PSYC317 Sleep 3 Notes
Reticular Formation and Sleep-Wake Control (Pages 1–3)
The reticular formation spans the length of the midbrain, the medulla, and the pons. It contains about 1000 small clusters of cells, and different nuclei within this reticular system are responsive to different aspects of sleep stages. When the midbrain is lesioned, the pathway traverses various reticular nuclei, and evidence suggests there are distinct nuclei that contribute to different sleep-stage characteristics. A caudal (caudal) level lesion can produce a normal sleep–wake cycle, whereas damaging the core of the reticular formation in the midcollicular region yields a continuous slow-wave sleep (SWS) pattern on the EEG. Stimulating the pons portion of the reticular formation induces wakefulness, with desynchronized EEG activity and small amplitude, delta-free patterns; if the cat is asleep when stimulated, it wakes. Taken together, these findings support the existence of a reticular activating system: low activity in the reticular formation corresponds to sleep, while high activity corresponds to wakefulness. There appears to be a sleep–wake switch localized somewhere within the reticular formation.
In addition to the general reticular system, there are specific brainstem nuclei involved in wakefulness and others involved in REM sleep. Wakefulness is supported by particular reticular nuclei that generate arousal and help to rouse us. There are also REM sleep nuclei; severing these during sleep disrupts REM sleep and leaves only SWS activity on the EEG. Two important clusters of cells within the reticular formation are especially implicated in generating sleep, notably REM sleep. A central question remains: what signals trigger sleep onset and entry into REM sleep?
Sleep Pressure, Sleep-Wake Cycles, and Binary States (Page 4)
Humans experience a build-up of sleep pressure, making it progressively harder to stay awake after a certain point. Sleep and wakefulness are binary states, but there must be a signal that gradually drives sleep when we need to rest. There is also a need to override the urge to sleep in situations requiring action, such as escaping a predator. Sleep follows circadian rhythms and sleep–wake cycles operate on a roughly 24–25 hour cycle, indicating an intrinsic rhythm that entrains to environmental cues.
Adenosine and Caffeine as Sleep-Pressure Modulators (Pages 5–6)
Adenosine is the neuromodulator that drives sleep pressure. It is formed during the breakdown of ATP, the energy currency of the cell. Glucose is metabolized to pyruvate in mitochondria, where it is used to generate ATP; ATP breakdown releases adenosine (ADO), which can be released synaptically as a neurotransmitter. During wakefulness and sustained activity, energy use is high, leading to substantial ATP consumption and increased adenosine formation. As wake time lengthens, adenosine levels in the brain ramp up, contributing to the sensation of sleep pressure. During sleep, particularly slow-wave sleep (SWS), adenosine is not released to the same extent; instead, it is converted to inosine, which can be reconverted to ATP, helping replenish energy stores. Enzymatic differences among individuals can affect adenosine production and clearance, leading to differences in sleep and wake durations.
Numerous people experience variability in sleep pressure due to these enzymatic differences. Caffeine, commonly consumed orally, modulates sleep pressure by acting as an antagonist at the A1 adenosine receptor, . This receptor blockade prevents adenosine from exerting its sleep-promoting effects, functioning as a central nervous system (CNS) stimulant. Caffeine’s stimulant effects include sympathetic nervous system activation, increased heart rate, heightened physiological arousal, and extended wakefulness. In short, caffeine supports wakefulness by blocking the receptor, thereby reducing adenosine-mediated sleep pressure.
Acetylcholine and Sleep–Wake Cycles (Pages 7–8)
Acetylcholine (ACh) is a key neurotransmitter in cholinergic nuclei located in two major regions: the brainstem and the basal forebrain. Cholinergic neurons exhibit diffuse projection patterns, with axons spreading widely through the cortex and some descending to the peripheral nervous system. The level of ACh in the cortex and hippocampus varies across sleep–wake states. When awake, cortical and hippocampal ACh levels are high; during sleep stages 1 and 2 (N1–N2), ACh levels fall; REM sleep is characterized by moderate to high ACh levels in the hippocampus and cortex, with REM levels resembling wakefulness. In particular, REM sleep involves hippocampal theta rhythms that depend on ACh to be generated, and dreaming is thought to be closely linked to this wake-like cortical activity facilitated by ACh.
Monoamines, Neuromodulators, and Sleep (Pages 9–12)
Norepinephrine (NE) systems originate from cell bodies in the locus coeruleus (LC) in the brainstem and project diffusely across the cortex. NE is involved in sensory processing, attention, and vigilance. During wakefulness, NE promotes alertness; as sleep onset occurs, NE firing diminishes and NE activity is reduced during all sleep stages, remaining low through SWS and REM sleep. Upon waking, NE exhibits a spike, indicating a re-engagement of attentional systems.
Serotonin (5-HT) originates from the raphe nuclei in the brainstem and reticular formation, with widespread projections influencing automatic movements (e.g., neck movements) and cortical arousal. Across the sleep–wake cycle, 5-HT activity tends to be lower during sleep and rises upon waking. Notably, if one wakes from REM sleep, 5-HT levels can increase rapidly within about 1 second after waking. The serotonergic system interacts with other neuromodulators to regulate arousal and movement.
Dopamine (DA) modulates wakefulness and arousal, with projections from the periaqueductal gray (PAG) region influencing cortical activity and motivation. Dopamine levels are generally low during sleep and higher when awake. The PAG also contributes to dopaminergic signaling to the raphe nuclei, which can in turn influence 5-HT release. The dopaminergic system interacts with NE, 5-HT, and ACh in a coordinated manner to regulate vigilance, locomotion, and arousal.
Histamine, produced by histaminergic neurons in the tuberomammillary nucleus (TMN) of the posterior hypothalamus, promotes cortical arousal and interacts with acetylcholine to sustain wakefulness. Histamine fibers diffuse widely to cortex, contributing to waking states. Antihistamines historically cause drowsiness by blocking histaminergic arousal. The TMN is part of the wake-promoting system alongside LC, ACh nuclei, and orexin systems. The anterior hypothalamus generally supports sleep, while the posterior hypothalamus, including the TMN, supports wakefulness.
Orexin System and Narcolepsy (Pages 13–16)
Orexin-producing neurons are located primarily in the lateral hypothalamus (LH) and the posterior hypothalamic nucleus. Orexin promotes arousal and wakefulness and is broadly excitatory across wakefulness centers, including the LC and TMN. Orexin excites multiple wake-promoting systems, and widespread orexin release helps coordinate arousal across the brain. Narcolepsy is often characterized by a profound deficit in orexin signaling, leading to abrupt transitions into sleep and poor maintenance of wakefulness. The orexin system functions as a master regulator of wakefulness; when orexin neurons are active, they help sustain arousal across other systems. Orexin also supports the ability to override sleep when needed, for example to pursue food during hunger, linking sleep regulation with motivational states.
Orexin neurons exert widespread excitatory effects on wakefulness centers; thus, activating orexin pathways tends to drive the entire arousal network. This interconnectedness helps explain why a lack of orexin can destabilize the sleep–wake switch, increasing susceptibility to sudden sleep episodes. The orexin system also integrates with fight-or-flight responses, suggesting a role in preparing the organism to act in demanding situations. If orexin is active, arousal centers such as the LC and TMN are turned on together, creating a coordinated wakeful state.
The Flip-Flop Switch: POA, Arousal Centers, and Orexin Stabilization (Pages 17–19)
The anterior hypothalamus houses sleep-promoting neurons, notably in the preoptic area (POA). Stimulation of the POA induces drowsiness and sleep, while lesions to POA cause difficulty in falling asleep. Adenosine generally inhibits neuronal firing, but POA sleep neurons are activated by adenosine; as sleep pressure rises, adenosine helps recruit POA neurons to promote sleep. POA neurons project to major arousal centers in the brainstem and release GABA (inhibitory neurotransmitter) onto those centers, thereby suppressing arousal.
The flip-flop circuit is a reciprocal, bistable mechanism that alternates between two states: sleep and wake. In one state, the sleep-promoting POA actively inhibits arousal centers (including dopamine and orexin systems), producing sleep; in the other state, the arousal systems dominate and inhibit the POA, producing wakefulness. This switch is fast but inherently unstable without stabilizing influences. Orexin plays a crucial stabilizing role by maintaining a level of arousal across the system, preventing abrupt transitions into sleep (as seen in narcolepsy when orexin signaling is deficient). Orexin’s broad connectivity with waking centers (e.g., LC) ensures that wakefulness can be sustained in situations requiring vigilance and action, linking sleep regulation with the organism’s motivational and survival needs.
Motivation, Feeding, and Sleep Regulation (Page 20)
If there is a strong motivation to stay awake, orexigenic neurons in the LH are activated, which robustly stimulate the arousal systems and further inhibit the POA, reducing sleep pressure. When energy is abundant and the organism is full, affiliative signals such as leptin may promote sleep by reducing orexigenic drive; individuals with high body fat and high leptin receptor activity may experience greater difficulty staying awake, leading to increased daytime sleepiness due to adaptability of leptin signaling. In summary, wakefulness results from the coordinated activity of orexin-driven arousal systems, while sleep is promoted by POA activity and adenosine-mediated sleep pressure. The balance among these systems—modulated by hunger, energy status, and motivational drivers—determines the overall state of wakefulness or sleep at any given time.