The Sympathetic and Parasympathetic Nervous Systems: How Do They Work Together and Support Each Other?
So, you may recall that the somatic nervous system is the part of the nervous system that voluntarily responds to external stimuli and that the autonomic nervous system is the part of the nervous system that involuntarily regulates internal body functions. Additionally, the autonomic nervous system can be further subdivided into two divisions: the sympathetic nervous system and the parasympathetic nervous system. Both of these systems control the same group of body functions, but they have opposite effects on the functions that they regulate. The sympathetic nervous system prepares the body for intense physical activity and is often referred to as the fight-or-flight response. The parasympathetic nervous system has almost the exact opposite effect and relaxes the body and inhibits or slows many high energy functions. The effects of the parasympathetic nervous system can be summarized by the phrase 'rest and digest'. Let's look at an example of how these two systems function in response to changes in the environment.
The nervous system has several divisions: The central division involving the brain and spinal cord and the peripheral division consisting of the autonomic and somatic nervous systems. The autonomic nervous system (ANS) has a direct role in physical response to stress and is divided into the sympathetic nervous system (SNS), and the parasympathetic nervous system (PNS). When the body is stressed, the SNS contributes to what is known as the "fight or flight" response. The body shifts its energy resources toward fighting off a life threat, or fleeing from an enemy. The SNS signals the adrenal glands to release hormones called adrenalin (epinephrine) and cortisol (Endocrine System). These hormones, together with direct actions of autonomic nerves, cause the heart to beat faster, respiration rate to increase (Respiratory System), blood vessels in the arms and legs to dilate, digestive process to change and glucose levels (sugar energy) in the bloodstream to increase to deal with the emergency. The SNS response is fairly sudden in order to prepare the body to respond to an emergency situation or acute stress, short term stressors. Once the crisis is over, the body usually returns to the pre-emergency, unstressed state. This recovery is facilitated by the PNS, which generally has opposing effects to the SNS. But PNS over-activity can also contribute to stress reactions, for example, by promoting bronchoconstriction (e.g., in asthma) or exaggerated vasodilation and compromised blood circulation. Both the SNS and the PNS have powerful interactions with the immune system, which can also modulate stress reactions. The central nervous system is particularly important in triggering stress-responses, as it regulates the autonomic nervous system and plays a central role in interpreting contexts as potentially threatening. Chronic stress, experiencing stressors over a prolonged period of time, can result in a long-term drain on the body. As the autonomic nervous system continues to trigger physical reactions, it causes wear and tear on the body. It's not so much what chronic stress does to the nervous system, but what continuous activation of the nervous system does to other bodily systems that become problematic.
As the Sympathetic and Parasympathetic Branches of the Autonomic Nervous System work, for the most part, in opposition to each other, there must be some higher control coordinating them to work in harmony, and the first step of this is in the Nucleus of the Solitary tract (Kandel et al. 2000); here, the nucleus receives afferent input from the Facial, Glossopharyngeal and Vagal nerve, and firstly sends this information to both the brainstem and the spinal cord, where basic functions of the Autonomic Nervous System are carried out, but more cleverly, the Nucleus of the Solitary Tract also takes in other information and combines it, the Nuclei of the Solitary tract also project to the Periaqueductal grey, which also receives information from the hypothalamus, the periaqueductal grey takes all this information and then projects to the Reticular Formation of the Medulla, where it controls the co-ordination between behavioural activity and the autonomic nervous system, and example of this is that when doing heavy exercise (behavioural), it’s important your heart-rate increase so your muscles can get a greater oxygen supply from the blood (autonomic). Another important control of the Autonomic Nervous System is in homeostasis, there needs to be cooperation between the baroreceptors or chemoreceptors for example, and the most useful branch of the autonomic nervous system, so hair can stand on end on cold days to preserve heat, for example; this is the job of the hypothalamus (Kandel et al. 2000) as the hypothalamus receives input from pretty much every sensory pathway in the body; a dated study from Swanson and Sawchenko (1983) proved that the Paraventricular Nucleus of the Hypothalamus had descending pathways to the Autonomic Nervous System. As well as direct control over the Autonomic Nervous System, the Hypothalamus also has an indirect influence over it through the use of relays in other parts of the brain (Squire, Berg, Bloom, du Lac, Ghosh and Spitzer, 2008).
As shown above, the endocrine system uses chemical signaling (hormones, produced by glands) while the nervous system uses electrical signaling (neural impulses). The signal transmission of the nervous system is fast because neurons are interconnected, but the functions are more short-lived. Signal transmission in the endocrine system is slow, since hormones must travel through the bloodstream, but the responses tend to last longer.
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