Stress Response Related to Vagus Nerve

Stress Response Related to Vagus Nerve Stress is a body response generated by several environment or psychological changes, and it is characterized by a feeling of tension and anxiety. In other words, stress can be described as the human body's abrupt attention toward a significant event or situation (Koolhaas et al., 2011). If stress is not controlled, it can lead to severe physiological consequences for the body, including depression, hypertension, and stroke. In most cases, stress is detectable via a set of signs and symptoms, such as abrupt mood swings, insomnia, indigestion, giddiness, excessive sweating, and anxiety (Koolhaas et al., 2011). To tackle stress, doctors and psychologists recommend various remedial measures that involve moving away from the stressful situation or shifting the mode of response toward such situations. However, the human body also has an innate mechanism to tackle body stress that automatically triggers a set of physiological responses that shift the body to a "rest and digest" state. The part of the central nervous system that controls this stress-decreasing response is called the parasympathetic nervous system.
Human Parasympathetic Nervous System and Stress Management
The parasympathetic nervous system is a part of the autonomous nervous system that controls involuntary actions. This system comprises the nerves originating from the brain (cranial nerves), including the lumber spinal nerves, facial nerves, glossopharyngeal nerves, and the most crucial vagus nerves (Tadi & Tindle, 2021). This part of the human nervous system has specific features that enable it to trigger and maintain a stress-managing response. For example, the axons of preganglionic neurons of the parasympathetic nervous system are longer than that of the sympathetic nervous system. They make synapses with the postganglionic neurons in close proximity to the effector organs. These nerves then communicate sensory signals to the effectors via a very short postganglionic axon (Tadi & Tindle, 2021). So, the communication of sensory information in this form of the nervous system takes place via two motor neurons: postganglionic and preganglionic neurons.
The parasympathetic nervous system is crucial in triggering and maintaining the autonomous stress response mechanism of the body as both preganglionic and postganglionic neurons use acetylcholine as a neurotransmitter to communicate the sensory message to their target organs. This neurotransmitter significantly facilitates effective communication. The primary function of this system is to generate the "rest and digest" state in the body, and for this purpose, it sends sensory messages to particular effectors. These sensory messages generate effective stress coping responses, including slow heartbeat, constriction of pupils, increased bronchial secretions, and enhancement of gut motility to facilitate digestion (Rea, 2016). The working mechanism of the parasympathetic nervous system listed below shows how nerves of this system facilitate the initiation of stress coping mechanisms in the body (Tadi & Tindle, 2021).
* The stimulation of M3 receptors triggers pupil constriction since the sensory information supplied by the PNS nerves stimulates the contraction of the sphincter muscles of the iris.
* The stimulation of M2 receptors in the heart triggers a coping stress response that lowers the heartbeat.
* The nerve impulses of PNS also trigger vasodilation by stimulating the vasculature.
* Similar stimulations of M3 receptors in the lungs increase bronchial secretions and facilitate breathing.
* The PNS also stimulates the salivary gland to produce a large amount of potassium ions, water, and amylase to enhance digestion and lower blood pressure.
* The PNS also stimulates the pancreas to release digestive enzymes and increase the pace of digestion.
These functions of the PNS system make it clear that this system is the core physiological mechanism of the body that creates lasting stress coping responses in the body. However, out of all nerves of the PNS, the vagus nerves play the core role in regulating the body's stress response mechanism. This nerve consists of nearly 71% of all nerve fibers of the PNS and controls the whole area of the viscera and abdomen (Tadi & Tindle, 2021). The following section covers the significance and role of the vagus nerve in the body’s stress coping mechanism.
The Vagus Nerve: Its Role in Stress Management
The vagus nerve is the central nerve of the PNS as it controls a wide range of body functions, including mood response, immunity, digestion, and heartbeat. It is the longest cranial nerve of the body as it starts in the brain and ends in the intestine. This nerve is also associated with facial expression, speech, urine output, and skin and muscular sensations (Breit et al., 2018). Therefore, activities such as taking a deep breath, hugging and cuddling someone, stretching the body, smiling, and laughing activate this complicated network of the vagus nerve that the all ok signal to the brain. The malfunctioning of this nerve system results in various disorders, including depression, epilepsy, irritating bowel syndrome, and others. According to research, several branches of the vagus nerve are spread to the muscles controlling facial and voice movement. The same study also concludes that low vagal activity is responsible for the depressing tone and dismal facial expressions (Green, 1990).
Another medical research verifies that high levels of vagal stimulations are responsible for lowering the heartbeat and minimizing inflammation. This nerve's activity stimulates the release of hormones that calm the immune system's inflammatory response (Henton, 2020). All these functions of the vagus nerve link it with the limbic system of the body; the limbic system is the component of the human brain that is associated with controlling and maintaining essential behavioral responses, especially those linked with fight and flight and rest and digestion responses. This system is essential for human survival since it regulates all the responses and functions necessary for survival.
Thus, by controlling speech, digestive activities, calming facial expressions, lowering heartbeat and inflammatory response, and mood, this nerve plays the most crucial role in stress management in the body. This nerve's active and healthy state ensures a satisfactory stress management response from the body, and its low activity creates several psychological issues, such as depression, anxiety, and high blood pressure that enhance stress. Thus, the role of the vagus nerve in stress management is indispensable. In other words, it is the vagus nerve that keeps the limbic system posted about the response it has to generate; depending upon the body's physiology, it sends either the stop signal to remove stress or the go-ahead signal for fight and flight response.
According to some scholars of medical science, the vagus nerve is very instrumental in establishing the sensory link between the brain and the digestive tract. This link is very significant in keeping the brain updated about the inner physiological and physical conditions of the organs of the alimentary canal via the sensory impulses sent through afferent nerve fibers. Thus, these scholars affirm that the vagus nerve's vital role in the limbic system makes it the favorite target in treating disorders related to psychology and the gastrointestinal tract. According to their research, the manual stimulation of vagal nerve fibers activates the monoaminergic brain systems associated with the limbic system. Consequently, this stimulation minimizes the symptoms of various psychiatric disorders, including mood and anxiety disorders (Breit et al., 2018). Thus, once again, this research study's finding also verifies the significant link between the vagus nerve and limbic system and reinforces the vagus nerve's importance in stress management.
In the same way, in another research study, some patients with epilepsy were treated with electrical stimulation of the vagus nerve to understand its effect on their condition. The result of this stimulation was recorded using single-photon emission computed tomography (SPECT). The result of the treatment showed that at the time of stimulation, positive changes in the regional cerebral blood flow appeared in the patients' brains. Consequently, the prolonged treatment of patients via vagus nerve stimulation produced permanent changes in the cerebral blood flow and significantly improved the patient's medical condition. The study concluded that Acute limbic hyper-perfusion and chronic thalamic hypo-perfusion correlate with positive clinical efficacy (Vonck et al., 2008). This study also verifies that the vagus nerve and limbic system are strongly correlated in terms of functions, and this finding reinforces the vagus nerve's importance in stress management.
Due to the immense significance of this nerve in stimulating relaxing and healthy physiological behaviors, several scholars have proposed The Polyvagal Theory to enlist the broad range of effects of this nerve. The following section deals with the discussion of this theory
The Polyvagal Theory: A New Way to Look into Stress Management
The Polyvagal Theory discusses the vagus nerve's role in regulating emotions, social behavior and activism, and fear management in humans and other vertebrates. Before the introduction of this theory, the central nervous system of vertebrates was considered a diagonal structure consisting of two antagonistic systems. These systems were supposed to work antagonistically, where high stimulation generates less calm and low stimulation generates more calming (Wagner, 2016). However, the Polyvagal Theory, as introduced by Stephan Porges, reveals the existence of an intermediate type of nervous response of the CNS: the social engagement system. This system works in a curious way involving the interplay of alternative calmness and activation. This alternative nerve stimulation pattern is operated by exclusive nerve stimulation by the vagus nerve (Wagner, 2016). According to this theory, this unique social engagement system is beneficial for maintaining and shaping social relationships via adopt...