What is the Autonomic Nervous System?

What are we talking about when we talk about the autonomic nervous system? This is a classic diagram of the organization of the autonomic nervous system, and to a novice in the area it’s just impossibly, impossibly complex.  So, we are going to try to begin from scratch here and build up and I’m hoping that by the end of this segment on  “What is the Autonomic Nervous System?” you’ll be able to look at that diagram and make some sort of sense out of it.

I conceptualize the central nervous system as being analogous to a Tootsie Roll Pop. Anybody here not know what a Tootsie Roll Pop is?  I have given this talk in Europe and they have no idea what I am talking about. But the concept is that the central nervous system is like a Tootsie Roll Pop:  you’ve got the stick, well that’s the spinal cord, and you’ve got this crusty candy shell, well that’s the cortex of the brain. But as everybody knows the key to the Tootsie Roll Pop is the chewy chocolate center, right? And that is the brainstem.  So, this is the Tootsie Roll Pop analogy and as you’ll see the autonomic nerves, or nerves that are a part of the autonomic nervous system, come from the brainstem and the thoracolumbar spinal cord and the sacral spinal cord, not really the cervical spinal cord.  So far I haven’t talked about where the autonomic nervous system is.  That’s the central nervous system.

I divide the peripheral nervous system into two parts: the somatic nervous system, which is responsible for interactions between the organism and the outside world especially by way of skeletal muscle, and the autonomic nervous system, which I think of as the automatic part of the nervous system, that’s involved with regulation of the inner world inside the body by smooth muscle and glands. And this is where the autonomic nervous system fits in.  Now I want to say right off the bat that the autonomic nervous –  it’s a terrible phrase –  because it implies some sort of autonomy between the autonomic nervous system – I have not shown you where the autonomic nerves are yet – but the autonomic nervous system and the central nervous system, which as you just saw doesn’t involve the autonomic nervous system.  But this autonomy is…well, there isn’t such autonomy.  If I wanted for instance to increase the sympathetic outflow to my heart let’s say, I could easily do so voluntarily by my cortex. All I have to do is go like this (fist squeeze) real hard, and my blood pressure is going to go up and my heart rate is going to go up, sympathetic outflow of the heart is going to go up.  So, this idea of autonomy I think is wrong.  I think of it as automatic, but it’s just built into the way autonomics has been taught – that there’s autonomy – and I can’t get around it.  So, I am going to call it “autonomic nervous system” but just recognize it is not autonomous.

Here is where the autonomic nervous system actually is.  This is a picture of a sympathetic chain in a plasticized person.  I was at an experimental biology meeting, and it came across this display – really amazing, amazing thing. There is a program in Holland where if you want to donate your body for medical science and so forth instead of “okay there you, on a gross anatomy table” and then that is pretty much it.  Instead, you can be plasticized and then your insides are available for teaching purposes indefinitely. Amazing program.

Anyway, I came across this which shows beautifully the sympathetic chain.  You can see, it’s outside the central nervous system and is kind of arranged along the front of the posterior ribs and you can see these accumulations of nerve cells and those are the ganglia.  So conceptually, the sympathetic chain looks like pearls on a necklace on each side of the spinal column.  So, that’s where that the autonomic nerves are.  What do these ganglia do? And to understand what the ganglia do I used the analogy of how electricity comes to your house.  Of course, there’s a generator plant and distribution center and then these towers that can convey thick high voltage electricity.  But that’s not what goes to your house.  Instead, outside your house, at least in older neighborhoods, you have a utility pole and on the utility pole there’s a transformer box and from that utility pole and transformer box there are thin kind of wispy wires that go to your house.  And similarly, or analogously I should say, the ganglia are like the transformer boxes on the utility pole outside your house.  So, you have these thick rapidly conducting preganglionic fibers and then you have these wispy slow conducting thin postganglionic fibers and that’s what actually goes to the target organ and the nerve terminals and target organs.  So, this introduces the idea of preganglionic and postganglionic nerves.