A systems view of biological health

Section 2: Theory

35 : The Three-Compartment Body-Plan

This work is licensed under CC BY-SA 4.0 : also see my full Copyright statement.

The 3-compartment body plan is a basic template for the anatomy and physiology of all "higher" animals (i.e. more complex, larger, and more recently evolved than a shrimp).

Human anatomy and most human physiology and biochemistry conforms to this basic body plan with a relatively small number of adaptations.

Understanding the ANS requires some view of its evolution.

Allowing for the way that it constantly back-tracks, discards, and re-invents itself – evolution has proceeded by a series of major "decisions" that set certain attributes in stone for all future adaptations.

Early Life

The first stage of evolution set up single cell organisms with a basic set of organelles, along with all the necessary tools to survive and self-replicate. Those basic features [1] are passed on to all descendants because that plan has been tried and tested to destruction trillions of times over the first 2.5 billion years of evolution. It is so refined and optimised that it is impossible to discard or replace any one part without everything failing[2]. This plan includes the choice of specific proteins to serve as building blocks, for signalling, and to perform vital metabolic functions. For instance, the molecular form of chlorophyll (the basis of plant metabolism) is almost identical to that of haemoglobin. The choice requires that these molecules are easily synthesised with minimal use of energy or raw materials, easily broken down, stable enough, and they or their parts can participate in as many other functions as possible. There is an integrated optimisation to these bottom-level processes and structures that – once in place – hardly changes across billions of years, and the basic cellular chemistry in archaea (pre-mitochondrial bacteria) is not so different from the basic cellular chemistry in your own body.

Mitochondria

Another major "decision" was to absorb a small bacterium and farm them to produce energy. This proved to be so efficient that most of cellular Life is Eukaryotic – i.e. each cell contains an average of about 200 mitochondria. Each of these tiny bacteria spend its life happily making little energy batteries in the form of ATP. This relationship between the cellular host and its mitochondrial garden is symbiotic, and so is a little ambiguous. Have Eukaryotic organisms domesticated their mitochondria; or have mitochondria found a set of larger hosts that allow them to be the dominant life-form on the planet? It’s an experientially interesting question. If I place my attention on mitochondria then I experience something remarkably similar the luminous clear "Shen" mental state that is "cultivated" in Daoist practice. If I place my attention on muscle and connective tissue then the impression is closer to a normal human sense of self-identity and physical strength (and it’s remarkably difficult to have a strong sense of self if there is any significant loss of embodiment). If I place my attention on the microbiome of the gut, then the net effect is a feeling of something relatively slow and sluggish. So maybe identity is partly dependent on the habitual resting place of attention, and if that is on mitochondria, we become mitochondri-al.

Cooperation

Multicellularity was another important step – the adaptation of cells to live in self-regulating undifferentiated colonies. Cooperation requires trust – an attribute that is no longer trusted in many human affairs. Cooperation also implies the rule of proximity – the fact that organs, cells, and any other anatomical features (Forms) that are adjacent inevitably in-form and affect each other’s Function.

The inverse is also therefore true – so (as one example) the adrenal glands are an adapted branch of the Thoracic sympathetic chain, whose original purpose was to release the substantial amount of energy necessary to separate fresh(er) water from saline sea water – to maintain the correct osmotic environment within the body. So they are ad-renal – they sit on top of the kidneys, because their original function was to provide energy to the kidneys. Water-salt stress (osmotic stress and dehydration) is therefore a major strain on the adrenal system. But it may also be one reason that a salt bath helps people to relax – the saltiness of the water putting the adrenals into an osmotic response, causing their danger-stress function to subside.

The cort-isone-producing cort-ex sits on the outside of the ad-renal-ine producing medulla, and has an immune function. Which is reasonable because all high-energy external action runs the risk of injury, so the immune system must be linked to the way that energy is released for action. Immune response of this kind will be a "bodge and fixit" and mainly interested in containment, because whilst the action continues there is no time for proper healing. Further, when bony animals move then the stronger movements stress bones, and so it is perfectly sensible that adrenal output is supplemented by adrenaline production triggered by mechanical bone stress[3]. Likewise, the gut is most motile when we are relaxed, and so some bacteria in the gut (who particularly thrive when peristalsis is strong) produce and store about 70% of the body’s serotonin – a neurotransmitter with diverse uses, associated with a "chilled" mood.

Specialisation

Then cells started to specialise. Specialisation very quickly led to a mouth being formed (sometimes with an anus at the other end of a tube, and sometimes not). If you have a mouth, then it is useful to have sensory equipment near it, and so the first compartmentalisation is to have a sensory head and mouth sitting on top of a digestive system – such as a sea anemone or a coral polyp. The senses and mouth need to be at the "front" to do their work, so if there were tentacles or limbs to draw the food in, these had to sit slightly behind the mouth, reach forwards, and also have some sensory capacity. Now sensing and movement also require some level of meaning-making (requiring a brain or central nervous system, near the main sensory platform – i.e. the head) and coordination through the whole body – a "peripheral" nervous system. And they use more energy and require more physical anchoring – so have a special place in the way that the body is put together and the energy management is organised and optimised. Each stage builds on, adapts and modifies (and sometimes discards part of) the previous stage. If you were to watch this like a film you would notice that every stage was complete and fully viable in itself.

The Three Compartments

The development of a central nervous system did not replace the distributed nervous system, but added to it. Anything with a CNS also has substantial "peripheral" nerve plexi that organ-ise local compartments. Once we became mobile over 600 million years ago, sometime before the beginning of the Cambrian, the basic human body plan was already devised …

1. A mouth and head with external sensory organs "at the front" with a nervous system devoted to meaning-making in the external world. The fact that it is mobile in a 2-D environment (a shallow sea lit by the sun from above and with a sea bed below) results in bilateral symmetry of senses and limbs.

2. A digestive and reproductive ("vegetative") compartment trailing at the back – the Abdomen, again with its own nervous system.

3. A motor (Thoracic) compartment lying between head and abdomen, with appendages that propel. As the outer shrimp-limbs moved they also circulated internal fluids and powered the gills. As we became larger with more complex energy management the fluid circulation and respiration remained in the Thoracic, but also developed their own self-motility and more complex adaptations of the Thoracic nervous system[4].

The three embryonic layers

Each of these three compartments more or less looks after itself and so has its own nervous system, but also has to work in an integrated way with the other two compartments, because each contributes something unique to the entire organism. Here we have a specific and universal example of Varela’s "Not One, Not Two" in action. This arrangement is so fundamental that the embryonic plate is organised into three layers that represent these three functional body zones:

1. Ectoderm: nervous system (inc. central nervous system) and skin (the skin being a sensory organ and a place of communication with the external environment)

2. Endoderm: the tube of the gut, from mouth to anus

3. Mesoderm: Everything else but particularly connective tissue, bone and muscle

Connectivity between compartments

So in every animal from the evolutionary stage of a trilobite or shrimp (maybe 650 MYbp) there is:

1. A nerve trunk connecting through the whole body. Later, in vertebrates (about 500 MYbp), this becomes the spinal cord.

2. A distributed nerve plexus in the Thoracic that attends to high energy functions (originally the osmotic separation of salt and water, followed by movement). This later becomes the para-vertebral Sympathetic chain.

3. A Thoracic nerve plexus for the gills and fluid circulation that eventually (about 350 MYbp) came to regulate the lungs and heart – the so-called "Ventral Vagus"

4. An abdominal nerve plexus that regulates digestive and reproductive functions – the so-called "Dorsal Vagus"

5. All of these interact with each other (have conversations) not only in the central nervous system, but also in large local Plexi, leading to a 6-way conversation.

Humans share the three-compartment body plan with all animals, birds, fish, insects, and many sea creatures (such as shrimps and crabs). Most neurotransmitters we use are also common across this vast range of Life-forms, to the extent that space travel hibernation research uses grasshoppers as a human analogue.