The nervous system:
The nervous system is like the central command center. It coordinates and dictates the actions of the other systems. It is most relevant to our discussion of anxiety disorders. When the brain initiates a command, it is like watching dominos fall: when you knock one down, it starts a cascading effect. So, let's begin by describing what is happening when the mind perceives danger, or anticipates a threat of danger.
Recall that the difference between fear and anxiety is that fear is a reaction to a danger presently detected in the environment, while anxiety refers to the anticipation of some potential threat, that may or may not happen in the future. However, the body systems do not distinguish between the two. Once the mind perceives a threat, and fear is activated, the cascade will begin. Whether or not the danger is real or imagined, the body begins to marshal the resources it needs to protect itself from this danger (real or imagined). Once fear is activated, both electrical and chemical messengers are sent out from the brain to automatically and immediately prepare the body for protective action whether by fight, or flight. This is how it works:
A portion of the brain, called the limbic system, is primarily responsible for initiating the chemical messenger chain (or domino) that informs the rest of the body, "DANGER-DANGER." As previously mentioned, both fear and anxiety are emotions. The limbic system represents emotional system of the brain. Among the many structures of the limbic system are the hippocampus and the amygdala.
The hippocampus is primarily responsible for memory functions. During anxious arousal, the hippocampus is activated. The hippocampus involvement suggests that previous experiences, and memories of those experiences, can initiate or increase anxiety symptoms. From a survival perspective, this makes sense. It would be helpful for us to remember that being chased by a bear did not result in a pleasant experience and should be avoided. Indeed, certain anxiety disorders are related to memories of past experiences. The amygdala is primarily responsible for regulating emotions such as fear. In addition to regulating emotions, the amygdala is thought to be responsible for detecting potential threats in the environment, and sounding the "DANGER-DANGER" alarm.
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The amygdala leads to another structure called the hypothalamus. The hypothalamus is extremely important with respect to anxiety. It serves to control the autonomic nervous system and several types of chemical messengers including many hormones, and neurotransmitters. The role of these chemical messengers will be discussed in just a moment. The autonomic nervous system, along with these hormones and neurotransmitters, all have key roles in the production of anxiety symptoms as the body prepares for action. So when our bodies sense some sort of danger or threat, the amygdala, via the hypothalamus, sends a message to the autonomic nervous system to prepare for action (fight-or-flight).
The autonomic nervous system (ANS) consists of two opposing sub-systems. These are the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS). Only one of these sub-systems (SNS or PNS) can be active at any one time. To illustrate this idea, the autonomic nervous system is like a light switch-it has an "on" and an "off" position. It cannot be both "on" and "off" at the same time. The SNS is the equivalent of the "on" position of the switch, while the PNS is the equivalent of the "off" position. The SNS is responsible for the fight-or-flight response and preparing the body for action. The activation of the SNS is similar to a military commander, shouting, "Man your battle stations!" Conversely, the PNS is like the "off" position of the switch. It is like the military commander ordering, "Stand-down," or "At ease." The PNS initiates the rest-and-relaxation response. It brings your system back to its normal, "at ease" state.
The SNS has an all-or-none response. This means, as with dominos, if you knock over one, a cascade begins. When the SNS is activated, it causes the adrenal glands to release the chemicals adrenaline and noradrenaline. These are also known as epinephrine and norepinephrine. These chemicals act like the body's fuel. They "rev" up your engine so-to-speak, just like gasoline in a car. As we will soon see, when the body's engines are revved-up, this produces many of the unpleasant physical symptoms of anxiety. Just like a car, your body eventually runs out of fuel. In other words, it cannot be in fight-or-flight mode forever. Your body can run out of these fuels in two different ways. First, a chemical clean-up crew can be dispatched to rid the body of unused adrenaline and noradrenaline. Second, the body can be put into the "off" switch position, meaning the PNS begins to do its job of bringing about rest and relaxation. However, just like a car, the body cannot come to an immediate stop. It must decelerate, sometimes more slowly than we would like. This gradual deceleration actually has a protective function. Imagine the cave man again. If a wild animal attacks him, that animal may return, or may return with reinforcements! So, it is better for the body to remain a bit prepared and on edge just in case the threat returns, rather than returning to a completely relaxed, normal, "at-ease" state too quickly.