Abstract

Located within the ascending reticular activating system are nuclei which release neurotransmitters such as acetylcholine, serotonin, dopamine, and norepinephrine. These nuclei have widespread projections that extend into the limbic system and throughout cortex. Activation of these neurotransmitters during awake states leads to arousal, while inhibition leads to the loss of consciousness experienced during slow-wave sleep. Previously, we proposed a mechanism in which cardiorespiratory synchronization may underlie the widespread hyperpolarization that occurs throughout the brain during slow-wave sleep. We further propose that a similar homeostatic mechanism may be involved in sleep-wake transitions and maintaining various arousal states including rapid eye movement sleep, waking, and anxiety. Widespread depolarization associated with more rapid, shallow breathing and desynchronized cardiorespiratory oscillatory activity may underlie waking, anxiety, and rapid eye movement sleep states. The exact voltage values of these widespread membrane potential changes remain unknown and possibly highly variable between different neural areas and cell types. Here, we place these consciousness states on a spectrum of approximated widespread membrane potential values with anxiety states being the most depolarized, followed by waking states, and rapid eye movement sleep. We propose that although these widespread membrane potential changes are minor, they may underlie transitions between and maintenance of varying levels of arousal. Further research on these mechanisms could provide insights into how the brain functions. This homeodynamic arousal mechanism involves the established feed-forward and feedback signaling between the ascending reticular activating system and the hypothalamus, as well as the modulation by cardiorespiratory oscillatory feedback from the body. Understanding the basic mechanisms responsible for the states of sleep, waking, and anxiety could lead to better treatment options in health and disease.