Important Components of the Autonomic Nervous System

The main parasympathetic nerve outside the brain is the vagus nerve, that’s the tenth cranial nerve, and importantly it supplies the heart and it supplies most of the GI tract – not all because of that sacral innervation – but most of the GI tract as well as the lungs and splanchnic organs such as the pancreas and spleen.  You can see that there are two types of receptors for acetylcholine: one is nicotinic (nicotinic receptors are responsible for the ganglionic transmission as I’ll be getting into later on), and muscarinic (muscarinic receptors in the heart or in these end organs).  So, if you smoke a cigarette and now you have or you swallow some nicotine, chaw tobacco, or something like that, you are going to be stimulating ganglionic neurotransmission.  If you eat a toxic mushroom, that’s sort of where muscarine comes from, then the effects are going to be because of stimulation of the muscarinic receptors and the parts of the sympathetic nervous system that use catecholamines as a transmitter wouldn’t be affected. But I wouldn’t recommend eating poisonous mushrooms.

The GI tract receives complex, complex multi‑transmitter innervation.  First of all, you’ve got the enteric nerves that are in the walls of the gut and then you have parasympathetic innervation which in general increases the tone of those organs…of the stomach, and sympathetic inhibition.  It’s kind of interesting to think about sympathetic inhibition.

So, the enteric nervous system – nobody knows what the chemical messengers are of the enteric nervous system, how they’re organized…it’s very complex.  There are numerous putative transmitters. They interact.  And of course, the enteric and sympathetic and parasympathetic nervous systems also have interactions in the gut.  So, the gut is really, hard to understand in any kind of a simple way.  There’s an example of what I am talking about.  These are enteric nerves, but they are impinged on by parasympathetic postganglionic neurons.  Now the parasympathetic ganglia are near or even inside the target organ, so very different from sympathetic postganglionic innervation where these are very long wispy fibers that come from the ganglia.  Here most of the parasympathetic innervation is preganglionic, it’s myelinated, the vagus nerve is myelinated, and it is only very short postganglionic fibers that go to the target organ.

So, this is an overview of the components and messengers of the autonomic nervous system and this is an overview of the organization.  Hopefully when you think back to that initial diagram where I wrote “uh oh” maybe you can get a little sense of what we are dealing with.  There are five different organizations. You’ve got the vagus nerve and pelvic nerves that are preganglionic parasympathetic nerves.  You have got ganglia that use acetylcholine as the chemical messenger…that’s true for all the parts of the autonomic nervous system.  You’ve got three types of sympathetic neurons where norepinephrine is from a postganglionic fiber, acetylcholine mitigating sweat is a postganglionic non‑myelinated fiber.  You’ve got the direct adrenal medullary innervation by myelinated fibers.  That’s kind of the overview of the organization of the sympathetic nervous system.