Okay. So, let's get into the structure of our parasympathetic nervous system. Here, fibers are going to be arising from the brainstem and the sacral region. This is sometimes also called our craniosacral division. Arguably, the most important one of these nerves is our vagus nerve or cranial nerve 10. This nerve provides about 90% of parasympathetic innervation, so it is very important. They're all doing a great job. They're all important, but this is, like, the MVP.
And then what we find in this division is that many preganglionic fibers actually synapse in ganglia within their target organs. So they're actually synapsing within the effector organ rather than in ganglia outside of it. Sometimes they do synapse in external ganglia, and in this division, these are called terminal ganglia. These are ganglia where preganglionic fibers can synapse with postganglionic fibers. You can think of these as being functionally very similar to collateral ganglia. They serve the exact same purpose. It's a location where that synapse can take place. The difference is these terminal ganglia are very close to their target organs. So those collateral ganglia are certainly closer compared to that sympathetic trunk. Right? But these terminal ganglia are extremely close. The way I always think of this is I think of these terminal ganglia as being like that terminal stop or that final stop right before the ultimate destination of that organ because they're so close to each other.
What we find here is that the preganglionic fibers are of course very long because they're extending all the way from the spinal cord or the brainstem all the way to the effector organs. And then our postganglionic fibers tend to be very short, and you can see that here in our image. You can see our purple fibers are all very long and our green fibers are all very short. Again, those synapses are going to be taking place either within the target organs or in terminal ganglia very close to them. And you can see that our fibers are arising from the cranial nerves up here as well as the sacral nerves down here.
Alright? So that's the structure of our parasympathetic nervous system and I will see you guys in our next video. Bye bye.