The Hypothalamic-Pituitary-Adrenal (HPA) Axis.
The other, the second, communication pathway is the hypothalamic-pituitary-adrenal (HPA) axis. The HPA tweaks both the physiology and immune functions by cueing the endocrine system into release of informational mass called hormones. (1) It is called indirect because of:
the way the body and health outcomes are affected. The brain must prompt the the release of hormones before adjusting both physiologic and immunologic responses. All changes to the CNS stemming from internal or external stressors or conditioned stimuli culminate the creation of hormones.
This is paramount: Because of the way we elucidate events, distinguished by the body as messages for action, DICTATES which hormones are released!
After the overview of the nervous system and it's workings, let's take an in depth account into mind-body communications beginning with…
The ANS (autonomic nervous system) which is activated by the direct pathway, SAM (sympathetic adrenal medullary). Also remember that the ANS is part of the PNS (peripheral nervous system, 31 pairs of spinal nerves, 12 pairs of cranial nerves which branch of the brain and spinal cord.) Further recollect that the CNS (central nervous system) is not considered a subdivision of the PNS, rather the PNS and CNS make up the nervous system.
The Autonomic Nervous System
The ANS deals with the motor or efferent neurons that are embedded in smooth and cardiac muscles and glands. Efferent neurons or motor neurons carry signals away from the CNS (brain, spinal cord) to return to organs and glands. Afferent or sensory neurons carry signals from the organs and glands back to the CNS. The ANS is constantly bombarded with signals from internal organs. This huge amount of information has to be constantly processed, and the body has to adjust to these signals in order to perpetuate homeostasis and support changing bodily activities and needs. In this dimension the ANS perform awe-inspiring.
In reaction to these alterations the ANS directs blood to areas needed, increases or decreases breathing and pulse, readjusts blood pressure and heart rates, body temperature and stomach secretions. The ANS regulates the adaptation to fight-or-flight or rest-and-digest situations or conditions in between. The rock climber's experience is a perfect description of the fight-flight scenario. On the other hand, sleep is a rest-and-digest scenario.
The ANS is often referred to as the involuntary or automatic nervous system because we are typically not aware of when our pupils dilate or contract or when our blood pressure rises and so forth. Scientist believed for the longest time that we were unable to consciously alter the functions of the ANS.
I hope slowly but surely my intend about these detailed descriptions is becoming a little transparent. The purpose of leading you through this exhausting journey of the nervous system has to be clearly the fact that we can modulate all these functions at will! Sounds preposterous I know.
Again, it is the purpose of Alternative Medicine to keep, or bring back the body into homeostasis (read health). And as mentioned before: The mind is our most powerful tool in achieving this objective.
Next: Which parts of the brain regulate the ANS?
1. Bulloch K., Pommerantz W. Autonomic Nervous System Innervation of Thymic-Related Lymphoid Tissue in Wild Type and Nude Mice. J Comp Neurol. (1984):228:57