## 5 Discussion
### Potential Mechanistic Link between ENS Hypoplasia and the CNS  
Of course, an important link that is still unclear is how hypoplastic ENS translates into effects on the CNS. The vagus nerve (VN) is the primary link between the ENS and CNS, consisting of afferent VN fibers going \"up\" from the intestinal wall to the brain and accounting for 90% of all VN fibers, and the remaining 10% of efferent VN fibers going \"down\" from the brain to the gut. Afferent VN fibers impact signals relating to inflammation, satiety, and energy metabolism, while efferent VN nerves affect secretion of gastric acid and digestive enzymes, as well as gastric capacity [21]. Some studies have suggested that vagus nerve stimulation (VNS) may be a promising adjuvant therapy for ASD and other neurodevelopmental disorders [22]. Additionally, mitochondrial dysfunction and impaired energy metabolism may be observed in ASD patients [23; 24]. Given that the brain accounts for nearly a fifth of the body's metabolic demands, and that neurons depend on oxidative phosphorylation, impairments in mitochondrial function would make a developing brain uniquely susceptible to neurodegenerative disorders [24]. Interestingly, it appears that children prenatally exposed to valpro acid (VPA), a medication used for epilepsy and mood swings, might have an increased likelihood of being diagnosed with autism [25; 26; 27], and that VPA may promote mitochondrial dysfunction at least in primary human hepatocytes in vitro [28]. Finally, [PERSON] et al. [29] have shown that VPA administration during pregnancy in a rat model additionally affected gut microbiota, with prenatal exposure to VPA mimicking not only anatomical and behavioral aspects of an autistic brain but also its microbiota. Gut microbiota has also been a focus of intense study in the ASD field. [PERSON] et al. [30] reported that germ-free mice tend to exhibit atypical social behaviors, including avoidance of social interactions and excessive grooming. Some bacterial pathogens, such as _Clostridia_, _Bacteroidetes_, and _Desulfovibrio_, produce propionic acid, which interferes with serotonin synthesis in the gut [31]; other bacteria species have been reported to differ in gut microbiota between autistic and non-autistic children as well [32]. Impaired metabolism of tryptophan, a precursor to serotonin, has also been reported to be associated with an autistic phenotype [33; 34; 35]. Together, these observations suggest that the link between the ENS and CNS perhaps lies in the ability of the vagus nerve to support energy and mitochondrial metabolism in the brain, and a hypoplastic ENS, which occurs because of a combination of factors affecting serotonin's function as a growth factor in the ENS, may provide insufficient signaling and thereby affect neurological function. These considerations are summarized in Figure 6.  
Interestingly, transcranial photobiomodulation has been investigated as a possible therapy for ASD [36; 37], which is hypothesized to work through improving mitochondrial function in the brain [38; 39]; studies evaluating this hypothesis are ongoing [40]. The analysis presented here of course has limitations, including numerous assumptions of mechanism translatability between mice and humans with respect to the impact of serotonin on the ENS. Nevertheless, it does highlight the multi-factorial nature of ASD even within the framework of a simple two-dimensional mathematical model and provides a possible interpretation of the ASD spectrum as a function of ENS size relative to the norm. Further investigation of this hypothesis might involve the development of a genetic panel that focuses on possible genes that contribute to the three mechanisms described here, and then conducting a prospective trial to evaluate the predictive capacity of such a panel. A complementary analysis should be conducted to further elucidate connection between ENS size and its potential impact on metabolism and mitochondrial function in the brain.