First, the Sympathetic Branch — Page 129, Lesson 201

I want to walk you through the stress chapter. We're going to start with the two branches of the autonomic nervous system that handle stress, then move into the three phases of the General Adaptation Syndrome. First, the Sympathetic Branch. This is the part of your nervous system that provides the body with resources to cope with a new and sudden source of stress. It's known as the fight or flight response, and its purpose is to prepare the body and mind for immediate physical activity. Think of it as the instant-reaction system. Then we have the Parasympathetic Branch. This branch does two things: it prolongs the body's mobilisation, giving it time to find a solution to the stressful situation, and it restores the body to normal functioning once the perceived danger has passed. So the sympathetic branch gets you ready to act fast, and the parasympathetic branch either keeps you mobilised longer or calms you back down. Now, the core model here is the General Adaptation Syndrome, which has three phases: Alarm reaction, Resistance, and Exhaustion. Let's go through them one at a time. Phase 1 – Alarm Reaction. In this alarm phase, the brain starts a reaction to the stimulus, and that reaction depends on past experience. The brain will adapt and colour the perceived event with pleasure or displeasure. The sympathetic branch is triggered to mobilise the body and allow it to react. The suprarenal glands — also called the adrenal glands — play an important part here because they secrete the stress hormone adrenaline. Adrenaline causes a massive release of sugar reserves from the liver, and it also brings about a whole set of bodily reactions. Let me list them for you: - The pupils of the eye dilate. - The flow of saliva is inhibited. - The heart rate increases. - The rate and depth of breathing increases. - The bronchi dilate to allow a greater volume of air — that is, oxygen — to the alveoli. - Peristalsis — the movement of food along the digestive system — is inhibited. - Bladder contraction is inhibited. - Blood pressure increases. - Blood flow to the muscles increases. - Muscles are tensed. - The senses are sharpened. So adrenaline causes all of these to increase the body's resources by increasing the energy available. This initial defence mechanism is common to all animals, and it's relatively primitive — it allows you to react physically. It has only indirect effects on the brain, and those effects are: - Acceleration of the activities of the brain. - Improved quality of immediate decisions. - A speeding up of the decision-making process. - Memory improvement. - Improved alertness. So in the alarm phase, you get a physical and mental boost, all driven by adrenaline. Phase 2 – Resistance. This is when the parasympathetic system takes over. It attempts to prolong the mobilisation of the body's resources to give time to find a solution. A different stress hormone is released here: cortisol, also called cortisone. Cortisol assists the body to quickly convert fats to sugar, maintaining the supply of energy to the muscles. So while the alarm phase is the instant adrenaline burst, the resistance phase is the sustained effort, managed by the parasympathetic branch and cortisol. We'll cover Phase 3 – Exhaustion – when we get to it. For now, you have the two branches of the autonomic nervous system and the first two phases of the General Adaptation Syndrome.