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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). This is balanced by the Parasympathetic nervous system – mainly the Vagus Nerve – that mediates vegetative processes – recuperation and repair, rest, digestion.
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 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 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. But in order to do this it also needs the cooperation of the Sympathetic nervous system, at the very least to organise blood flow or regulate body temperature and osmotic balance. 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.
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.
References & Notes
1 Alternatively called the Ortho-sympathetic
