In this video, we're going to be talking about the different pathways of sympathetic innervation. So in our sympathetic division, a preganglionic fiber can synapse with a postganglionic fiber in one of 3 ways. First, it can synapse in a trunk ganglion at the same level, and I'll show you what I mean by "same level" in just one second. We're going to follow along with this blue fiber here. I'm going to highlight it in a nice dark gray as we talk about it. Our preganglionic fiber is going to start in the spinal cord and it is going to travel through the spinal nerve like this. It is going to enter the trunk through the white ramus and then at that same level that it entered, it is going to synapse with a postganglionic fiber, and then that postganglionic fiber will leave through the gray ramus, travel through the spinal nerve, and head out to its effector organ. The preganglionic fiber will enter at that level and then synapse at that same level. That's what I mean by "the same level." Alright. So that's the first method, pretty straightforward.
What we can also have is that the preganglionic fiber and postganglionic fiber will synapse in a trunk ganglion at a lower level or at a higher level. Following along with this red nerve fiber now, we are again going to start in the spinal cord with that preganglionic fiber. It is going to travel through the spinal nerve, enter the trunk through the white ramus, but now it is going to travel down to a lower level trunk ganglion and that is where we are going to have that synapse, and then that postganglionic fiber will exit through the gray ramus, travel through the spinal nerve, and head out to the effector. Basically, that preganglionic fiber will either travel up or it's going to travel down before synapsing in a lower or higher level ganglion. That's the second one. The first and second are pretty similar; we're just talking if they're going to be synapsing at the same or a lower or higher level.
The third one is quite different. For this third one, what's going to be happening is our preganglionic fiber is going to pass through our sympathetic trunk without ever synapsing. It is going to pass through the trunk, and instead, it is going to synapse in what's called a collateral ganglion. So that's what we have depicted right there. Following along with this purple fiber, our preganglionic fiber again will start in the spinal cord like always. It is going to travel through that spinal nerve, enter the trunk through the white ramus, so that's always the same, it is going to travel right on through that sympathetic trunk, never stopping, never synapsing, and head all the way out to this collateral ganglion which will be closer to the effector organ, and then it is going to synapse right in there. Then from there, our postganglionic fiber will head out to the effector. That one is quite different. And when this happens, the preganglionic fiber is actually called a splanchnic nerve, and that is going to be the topic of our next video. So I hope to see you there. Bye-bye.