Home > Conscious > Chapter 7 > 7.3. The Ascending Arousal System



Figure 7-5. The ascending arousal system. The first branch involves LDT and PPT, projecting to the thalamus. The second branch extends to the basal forebrain (BF) which has direct connection with anterior cingulate cortex (ACC). [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 7-5). The first branch involves laterodorsal tegmental (LDT) and pediculopontine tegmental (PPT). 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 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 mAChR to generate α rhythms (Section 7.5). The thalamus is one of two major sources for α rhythms (Schreckenberger et al., 2004). Another one is anterior cingulate cortex (ACC) (Connemann et al., 2005). Incidentally, the basal forebrain has direct connection with ACC (Koliatsos et al., 1988). Orexin neurons, which are implicated in narcolepsy (De la Herrán-Arita et al., 2011), also project to medial prefrontal cortex, including ACC (Jin et al., 2016). Hence, the ascending arousal system fits very well with the Alpha Hypothesis: consciousness arises from globally synchronized α oscillations.

The α wave, in addition to creating a basic conscious state, also plays critical roles in adding contents. During the REM stage, the activities of both LDT/PPT and basal forebrain are high (Schwartz and Roth, 2008). Consciousness should be largely recovered at this stage. The "dreams" that occur at the REM stage could be the conscious perception of internal brain activities.

During waking and REM sleep, the cholinergic neurons in the basal forebrain fire at the θ band (Lee et al., 2005 ). In Section 5.5, it has been pointed out that the content of consciousness is mostly encoded in the θ-γ nested oscillations. Gamma rhythms are typically produced by the pyramidal neurons coupled with GABA interneurons (Appendix A). They are commonly observed in the cerebral cortex. Thus, the cholinergic neurons projecting from the basal forebrain to the cortex may modulate γ rhythms, creating θ-γ nested oscillations in the cortex.