Basics of Nervous System Anatomy with an Evolutionary Perspective

For our journey into the world of brains and success it is useful to have some basic ideas about how the brain is organized and how it works.  I’ll present this material by using a story regarding how evolution put the anatomy together.  We cannot really understand our brains and our behaviors without giving at least a little consideration to this.  So, with sweeping strokes of broad generalizations let’s have a look at how your brain is put together. I’ve provided links to some resources that have nice pictures and further information.  The goal here is to give you a brief overview so that you can begin to see the logic (or apparent illogic) of why we do what we do.

First, the overall anatomy.  The human nervous system can be thought of as having two main divisions: a peripheral nervous system in the arms and legs and a central nervous system in the spine and skull.  The peripheral nervous system consists largely of “somatic nerves” and an “autonomic nervous system”.  The somatic nerves are primarily responsible for sensation, movement, and coordination.  In the head some of the somatic nerves have become specialized to serve the special senses (smell, vision, hearing, taste).  The autonomic nervous system controls overall states of function in relation to stress responses, digestion, blood flow and other “vegetative” (more or less automatic) functions.

The central nervous system consists of (from the bottom up) spinal cord, brainstem, cerebellum, deep brain systems (for emotions and quick-response orchestration), and the cerebrum (the “new” brain for “thinking”).  The spinal cord carries information from the peripheral nervous system into the brain (mostly sensation) and information going from the brain to the body (mostly muscle) .   The brainstem is an one of evolution’s early centers for integrating information for complex response which includes responses to advanced sense information (smell, vision, hearing, taste, etc).  The cerebellum is primarily concerned with orchestrating complex movement.  The deep brain systems (located deep in the cerebral hemispheres) broadly include the limbic system for emotions and the basal ganglia for “subconscious” integration of information.   And last, the “new brain” (cerebrum) is the large, two hemisphere, structure of “wrinkled” tissue you might see in a basic picture of the brain.

The next issue is basically how they work.  For this we must consider the journey of evolution.

Imagine you are on this planet back when the first one-celled organisms appeared.  One of the earliest problems was that of movement.  Organisms could stay in one place and take whatever comes at that location, or move to find better options.  So, some early organisms which would become the ancestors of animals figured out how to move, a major development.  However, a problem soon developed with movement – running into things.  Movement without guidance can become self-destructive.  So, sensation developed.

Armed with movement and sensation early organisms could begin to get around in their environments.  As multicellular organisms developed, movement and sensation needed to be orchestrated across groups of cells; so, the beginnings of a nervous system developed.

As organisms became bigger and movements became more complicated  a spinal cord developed to connect information about various parts of the organism and to orchestrate movement.  So, thus far in the story, early organisms are now moving, sensing the results of movement, and orchestrating more complex responses for the next movement.

Next is the development of special senses.  The ability to smell begins to add more utility to movement.  Later, the development of rudimentary sight adds still further utility to movement.  Hearing, balance, and taste also develop.  These senses develop, not surprisingly, at the front of the organism (the future “head”) – where the information will be useful to aid selections of movements at first contact with the new information.

As senses are developing there is a need next for orchestrating the information of these new senses.  The expansion of the the front of the primitive nervous system became the primitive brain.  This remains in us now as our brainstem.   In the primitive brain there was further need to develop a system for orchestrating increasingly complex movements in response to increasing types of sensory information.  So, a cerebellum begins to develop and specialize as a system for orchestrating complex movements.  It is attached to the primitive brain at the early center for integration, the part we now call our brainstem.  It still functions primarily to orchestrate sensory information and complex movement.

This is when things really begin to get interesting from the perspective of understanding social issues of interaction.  So far, we’ve developed organisms which can move, sense, react, and incorporate information from some developing special senses.  But there is a problem: learning.  It is one thing to react again and again in the same automatic way, but what about learning how to react differently!  This requires memory.  So now its time to start building systems for memory.  Thus we begin the early development of what will later be called the basal ganglia: systems which can remember past consequences of action and select the past action which worked out best the next time a similar circumstance comes along.  By this stage we’re beginning to have organisms which can – in primitive ways – select responses according to circumstances.

However, there is another problem.  As early organisms are developing these increased abilities to react, there must be some way to categorize the increasing fund of memories of past reactions.  It would be impractical to “review” all of the past memories to decide every time how to react.  There needs to be some way to form generalizations about reactions.  There needs to be some way to differentiate what happens in an emergency (“run away!”, where damage may occur) from what happens where there is a great feeding opportunity (“chase it”, where attraction is the focus).  Therefore, another system develops – the primitive emotional system: the limbic system.   Not surprisingly, in anatomical terms this is wrapped around the developing early memory-choice system (largely centered in the basal ganglia).

Look how far we’ve come!  We’ve now got organisms that can move, sense, react, coordinate primitive reaction, sense in more complex ways, remember, place values on what is remembered, and even orchestrate more complex responses to varying situations! The nervous system is really becoming more sophisticated.  Yet, there is more.

While all of this was happening the overall organism is becoming more complex and bigger.  While a one-celled organism carries on all of life’s activity in a single cell, complex multicellular organisms develop specialized organs to perform special functions –  like pumping blood, processing food, getting rid of wastes, and so forth.  With the development of these special organs there becomes a need to orchestrate when these organs are active and how active they are.  For example, if an organism is running away from a predator that is not the time to be eating food or getting rid of wastes.  So, another system develops – the autonomic system.  This is a portion of the nervous system for coordinating the levels of activity within “vegetative” organs – those that basically manage energy and keep the organism going.  This autonomic system becomes integrated into the developing spinal cord, brainstem, and limbic system.

At this point in evolution reactions are pretty good.  But, there is another problem.  Reaction only goes so far.  The next thing that is needed is the ability to consider response that is independent of current incoming stimulus.  The next need is imagination: to imagine stimulus and imagine an appropriate response to it, to imagine what might happen, to imagine where something might be going, to imagine what might be meant by some new circumstance.  Of course, there are differing levels of sophistication in imagination.  So, our current human level of imagination was certainly not present in early creatures.  Nonetheless, creatures needed to go beyond reaction to circumstance; and that required the ability to project what incoming information might mean in an imagined future, or even imagine reaction to an imagined stimulus.  So, the cerebrum began to develop as evolution’s next gift – a place for thinking rather than just reacting, a place for imagination.  Literally, a place for dreaming.

This development happened earlier than we often like to think.  The concept was even subtly woven into the famous movie Jurassic Park (written by Michael Crichton, who was trained as a doctor, and directed by Steven Spielberg).  In the movie, the people trapped on the Jurassic island were shocked to discover that the dinosaur velociraptors planned coordinated attacks.  While Jurassic Park is just a movie it provides a subtle commentary on the realization that imaginations are not solely our ability.  We do it best.  But, with the development of imagination to move beyond reaction, the foundations of conscious planning were set in motion long ago.

Comparatively, imagination takes a lot of brain power.  It is relatively simple to take concrete incoming data and orchestrate a reaction to it.  But, to imagine means to consider various possible situations which might be present, and even to play out what might be done in response to each of them.  This takes a lot of computing power and also a lot of energy.  We can see this difference even in ourselves.  Most people can react to something which has been invented, but only few people can invent it.  From another perspective, we can ski all day, reacting to a huge stream of physical data, and while we might be physically tired at the end of the day we are typically not mentally tired.  However, if we take a test for just a few hours or do creative work all day we may be totally “wiped out” by the end of the day.  Therefore, as we look at the brains of advanced creatures (with lots of imagination) we see that they have much larger cerebrums in comparison to early brain structures and that organ has a large blood supply to deliver the needed energy.

We think of “thought” in some complex ways.  When we use the word “thought” regarding ourselves it includes advanced abstractions, planning far into the future, and imagining even whole worlds.  When we think of imagination and thought in evolution’s earlier animals we need to use the word more broadly, including lower levels of sophistication in the concept.  Rats learning in a test can “imagine” that food is at the end of a maze and work through the maze to find it.  Hyenas hunting in a pack can imagine the role of other pack members for a coordinated attack.  So, in the evolutionary development of imagination we need to remember that it has been a progressive process.  There are other animals which also think.  Animal stories (of dog, other pets, or research animals) are full of examples where animals plan and think.  We often most notice the events which are to our benefit – like when a dog saves its owner by going to get help.  Research now shows that other primates and non-primates (for example, dolphins) can think and use symbols for representing ideas.  Our thinking is more sophisticated; but, other animals do it also.  Evolution has been building this system for a long time.

In our brains the cerebrum has become the commanding structure.  Its billions of neuron cells have become specialized in numerous ways: neurons for processing complex visual information (like the text you are reading); neurons for creating and understanding speech; neurons for considering complex spacial relationships; neurons for creating a myriad of objects which have never existed before; neurons to imagine things which cannot be seen and which can never be seen (like the beginning of the universe); and neurons to create complex systems of information organization in disciplines like science.  We now have capabilities which have not existed in any prior species – a differentiation generated by the opportunities afforded by our tools.

The next piece in this story is critical to understanding our current overall behaviors, our abilities to adapt (and limits for adaptation), and our continuing tendency to show some primitive behaviors (like war).  Through all of this evolutionary history of brain development the over-arching theme has been “add to what you’ve already got”.  Our new capabilities are built upon the older systems.  And here is the punchline for this story: the new systems remain subjugated in some ways to the older systems.  For example, you cannot do complex abstractions if you are in severe, response-needing pain.  If you are literally dying of thirst you cannot focus on writing an opera.  We can conceive of logic and understand its concepts, but our use of it remains subjugated to more primitive brain systems.  Thus, while some individuals may have the ability to control self-interest sufficiently to  choose against self-interest in favor of life-propagating common-good, most individuals of this era have limited ability in this regard.  As is obvious from reading any newspaper, as a species we are not yet able to move beyond greed, war, and destructive self-interest (even though some individuals of the species are at the front edge of evolution and do have this ability in this era).  Our new systems of insight and complex thought are, more or less, subjugated to the primitive brain.  Yet, we are often unaware when our primitive brain is subverting the more advanced brain.  We often succumb “unconsciously”.

In his book, “Influence: Science and Practice”, Robert Cialdini discusses numerous examples of techniques used by salespeople to encourage a “sale” where logic might dictate against it.  Largely these techniques call upon more primitive behavioral drives to subvert what logic would advise.  In fact, this has even been codified in the law in the form of the ability to cancel a major purchase (such as a car or a home) within a few days – after more “cool headed” thought may allow reason to emerge.  Emotions are commonly exploited in the sales industry to subvert logic.  Our brief neuroanatomy/neuro-evolution might now give you greater insight into how such vulnerability has developed.

We need a few more words here about consciousness, the subconscious, and self-awareness.  These are long topics and I’ll only touch on them here.

The word “conscious” typically means the ability to think about what is happening, or/and to imagine what might happen, and then think about that.  It goes beyond reaction, to a separate internal stream of processing about real or imagined events.  The “hardware” for that developed with the systems for abstraction.

Taking it a step further, when the cerebrum developed to allow conscious thought and imagination, then greater emphasis needed to be placed on managing reaction to incoming stimulus semi-automatically – so as not to interfere with conscious thought.  In essence, the brain needed a way to allow eating as an automatic behavior while reading a book, or doing assembly-line tasks as an automatic behavior while the mind wanders to topics of more interest.  So, data processing within the cerebrum versus within the deep brain structures became partly separable as conscious and unconscious processing.  While the exact anatomy corresponding to what we call “conscious” and what we call “unconscious” has not been worked out, our journey through the development of the brain provides some insight into how this may be segregated.

Last, we come to the term “self-aware” or “self-awareness”.  This is a state which derives from the ability to imagine (i.e. process theoretical information rather than reacting to actual incoming stimulus).  With the ability to imagine we gain freedom from the tethers of stimulus.  Thus we can develop concept structures which are beyond stimulus-contingent concepts.  This allows us to think of entities as units rather than as generators of positive or negative stimulus.  From that comes the ability to think of ourselves as “selves” – units of being.  We can then consider these “selves” in relation to one another.  In part this results in the ability to be “self-aware”.  (I hope it is evident that such awareness is not unique to us as a species, even if it may be more advanced in us.)

Our journey through the development of the nervous system partly serves to tell us what its components are and how they interact.  But, more importantly, our journey is also a starting point for learning what controls what, what limits what, and how our behaviors are a consequence of our anatomy.  We are an incredibly complex and wonderful invention; however, the process of our evolution is not done.  In certain ways we are quite “rough around the edges”.  For example, we are able to make incredible tools but less able to use them rationally (and, particularly, to be insightful about when not to use them).  Our less-admirable human traits are a testimony to work yet to be done on the path to the complete brain.  Some people pursue that path within their own lives.  They push the boundaries of our species as we move toward the coming opportunities.

We can work with the brains we have been given – taking them to their best way of being.  Happiness, satisfaction, intellectual flexibility, and peace are possible.  The above overviews of human brain anatomy provide one of the starting points for understanding why we are as we are, and what we can be.  We have a bright future – if our primitive brain and our inventiveness do not conspire to pave our own destruction.  I personally believe in the opportunity.  Our development is a slow and painful process; but, we are improving.

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