Activation of subcortical fiber systems such as the locus coeruleus and the ventral tegmental area, both of which have access to wide expanses of the neocortex, accelerates recovery from anesthesia and shortens sleep (Kaitin et al. 1986; Moruzzi and Magoun 1949; Solt et al. 2014). Sensory feedback from the heart, lungs, blood vessels, and digestive tract via the vagus nerve has long been known to be involved in highly emotive behaviors: fighting, fleeing, fornicating, and feeding (Klarer et al. 2014). It has recently been reported that electrical stimulation of the vagal nerve in a vegetative patient induced a 'conscious' EEG profile and some behavioral improvements (Corazzol et al. 2017; also see Dong et al. 2023). The patient had damage of midbrain and thalamic nuclei, which quasi-transects the neocortex from the motor nuclei in the brain stem (Tehovnik et al. 2021). Stimulation of the vagal nerve, when brain stem catecholamines are intact, activates the neocortex via the locus coeruleus and the ventral tegmental area (Dong et al. 2023), which send neocortical projection subjacent to the thalamus (Fig. 1). However, even if the EEG activity suggests neocortical alertness in a vegetative patient, once the motor system is disconnected from the neocortex, the neocortex will have an altered state of consciousness since it has no feedback from the cerebellum via the thalamus (Hasanbegović 2024). Moreover, even though patients in a vegetative state can exhibit ‘conscious’ EEG activity during REM sleep, this activity is severely curtailed as compared to that found in normal subjects (Oksenberg et al. 2001). Therefore, the evocation of a ‘conscious’ EEG by vagal stimulation is somewhat misleading, since the neocortex in a vegetative state cannot command body movements and in the absence of cerebellar connectivity, a subject’s automatic movements (if there should be any movements at all) are isolated from the neocortex thereby further degrading neocortical control (Tehovnik, Hasanbegović, Chen 2024). Accordingly, consciousness depends on an intact and integrated hippocampus, neocortex, and cerebellum, which together represent over 98% of the neurons in the brain (Herculano-Houzel 2009). We can live without our eyes, our cochlea, or our limbs (albeit with extreme difficulty), but the central nervous system is a ‘merger’ of information (Chomsky 1965) and therefore depends on a connected nervous system (Tononi et al. 2008ab). The extent of this integration is profound: one cortical neuron can send thousands of collaterals to vast portions of the neocortex and subcortex, including the cerebellum and spinal cord (see Fig. 2). Hence, much like consciousness, which cannot be broken up into smaller parts (Tononi et al. 2008ab), the brain too cannot be broken up, even though neuroscientists are notorious for reducing the brain into its component parts right down to a single neuron and further.
Figure 1: Projections from the locus coeruleus to the neocortex in humans. From figure 1 of Luppi et al. (2021).
Figure 2: A neocortical neuron following reconstruction of its cortical and subcortical collaterals with the main axonal branch destined for the cerebellum in the mouse. From figure 2D of Hasanbegović (2024, pp. 116).
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