So here's another view at how the highly evolved mammal engine that demands really high energy resources is regulated by the autonomic nervous system. The sympathetic nervous system permits the engine to have a variable speed beginning at a heart rate of 100 beats per minute. And the engine is naturally set to this speed. The engine can be turbo charged by the adrenal medulla up to around 200 beats per minute depending on the age of the athlete. The older the athlete, the lower the turbo charge effect will be. When the influence of the adrenal medulla is removed, the engine speed falls back under the influence of the sympathetic nervous system. However, the sympathetic nervous system keeps the heart beating at a natural rate of 100 beats per minute. And energies supplies in the body would quickly deplete at this heart rate if some mechanism was not available to turn the engine down to an idle mode when it was not needed.This is where the nucleus ambiguus branch of the vagus nerve comes into play. And nucleus ambiguus acts as a brake to cut the speed of the engine to a more energy-efficient idle during times of rest and low activity, or low levels of activity. The brake acts quickly to slow things down. And when it's running at idle speed, this is referred to as being under very high vagal tone. Now the brake can be just as quickly removed so that the engine speeds up to the sympathetic nervous system speed almost instantaneously. And this is an essential asset allowing the quick response to a high metabolic demand. Vagal tone is highest during resting situations, such as during sleep, and is reduced when they're metabolically demanding states, such as exercise and stress, and information processing. So when your brain is working, you have a slightly higher heart rate. In mammals, the ancient DMNX maintains its old slowing function. But in a healthy athlete, the slowing effect of the DMNX is usually minimal. It's not bought into play. But if the athlete's sympathetic nervous system is fatigued due to overuse, the DMNX has a higher influence on the athlete's engine. And one effect of the overactive DMNX is to cause expulsion of all undigested food in the gut. This is the ancient survival strategy to reduce the energy requirements of digestion. Now, an overactive DMNX may be one reason why some athletes experience diarrhea when they're overly anxious about a competition. It's possible that the DMNX has increased its activity in response to the stressful situation. And the athlete is experiencing its ancient effects. But It's unclear if that's what's going on in this particular case. Now, the existence of both the nucleus ambiguus and the DMNX can also explain the two types of heart rate responses when an athlete is chronically fatigued due to over training. And as you will soon learn, in some over trained athletes, their heart rate is higher than normal, and in other over trained athletes it's as much lower than normal. And when heart rate is higher than normal, the nucleus ambiguus is providing a lower braking effect on the heart and the sympathetic nervous system is having a higher influence. And when the heart rate is slower than normal, this possibly indicates the sympathetic nervous system is fatigued or even exhausted. And the DMNX is the next evolutionary line of defense. A high vagal tone that slows the athlete's heart rate right down below their normal resting heart rate reflects a fatigued sympathetic nervous system. And in this case, the athlete is possibly quite seriously sick.