So let's now return for a few minutes to the vagus nerve. If your athletes have an unexplained deterioration in their performance, in conjunction with unexplained fatigue, and they also show lack of motivation, the vagus nerve is likely involved. Now, Professor Steven Porges, has proposed one of the most interesting insights into how the vagus nerve works and he called it Polyvagal Theory. And it explains an awful lot about these symptoms of chronic fatigue, due to overtraining. The theory is elegant and provides really valuable insights for coaches. This slide here is showing a cross section of the brain stem with the vagus nerve and the organs that the vagus nerve innovates, the lungs and the heart and the stomach, etc., as it works its way through the body. The brain stem is located just above and is connected to the spinal cord. The cross section through the brain stem is at the dotted line here, where the vagus nerve actually exits from the brain stem. And here you see the brain stem when the other parts of the brain are stripped away. The vagus nerve is highlighted in green. Now the brain stem is the oldest part of the brain. It controls basic survival functions including heart rate, digestion, blood pressure, and arousal, that is, being awake and alert, and being ready for action. So let's go back to the cross section view and here's where Polyvagal Theory comes in. One portion of the vagus nerve begins in the dorsal motor nucleus X, or DMNX as it's shortened to. The nerve fibers extend from this nucleus to a number of organs, including the heart. The second nucleus of the vagus nerve is called the nucleus ambiguus and the fibers from this nucleus also link into the vagus nerve. The dorsal motor nucleus X is a very ancient nucleus and the nucleus ambiguus evolved much later. The nerve fibers coming from the nucleus ambiguus are fairly fast and therefore, they dominate the solo fibers that come from the DMNX. Both nuclei send branches to the heart and the significance of this will become apparent shortly.