It goes without saying, but I am not a neuroscientist. I do not study nervous systems. I do not have the prerequisites (e.g. laboratory and equipment) to do so.
I work with what I have.
Moreover, I do not have the need for such a specialized knowledge.
Introspection is enough for me.
Because of this, it’s a little bit irresponsible on my part to use the term “nervous system” to refer to what I am saying. Nervous system is a physical, three-dimensional object, you can see and touch. What I am speaking of, though related to nervous system, is not exactly nervous system.
I suspect that the object of my inquiry is much narrower than nervous system. Probably a subsystem, or layer, of some sort.
Perhaps it is none other than consciousness.
You asked what kind of activity can a “nervous system” be filled with? You were more specific than that, you asked what kind of activities can run in parallel. But these are related questions.
I suppose that anything we sense is already filling up the nervous system. The only question is to what degree.
Sight, hearing, smell, touch, feelings, desires, bodily micro- and macro- movements . . .
They all consume “nervous space”.
One is always conscious of nothing beside “nervous activity” within one’s “nervous space”. When there is no “nervous activity”, one is unconscious.
I divide consciousness into two types: focal consciousness, which is clear, and peripheral consciousness, which is vague.
The latter is known as “subconscious”.
Of course, in reality, it is a continuum. You have the focal point, which is the point of maximum clarity at any given moment, and multiple peripheral points which are vague to the degree of their distance from the focal point.
You can move the focal point through your “nervous space”, thereby changing what you sense with clarity and what you don’t.
Which activities can run in parallel, you ask? Any combination. There are many activities that are running but that we are not aware of. Until we decide to “switch off”. By “switching off” you create the opportunity to slow down the activation thereby creating the opportunity to get to know what would have otherwise run in the background of your mind. It allows you to get to know yourself.
But “nervous space” has a limit and when that limit is exceeded one becomes overwhelmed. This state, then, is what causes the person to lose control over himself.
Control can be regained by freeing up some of the “nervous space”. This entails locating some “nervous activity” within the “nervous space” by moving the focal point of consciousness to the corresponding place and then reducing that activity (which is sensed as relaxation, or rather, as decrease in sensation.)
This “nervous space” is sort of like a RAM (= working, operating) memory. And just as applications can use their RAM any way they want, so too can human brain use this “nervous space” any way it wants.
There are many reasons for clearing up “nervous space”. Regaining control is one but not the only one. Reason is another. In fact, any whim would do.