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The ANS is an important part of the central and peripheral nervous
systems, largely concerned with regulating physiology
One way to think of the body is to say that it is a substantial part of (what is called) the subconscious – the things that we know but don’t know how we know, or even don’t know that we know. However, if you spend even a little time being aware of the moment-to-moment interactions between your body and mind, it becomes clear that the boundary between conscious / cognitive wilful action and fully automatic (i.e. subconscious) activity – is very blurry indeed. In fact, it is exactly the same as the sensory window of attention – we can consciously regulate the breath cycle, can "think of a lemon" and initiate a salivary reflex, and many other things that show the boundary between conscious control and subconscious automatic response is strongly influenced by attention.
The ANS is the anatomical part of the nervous system that is supposed to handle all of this local autonomy (hence its name – "Autonom-ic").
The ANS can be described in a very generic manner as having two branches – one of which (the Sympathetic [1]) facilitates high-energy activity, releasing energy (via Thyroxine and Adrenaline), controlling vascular tension (and therefore directing blood flow) and affecting immune function via release of cortisol. This is balanced by the Parasympathetic nervous system – mainly the Vagus Nerve – that mediates vegetative processes – recuperation and repair, rest, digestion. Gabriel Natureza Kram [3] describes anatomy book ANS maps as like a lumberyard rather than a forest, due to the vast, dense mesh of autonomic fibers that interpenetrate every part of the body - that are so fine (about 15 microns) that they just disappear without trace under the conditions present in most anatomical investigations.
If you only want a very general working model of the ANS, then this division of tasks (high energy vs vegetation) is close enough. However, the anatomy of the ANS shows some rather peculiar features - that also give some measure of how much we consciously dip in and out of it during the day. For instance, the Trapezius and Sternocleidomastoids that turn your neck are major "voluntary" muscles - i.e. are supposed to be under conscious control. But they may also undertake the rapid reflexive automatic head movements called "startle-alert"- which are coordinated by motor efferent fibres of the Parasympathetic(!) system.
The Vagal ANS is about 80% afferent - i.e. is much more interested in receiving information, and much less so in returning information / sending out instructions. To provide some meaningful context, this ratio of information flow is directly comparable to (e.g.) an intelligent power grid with smart meters/smart devices. A central grid manager might send a message to a neighbourhood along the lines of “reduce power consumption by 12% for the next 10 minutes”. Just as a smart grid is called smart because it has distributed intelligence, living systems also possess distributed intelligence, and the “central processor” (in the case of humans - the cognitive mind linked to meaning-making functions in the mid and hind brains and motor functions in the cerebellum) is integrated into a whole body with many layers and fields of conscious or sentient intelligence. For a living system, the instruction might (possibly) be something like “(allowing for local requirements) the energy output level needs to be X until further notice” (where X is one of a set of activity bands defined by the Window of Normal Adaptation, and includes implicit information regarding the safety or otherwise of the particular moment and activity). Implicit because normal biological action (that is not over-cognitive) automatically includes risk in its meaning - a leopard is always at risk of being hurt by its prey, a dragonfly is always at risk of being picked off by a bird, but those risks are just normal background - they are part of living. [4]
The Vagus regulates mainly rhythmic motility via release of acetylcholine. Whilst the usual meme is that the Vagus is all about relaxation, its actual function in evolutionary terms is more related to the necessity for ambush predators to be (a) very still, (b) have a very low metabolic tick-over, and simultaneously (c) be aware of small changes in their environment (if this is sleep, it is a very light, hyperalert sleep), and (d) be able to release a very rapid motion as required. Therefore the Vagus is NOT really about relaxation in the normal meaning of the word. Looking at the Vagus Nerve in detail for the heart - the right branch of the Ventral Vagus nerve is the main connection to the sinoatrial node whilst the left supplies the atrioventricular node (slows the heart rate).
The fact is that – once we leave the simple level of a single mitochondrion, things start to get more complicated. The Parasympathetic nervous system (PNS) is the Vegetative part of the ANS, usually thought of as being about "slowing down and relaxing", that is mainly "interested" in running the internal organs of the body, and particularly organs related to assimilation and digestion of food. Assimilation of food includes chewing, and for primitive animals (such as a dragonfly) this also includes orienting the mouth to catch food so it can be chewed, and the startle-orient reflex so that we also see in order to catch the food we are going to chew. We seem to have retained that neural relationship between head turning, rapid (non-conscious or reactive) sensory orientation and catching food with our mouths – in the PNS of a modern human. But in order to do this it also needs the cooperation of the Sympathetic nervous system, at the very least to organise the relative distribution of blood flow or regulate body temperature and osmotic balance.
Similarly the Sympathetic Nervous system needs the PNS to (at the very least) optimally regulate cardiac activity or swallow or engage the startle-orient reflex. The real integrated requirements of the human organism may be described at a very basic level as active vs vegetative. But that simplistic view translated into the two major branches of the ANS doesn’t survive even a small amount of scrutiny.
So after reflectively talking to patients about the ANS for over 15 years I gradually fouud myself gravitating towards a very different understanding. The ANS - as a set of nerves - is an anatomical distinction used to attempt to explain a set of processes. This way of thinking is inherently misguided, because anatomy organises itself around the needs of process, and process is not subservient to anatomy. Process uses whatever anatomy is available, but does not limit itself to any one structure - rather, process seeks any and all available means to meet its ends. Indeed, one fundamental principle of Semiotics is that rich meaning-ful communication is only possible with at least several layers (or means of) of communication. In the end I have come to realise that the ANS is more like a set of telephone lines that facilitate conversations [2]. So if you have a conversation with your friend over the telephone, you would not attribute the conversation or the content to the telephone line (or mobile phone network) - would you?
As such, the vagus Nerve is a means to communicate, NOT an end in itself. Yes - there are things we can do to alter the degree and kind of communication that it transmits, but the issue is - do the parts using it for conversations actually want to hold a conversation - or is this temporarily a low prioroty for them? If you were talking with a friend and smelt burning in your house, you would say something like "I can smell burning - we can carry this conversation in later when I've investigated". This is exactly what is happening when Vagal activity is reduced. The issue is not "Vagal insufficiency" but rather - to do with how effectively the whole organism calibrates itself to the reality of its present environment.
References & Notes