The Limbic System of the brain is the "CEO" of your stress response. It's the region of the brain in charge of keeping all our organs running. It keeps all the amazing mechanisms in our bodies working on autopilot, controlling regulatory functions, such as temperature regulation, sleep-wake cycles, and appetite and hunger signals.
One of the most important jobs of the limbic system is making the subconscious decision for your body to be in fight-or-flight mode when necessary. Once that decision is made, it simultaneously triggers both the nerve and hormone sides of the stress response, resulting in the adrenal glands pumping out stress hormones.
The hormone side of the stress response is referred to as the HPA Axis (Hypothalmic-Pituitary-Adrenal). This axis ends with the triggering of the adrenals to release cortisol. The hypothalamus and pituitary sit just under the limbic system in the brain, although they are part of your hormone system and not your central nervous system.
The nerve side of the stress response refers to the Autonomic Nervous System (or ANS). These are the nerves that control your organs. They also send signals to your adrenal glands to produce adrenaline.
The ANS is a system of nerves; consider the ANS as two wires from your brain that travel to every organ in the body. One wire is a “fight-or-flight” and the other is a “rest-and-digest." These functions oppose one another; the ANS can only fire off one wire at a time. When a person is stuck in fight-or-flight mode, the body is hyper-focused on survival and nothing else.
I tease these two systems apart for two reasons. One, I need to understand the exact cause of my patients' symptoms.
For example, when a patient experiences a rapid heart beat under stress, it is primarily the result of the main fight-or-flight nerve to the heart, and therefore, an ANS issue. But if a patient has low blood sugar, which is the result of a cortisol imbalance, there is instead a hormone or HPA axis issue.
The second reason I address the hormone and nerve sides of the stress response individually may be obvious: Nerves are inherently different than hormones. That means the treatments are also different. In other words, if the rapid heart beat occurs because of an ANS issue, my treatment is going to be quite different than if it's a result of a hormone imbalance. Targeted treatment is essential to healing.
All that said, you absolutely cannot affect one of these systems without affecting the other. They work together intimately.