Synopsis
- Neurotransmitter, popularly known as brain chemicals, are also used by the nervous system of the gut. For many of these the primary place they are made is in the gut, not the brain.
- These various neurotransmitters have effects in the GI tract and in the brain, influencing digestive processes along with mental-emotional states.
- By examining the nervous system of the gut, and the connection between the gut and the brain, the Stomach in Chinese Medicine starts to make sense from a Western medical standpoint.
In part 1 I introduced the roles of the Stomach from the perspective of Chinese Medicine. Part 2 began to correlate the gastrointestinal tract overall to the Stomach via the gut hormones. Now we will examine the enteric nervous system and its similarities to the Stomach.
Neurotransmitters
In American popular culture neurotransmitters are known as brain chemicals, since many mental health issues have been ascribed to an imbalance of these substances in the brain. What many do not know is that the nervous system in the gut – the enteric nervous system – uses all these same neurotransmitters. It is helpful to examine how they influence the gut as well as the mind, giving biomedicine a way to talk about integrated organ function as it is understood in Chinese Medicine.
Serotonin
(1, 2, 3) – This well-known neurotransmitter plays a significant role in gut function. Serotonin increases gut motility and secretions. Other digestive functions are unclear, though there is speculation that serotonin is involved in functional gut disorders like irritable bowel syndrome (IBS). Serotonin syndrome, a state of excessive serotonin, causes nausea, vomiting, diarrhea, sweating, tachycardia, tremor, agitation, pressured speech, hyperthermia (fever), delirium, muscle rigidity, up to seizure, respiratory or kidney failure, coma, and death. Serotonin has been connected to anxiety, depression, and other mental health issues but definitive actions are not clear.
Serotonin syndrome as described above is an almost perfect picture of an acute Yangming state. Remember from part 1 that the Stomach is a Yangming organ; excess serotonin creates a state of yang excess due to Stomach counterflow. The significant roles that serotonin plays in gut motility and secretions points to its correlation with Stomach function. As serotonin is a precursor to melatonin, the well-known sleep hormone, and the Stomach has a role in the initiation of sleep, melatonin could be one of the carriers of Stomach function as well.
Melatonin
(4, 5) – Melatonin is known as a hormone related to our circadian rhythms and to sleep, but what most people don’t know is that most of the body’s melatonin in synthesized in the upper part of the GI tract. Since the majority of the body’s serotonin is synthesized in the gut it is logical that melatonin would also be highest there. Melatonin has a variety of actions in the gut, such as protecting against inflammation, regulating intestinal motility, regulating secretions, and modulating the immune system. Outside the gut it is involved in circadian rhythms, promotes sleep, regulates blood pressure, controls tumor growth, protects the bone, and much more. It helps regulate appetite by its interaction with some of the other hormones involved in hunger, such as ghrelin. Interestingly, melatonin from the pineal gland is secreted in a circadian fashion, but melatonin secreted from the gut is in response to food.
Melatonin makes a connection between digestion and sleep, something also ascribed to the Stomach; the rise in melatonin both after food and at night mirror’s the role of the Stomach in promoting digestion and sleep. Inflammation and ulcers in the GI tract are typically attributed to Stomach heat (or fire), which is a form of Stomach counterflow. The ability of melatonin to protect against inflammation correlates with the downward movement and containing effect of the Stomach.
Dopamine
(6, 7, 8) – This well-known brain chemical is also produced in the gut and has a number of effects on digestion. It stimulates digestive secretions, inhibits motility, protects against damage to the gut mucosa, and may inhibit insulin secretion from the pancreas. In the central nervous system dopamine is known for activating “wanting”, playing a role in movement, desire, and pleasure. Dopamine imbalance, especially excess amounts, has been implicated in mental health issues such as bipolar, anxiety, schizophrenia, attention deficit disorder, and addiction. Too much dopamine can also cause cardiovascular signs and symptoms like hypertension and arrhythmia.
Similar to some of the hormones discussed in part 2 there is a connection here between the digestive system, the brain, and the heart. The types of mania associated with bipolar and excess dopamine is a picture of a Yangming state, or Stomach counterflow. The role of dopamine in “wanting” also points towards the Stomach’s role in the activation of both physical and psychological desire.
GABA
(9, 10, 11, 12) – Gamma amino butyric acid, or GABA, is an important inhibitory neurotransmitter in the brain. It reduces neural activity and plays a role in the process of falling and staying asleep. Similar to the other neurotransmitters it also exists in the gut, where it regulates intestinal motility and digestive secretions. There is also evidence it can control inflammation and regulate autoimmune processes, protecting against autoimmune mediated inflammatory damage. Interestingly, GABA can help regenerate damaged pancreatic islet cells in people with type 1 diabetes.
GABA is an important carrier of Stomach function, as it helps control digestion as well as sleep. The influence of the Stomach on inflammation can be seen in how GABA reduces damage from autoimmune processes. The ability of GABA to balance the hyperarousal in manic states points towards this neurotransmitter as one form of the Stomach’s downward movement.
Acetylcholine
(13, 14, 15) – Acetylcholine is the main neurotransmitter for the parasympathetic nervous system (PNS); the PNS is generally known as the “rest and digest” aspect, in contrast to the “fight or flight” (sympathetic) aspect of the autonomic nervous system. Acetylcholine mediates the increase in digestive functions that happens when the PNS activates, and is therefore an integral part of the gastrointestinal tract. Acetylcholine is also used by the nervous system to interface with muscles; diseases that interfere with this tend to cause muscle atrophy. In the brain acetylcholine is associated with learning and memory. Decreased acetylcholine function is known to occur in Alzheimer’s disease, and common medications for dementia increase levels of this neurotransmitter.
The interesting aspect of acetylcholine and its relation to Stomach function is its role in the formation of new memories. The link to digestion is obvious, but the formation of memory can be seen as “digesting” experiences and integrating them into the self. As this type of long-term memory generation happens during sleep, the role of the Stomach in sleep is further reinforced. Additionally, muscle wasting diseases are attributed to the inability of the Spleen to nourish the tissues. Because the PNS is so closely linked to digestion it could be seen as a major link to Stomach function in Chinese Medicine. This suggests other links between the PNS and the Stomach, such as the role of the PNS in regulating immune function and inflammation.
Just as there are a variety of hormones there are also numerous other chemical messengers that function as neurotransmitters. The brief overview above points to how the Stomach in Chinese Medicine functions as an integrated understanding of multiple biomedical systems. In the next section, part 4, I will be examining the community of microorganisms that inhabit the gut, known as the microbiome. The microbiome is yet another system that can be understood through looking at the role of the Stomach.