness and REM sleep.

High voltage slow waves are found during non-REM sleep.

Stimulation of the ARAS produces EEG desynchronization by suppressing slow cortical waves (0.3–1 Hz), delta waves (1–4 Hz), and spindle wave oscillations (11–14 Hz) and by promoting gamma band (20 – 40 Hz) oscillations.

The change from a state of deep sleep to wakefulness is reversible and mediated by the ARAS.

The ARAS also helps mediate transitions from relaxed wakefulness to periods of high attention, and there is increased regional blood flow in the midbrain reticular formation (MRF) and thalamic intralaminar nuclei during tasks requiring increased alertness and attention.

Mass lesions in brainstem ARAS nuclei can cause alterations in level of consciousness.

Damage to the reticular formation of the midbrain may lead to coma or death.

There is a general decline in reactivity of the ARAS with advancing years.

Impairment of the ARAS has been implicated in the following disorders:



Post-traumatic stress disorder

Parkinson’s disease

REM behavior disorder

Progressive supranuclear palsy



Alzheimer’s disease

Attention deficit disorder

Premature birth induces persistent deleterious effects on arousal and sleep-wake abnormalities, attentional and cortical mechanisms throughout development.

The reticulospinal tracts, known as the descending or anterior reticulospinal tracts, are extrapyramidal motor tracts that descend from the reticular formation.

The reticulospinal tracts act on the motor neurons supplying the trunk and proximal limb flexors and extensors, and are involved mainly in locomotion and postural control.

The descending reticulospinal tracts are one of four major cortical pathways to the spinal cord for musculoskeletal activity.

The medial reticulospinal tract is responsible for exciting anti-gravity, extensor muscles.

The fibers of the medial reticulospinal tract arise from the caudal pontine reticular nucleus and the oral pontine reticular nucleus and project to the lamina VII and lamina VIII of the spinal cord

The lateral reticulospinal tract is responsible for inhibiting excitatory axial extensor muscles of movement, and also responsible for automatic breathing.

The ascending sensory tract conveying information in the opposite direction is known as the spinoreticular tract.

The reticulospinal tracts integrates information from the motor systems to coordinate automatic movements of locomotion and posture, facilitate and inhibit voluntary movement; influences muscle tone, mediate autonomic functions, modulates pain impulses, influences blood flow to lateral geniculate nucleus of the thalamus.

The reticulospinal tracts are inhibited by the corticospinal tract.

Damage that occurs at the level of or below the red nucleus is called decerebration, and causes decerebrate rigidity.

Decerebrate rigidity is an unopposed extension of the head and limbs.

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