Sunday, September 22, 2013

Alright, now that you know how nerves work, let's check out the nervous system as a whole. CrashCourse Biology has good video on the subject, but I'm going to go ahead and assume you're here for the usual reasons, so here we go.

When I say "Nervous System" you probably think of something like this:

"Ah, the missus of the house! Have you got a moment to talk about Firefly?"

Well that's right. That's what it looks like. Well done.

However, when we talk about the nervous system, it's important to understand that we aren't just referring to the physical structure of the thing, it's function as well as both network topology.

The Nervous System is the part of your body that controls motion, both conscious and unconscious, and is physically composed of your brain, spinal cord and all your nerves. Those first two make up the central nervous system, while the nerves extending from them to all other parts of your body make up the peripheral nervous system. The peripheral nervous system can be broken down into the somatic and autonomic nervous systems, and the autonomic nervous system into the sympathetic and parasympathetic nervous systems.

(There's also the enteric nervous system, but that's like... your gutbrain and not exactly relevant to this discussion but what you should know is that there are about a million neurons in your guts, which is about a tenth of what's in your brain-brain. 90% of your body's serotonin and 50% of it's dopamine hangs out in your digestive system, which is part of why your digestive system comes up so often when we're talking about drugs and other stuff that affects your brain.)

A diagram of the nervous system, broken down into the central and peripheral nervous systems, the latter of which is subdivided into the somatic and autonomic nervous systems.
Like so.

So, of the things we care about at the moment:

The Central Nervous System
The Central Nervous System is the part of your body that receives information from, and manages the actions of, the body. It is physically composed of the brain and spinal cord.

1. The Brain
This is your brain. We've been over this.

2. The Spinal Cord
The spinal cord is a bundle of nerves and support cells that runs along the length of an animal's body. Most of us multicellular animals only have one, but there are some worms that have two running in parallel, and things like cnidarians and ctenophores have nerve nets instead, and sponges don't have anything because they're sponges... but the point is that in most of us have one. It runs from the base of the brain down the length of our bodies and has three primary functions:

  • Sending motor commands from the brain to the rest of the body
  • Sending sensory information from the rest of the body to the brain
  • Coordinating certain reflexes 
In the case of that last one, these are called spinal reflexes, and make it so certain actions can take place without a signal needing to be sent all the way to the brain and then waiting for a response to be sent back. For example: The flexor reflex makes you automatically pull your hand away when you touch a hot stove, but you can still consciously override this if you wanted to burn your hand intentionally or something.

Damn do I love TF2.

Also, here's something cool, the physical segments of your spine can be directly tied to what they go to.  I mean, I know that's something that seems intuitive, but remember, this is biology. Biology is almost never that nice and clean. The ability to say "This thing goes directly to that thing." is kind of a big deal. Check it out:

Click to embiggen.

The Peripheral Nervous System
The peripheral nervous system is the part of your nervous system that extends beyond the central nervous system and into the rest of your body. It can be broken down as follows:

1. The Somatic Nervous System
This is the bit you're controlling when you consciously tell your body to do something. It's what carries signals from the brain to your skeletal muscles.

1A. The Cranial Nerves
The nerves that emerge directly from your brain. Your optical nerves, for example, are cranial nerves which come directly out of your cerebellum.

"Car broke. Phone. Yes?"

1B. The Spinal Nerves
These are the nerves that carry signals into the spine from the body. We humans have 31 pairs of the things that emerge from the spine and then kinda branch out and spread out to everywhere else.

1C. Association Nerves
When those spinal nerves branch out, these are what they're connecting two. These are the individual nerve cells that make up those connections and there are thousands of them.

2. The Autonomic Nervous System
Where the somatic nervous system is conscious movement, the autonomic nervous system is automatic. I would say "unconscious" but you can control some parts of it to some degree. It's what makes your heart beat, your viscera do their squishy business, and allows you to continue blinking and breathing without you thinking about it. It's also what controls your automatic reactions to the environment.

Physically, it's composed of your medulla oblongata and a few of its friends in the lower-brain sending signals to the rest of you, telling you to do things without you-you having to think about them.

2A. The Sympathetic Nervous System
Your sympathetic nervous system is responsible for making you ready to move. Put simply, although it's always active to a certain degree to keep things in balance, it becomes more active when you're stressed, and causes physical reactions such as pupil dilation, an increase in blood pressure and heart rate, constriction of blood vessels, an increase in sweating, etc...

2B. The Parasympathetic Nervous System
Your parasympathetic nervous system is the anti-sympathetic-nervous-system. When your sympathetic nervous system does things like spike your heart rate in response to people trying to kill you, it's the parasympathetic nervous system's job to bring it back down again.

Click to embiggen.

(Fun implication of those last ones, the fact that we have these systems in place means we can monitor them to get an idea of people's mental states, but more on that later.)


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