Soul > Part III > Chapter 31 > The Mechanism of Arousal

An adult sleeps for an average of 8 hours a day. During this period, brain activity changes periodically. Each cycle is about 90 minutes (Figure 31c). A cycle consists of four stages N1, N2, N3 and REM, where "REM" stands for "rapid eye movement", "N" for "non-REM" (NREM). From N1 to N3, the sleep deepens. Dreaming is a type of conscious perception that occurs mostly in the REM stage. During deep sleep, consciousness is lost.

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Figure 31c. Stages of sleep. [Image source: Wikipedia]

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Figure 31d. The ascending arousal system. The first branch involves LDT and PPT, projecting to the thalamus. The second branch extends to the basal forebrain (BF). [Source: Schwartz and Roth, 2008 ]

Wakefulness is regulated by the ascending arousal system which starts from the upper brainstem, and separates into two branches (Figure 31d). The first branch involves pediculopontine tegmental (PPT) and laterodorsal tegmental (LDT). The second branch consists of locus coeruleus (LC), dorsal raphe nuclei, tuberomammillary nucleus (TMN), ventral periaqueductal grey matter (vPAG), lateral hypothalamus (LH), and basal forebrain. Each of these regions comprises distinct populations of neurons that release specific neurotransmitters.

  • Cholinergic neurons (releasing acetylcholine) in LDT and PPT nuclei.
  • Noradrenergic neurons (releasing norepinephrine) in locus ceruleus.
  • Serotonergic neurons in dorsal raphe nuclei.
  • Histaminergic neurons in TMN.
  • Dopaminergic neurons in vPAG.
  • Orexin neurons in lateral hypothalamus
  • Cholinergic and GABAergic neurons in basal forebrain.

The cholinergic neurons in LDT and PPT project to the thalamus, releasing acetylcholine (ACh), which may activate its muscarinic receptor M3 to generate alpha rhythms in sensory thalamic nuclei such as lateral genciulate nucleus (LGN) and ventrobasal complex. PPT/LDT also innervate the anterior thalamic nuclei (ATN) which has direct reciprocal connections with anterior cingulate cortex (ACC) (Jankowski et al., 2013). Activation of cholinergic neurons in PPT/LDT is sufficient to induce REM sleep (Van Dort et al., 2015), which may consciously perceive internal brain activities, manifested as dreams.

When we are fully awake and have a clear consciousness, the activities of both thalamus and BF are significantly enhanced. Which one is the major factor for the emergence of consciousness? Early scholars believed that the thalamus is indispensable because external information first enters the thalamus and then transmits to the cerebral cortex through dense nerve circuits (Figure 31d). Recent research has revealed that both are important but play different roles (next section).

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Figure 31e. Thalamocortical network. [Image source: Bathelt J et al., 2016]

 

Author: Frank Lee
Posted on: 2020-08-25