Scientific Understanding of Consciousness
Schematic of key brain stem nuclei involved in life regulation (homeostasis)
Three brain stem levels are marked in descending order (midbrain, pons, and medulla); the hypothalamus (which is a functional component of the brain stem even if it is, anatomically, a part of the diencephalon) is also included.
Signaling to and from the body proper and to and from the cerebral cortex is indicated by vertical arrows.
Only the basic interconnections are depicted, and only the main nuclei involved in homeostasis are included.
The classic reticular nuclei are not included, nor are the monoaminergic and cholinergic nuclei.
The brain stem is often considered a mere conduit for signals from body to brain and brain to body, but the reality is different.
Structures such as the NTS (nucleus tractus solitarius) and PBN (parabrachial nucleus) do transmit signals, from body to brain but not passively. These nuclei, whose topographic organization is a precursor of that of the cerebral cortex, respond to body signals, thereby regulating metabolism and guarding the integrity of body tissues. Moreover, their rich, recursive interactions (signified by mutual arrows) suggest that in the process of regulating life, new patterns of signals can be created.
The PAG (periaqueductal gray), a generator of complex chemical and motor responses aimed at the body (such as the responses involved in reacting to pain and in the emotions), is also recursively connected to the PBN and the NTS. The PAG is a pivotal link in the body-to-brain resonant loop.
It is reasonable to hypothesize that in the process of regulating life the networks formed by these nuclei also give rise to composite neural states. The word feelings describes the mental aspect of those states.
Brain Stem Nuclei Involved in Homeostasis
(Damasio, Self Comes to Mind, 98)