The Role of the Reticular Activating System in Sleep, Wakefulness, and Attention

The reticular formation (RF) is a network of nerve pathways that acts as a bridge through the central core of the brainstem, connecting to the cerebral cortex. The brainstem is an ancient region located in the lower part of the brain that regulates critical functions for vertebrate survival, while the cerebral cortex is responsible for consciousness and cognitive processes. Prior to the mid-twentieth century, the prevailing notion held that wakefulness was a result of external and internal stimuli affecting the cerebral cortex, and inhibitory factors produced sleep. However, research conducted by Giuseppe Moruzzi and Horace W. Magoun at Northwestern University in Chicago in 1949 challenged this concept and provided new insights into the mechanisms underlying sleep and wakefulness by studying the RF.

As illustrated by feline experiments, one of the roles fulfilled by the reticular activating system is the facilitation of the transition from a relaxed state to a phase of heightened focus and arousal.

Moruzzi and Magoun's experiments involved electrical stimulation of the RF, which caused electroencephalographic (EEG) alterations mimicking arousal in cats. These changes persisted even after severing the ascending sensory pathways that led to the cerebral cortex. Conversely, when the RF was lesioned, cats entered a state of coma, regardless of the integrity of their sensory pathways.

The reticular formation (RF), a key constituent of the reticular activating system (RAS), plays a pivotal role in regulating the transition from deep sleep and relaxation to a heightened state of alertness and focused attention. The RAS acts as a filter, selectively processing external stimuli that are relevant or novel while disregarding familiar and repetitive ones (known as habituation). Additionally, the RF is involved in the transmission of pain signals from the lower body to the cerebral cortex and integrates cardiovascular, respiratory, and motor responses to external stimuli.

The reticular activating system (RAS) is subject to the influence of both cholinergic and adrenergic nerve systems. Cholinergic nerves, which use acetylcholine as their neurotransmitter, are thought to act as chemical modulators of wakefulness, arousal, and rapid eye movement (REM) sleep, with glutamate, the principal excitatory neurotransmitter in the brain, also involved in these processes. Conversely, adrenergic nerves, which rely on norepinephrine as their neurotransmitter, are active during deep sleep and quiescent during REM sleep. A dearth of norepinephrine in the RAS may lead to attention deficit disorder.