THE RAS ACTIVATES THE CEREBRAL CORTEX TO REGULATE ALERTNESS, CONSCIOUSNESS, AND ATTENTION

A. Verrotti^1* and G. D’Amario²

¹ Department of Pediatrics, University of Perugia, Italy;
² Child and Adolescent Neuropsychiatry Unit, Bambino Gesù Children’s Hospital, IRCCS, Rome, Italy.

*Correspondence to:
Prof. Alberto Verrotti,
Department of Pediatrics,
University of Perugia,
06129 Perugia, Italy.
e-mail: alberto.verrotti@unipg.it

Received: 06 June, 2026 Accepted: 04 August, 2026

ISSN 2279-5855 print / 3103-7364 [online] Copyright 2026 © by Biolife Publisher This publication and/or article is for individual use only and may not be further reproduced without written permission from the copyright holder. Unauthorized reproduction may result in financial and other penalties. Disclosure: All authors report no conflicts of interest relevant to this article.

ABSTRACT

The ascending reticular activating system (RAS) is a brainstem neuronal network that receives sensory inputs and projects them through the thalamus and other forebrain structures to the cerebral cortex. By modulating thalamocortical circuits and cortical activity through multiple neurotransmitter systems, the RAS is essential for maintaining wakefulness, awareness, and higher cognitive functions, and its dysfunction can lead to serious alterations in consciousness. The RAS integrates sensory information from external and internal sources to regulate the basic state of the awake brain, choosing what to think about, and self-awareness based on the relevance of incoming stimuli. The RAS activates cortical circuits, maintaining wakefulness and enabling the conscious processing of sensory and cognitive information. By continuously regulating brain activation through complex neurotransmitter systems, the RAS supports adaptation to environmental demands, while dysfunction of this system can lead to severe alterations in consciousness, including coma. The RAS influences cortical activity by modulating brain activation rather than directly transmitting sensory information to the cortex by integrating sensory information and adaptive responses to the environment. Ultimately, the RAS represents a fundamental neural hub that connects activation regulation with cognitive processing, ensuring a dynamic balance between wakefulness, awareness, and adaptive behavior in response to internal and external stimuli.

KEYWORDS: Ascending reticular activating system, RAS, neuronal network, brain, cerebral cortex

INTRODUCTION

The ascending reticular activating system (RAS) is a neuronal network in the brainstem that receives sensory information from various neural pathways and transmits it to the thalamus and other brain structures, which in turn project it to the cerebral cortex (1,2). However, the RAS does not simply spread “activation signals” but modulates thalamocortical activity and arousal through ascending projections and multiple neurotransmitter systems. In this way, the RAS helps regulate alertness, level of consciousness, and attention.

Projections to the thalamus, particularly to the intralaminar and midline nuclei, are essential because the thalamus serves as a relay center for information directed to the cortex (3). Through these connections, the RAS modulates the activity of thalamocortical circuits, promoting a state of alertness and enabling the conscious processing of sensory stimuli. When RAS activity decreases, as during sleep, thalamic and cortical activation also decreases (4). In addition to the thalamus, the RAS sends projections to numerous basal nuclei of the forebrain, including the basal nucleus of Meynert, the medial septum, and other structures involved in modulating cortical activity (5). These areas use neurotransmitters such as acetylcholine, norepinephrine, serotonin, dopamine, and histamine, which help regulate the excitability of the cerebral cortex, attention, learning, and memory.

Thanks to this complex network of connections, the RAS not only re-stimulates the brain, but also continuously adjusts its activation level based on the body’s needs (6). A lesion of the RAS or its projections to the thalamus can cause a marked reduction in consciousness, even coma, demonstrating the essential role of these pathways in maintaining alertness and higher cognitive functions (7).

DISCUSSION

The RAS receives input from numerous sensory pathways, both from the external environment and from the body itself. This allows it to continuously integrate sensory information and modulate the level of alertness and attention based on the relevance of the stimuli received. Unlike specific sensory pathways, which transmit detailed information to specific areas of the cerebral cortex, the RAS performs a general regulatory function of the brain’s activation state (8). Inputs to the RAS come from multiple sensory systems, including somatosensory, visual, auditory, vestibular, and nociceptive pathways (9). Many stimuli, such as a sudden noise, a bright light, a sharp pain, or a change in body position, can activate the RAS, causing a rapid increase in alertness (10). This allows the system to react promptly to potentially important or dangerous stimuli.

In addition to external stimuli, the RAS also receives information about the body’s internal state, such as changes in respiration, blood pressure, and other physiological parameters. This information helps regulate the sleep-wake cycle and adapt the level of brain activation to the individual’s metabolic and behavioral needs. The integration of all these inputs makes the RAS a key structure for selecting the most significant stimuli (11). Through its projections to the thalamus, basal forebrain, and cerebral cortex, the RAS increases attention to relevant stimuli and reduces the influence of irrelevant ones (12). For this reason, the RAS is essential in the processes of alertness, attention, consciousness, and the body’s adaptation to the surrounding environment (13).

The RAS activates the cerebral cortex through a complex network of connections involving the thalamus and the basal forebrain (8). These pathways transmit excitatory signals that increase the activity of cortical neurons (14). Activation of the cortex allows for the maintenance of wakefulness and an adequate level of attention (15), allowing the brain to consciously process stimuli from the environment and the body. If RAS activity is impaired, cortical activation also decreases, resulting in reduced consciousness and, in severe cases, coma (16,17).

RAS activity keeps the cortex sufficiently excitable to process sensory and cognitive information. This allows the individual to be alert, attentive, and aware of their surroundings. The RAS also helps regulate the transition between sleep and wakefulness, adapting the level of brain activation to the body’s different needs (18). Its function depends on the coordinated action of several neurotransmitters, including acetylcholine, norepinephrine, serotonin, dopamine, and histamine (19). The RAS does not directly transmit all information to the cerebral cortex. Its primary function is not to send specific sensory signals, but rather to regulate the overall level of brain activation (20). To perform this task, RAS projects primarily to the thalamus, particularly the intralaminar and midline nuclei, and to other structures of the basal forebrain, which serve as modulatory centers of cortical activity (21). Receiving projections from the RAS, it modulates the transmission of information to various cortical regions, regulating the excitability of cortical neurons (22). This allows the cortex to maintain a state of adequate alertness, necessary to perceive stimuli, maintain attention, and process information consciously.

In addition to the thalamus, the RAS sends projections to the basal forebrain, which is rich in cholinergic neurons, which help spread activation signals across vast areas of the cerebral cortex (23). The coordinated action of these structures allows the level of cortical activation to be modulated according to the body’s needs, promoting the transition between sleep and wakefulness and adaptation to environmental stimuli.

The RAS plays a fundamental role in regulating vigilance (24). It sends activation signals to the thalamus and cerebral cortex, maintaining the brain in an appropriate state of wakefulness (25). Vigilance represents the state of wakefulness and the general level of nervous system activation that allows an individual to be receptive to internal and external stimuli. Thanks to the RAS, the brain can effectively switch between sleep and wakefulness and maintain an adequate level of attention during daily activities. Proper functioning of the RAS is essential for maintaining consciousness, concentration, and the ability to respond to environmental stimuli (26). Consciousness requires an adequate level of brain activation, largely ensured by the activity of the ascending reticular system. Thanks to the RAS, the brain is able to integrate stimuli from the external environment and internal conditions of the body, enabling a conscious response (27).

The RAS contributes to the regulation of attention, or the brain’s ability to select and focus on the most important stimuli. The RAS modulates the level of activation required to maintain concentration. This system allows for the filtering of numerous environmental information, facilitating the processing of stimuli considered relevant (28). Adequate RAS activity allows for maintaining attention during a task, reducing distractions from secondary stimuli.

An alteration in the RAS’s function can can cause changes in consciousness and impair the ability to concentrate, causing difficulty paying attention and reduced mental alertness, drowsiness, and, in severe cases, loss of consciousness (29-31).

The RAS pathway is closely linked to several neurodegenerative diseases (32). RAS translation produces several dipeptide repeat proteins such as poly-GA, -GP, -Gr, -PR, etc., and some of these proteins, such as poly-GR and poly-PR, are highly neurotoxic (33). RAS translation with protein generation has been discovered in Huntington’s disease, spinocerebellar ataxia type 8, myotonic dystrophy types 1 and 2, and other forms (1). Toxic RAS proteins are produced in an unusual manner, accumulate in neurons, and form intracellular inclusions and protein aggregation. This causes cellular stress with alterations in nucleocytoplasmic transport, ribosome function, proteostasis, and stress granule dynamics, leading to neuronal death and neurodegeneration. RAS translation represents a promising strategy for new therapies.

CONCLUSIONS

The RAS plays a fundamental role in activating the cerebral cortex and maintaining the optimal level of brain activity required for daily functioning. By regulating alertness, consciousness, and attention, it allows the individual to remain awake, aware of the environment, and able to focus on relevant stimuli.
Therefore, the proper functioning of the RAS is essential for cognitive processes, adaptation to the environment, and interaction with the surrounding world.

Conflict of interest

The authors declare that they have no conflict of interest.

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