How and why does mental pain turn into physical pain?

Everything that my brain is "full" is, in fact, me, and this "everything" got there through my senses, and not in any other, mystical way. The structure of analyzers and the principle of their functioning are purely material formations and everyone knows how they are arranged and work. The principle of processing, storing and retrieving information in the neurons of the brain is not fully understood, but there is definitely no mysticism there. In this regard, can we say that there are any other phenomena, including in matters of mentality, besides physical? In other words, our body is a "slave" of our central nervous system, the brain in particular.

This issue is dealt with by such sciences as psychosomatics. She tries to connect our emotional experiences and the subsequent physiological changes. It is believed that 80% of our body's resources are spent on servicing the nervous system and only the remaining 20 on everything else. Stress stimulates areas of the limbic system, which also have nuclei responsible for pain. This is one scenario. In addition, negative emotions stimulate the release of catecholamines (adrenaline, norepinephrine, and others like them), which increase the heart rate and constrict blood vessels, therefore, increase blood pressure. Here the heart can already ache. But there is still no exact knowledge on this issue. Again, this refers us not only to medicine, but also to philosophy. The main question is "the relation of matter to idea". Experiences can, despite their neurogenic origin, be considered as an idea, so it turns out that sometimes an idea affects matter?