Going to finish up this section by talking about those splanchnic nerves as well as the adrenal medulla. As we discussed in our sympathetic division, the preganglionic fibers arise from spinal cord segments T1 through L2. What we find is that from T5 down, the majority of those preganglionic fibers are these splanchnic nerves, which are nerves that synapse in a collateral ganglion rather than in that trunk. So these collateral ganglia are just ganglia that are further out, closer to the effector organs. These nerves basically bypass the sympathetic trunk. They pass through it without ever synapsing and instead synapse in these collateral ganglia. These nerves mainly innervate the visceral structures of our abdomen, such as our stomach, intestines, liver, and spleen. We find that the preganglionic fibers tend to be long, and the postganglionic fibers tend to be short. This is the opposite pattern that we saw in the fibers above T5. So, if we look down here, we have our spinal cord, again working from T5 down with these splanchnic nerves, and you can see how the purple preganglionic fibers must extend all the way from the spinal cord to these collateral ganglia, which are depicted here as yellow balls. They will then synapse in those ganglia, and the now relatively shorter postganglionic fibers go from the collateral ganglia out to the effector. That is the opposite pattern from what we observed above T5.

Next, we are going to switch gears and discuss the adrenal medulla, which is a very important structure for our sympathetic division. To understand why this is so critical, we have to recall something called dual innervation. We have been discussing how our sympathetic nervous system innervates all of these organs, but we haven’t gone into detail about how most organs are actually innervated by both the sympathetic and parasympathetic divisions, which is referred to as dual innervation. However, there are exceptions to that, and one of those exceptions is the adrenal medulla, which is only innervated by our sympathetic nervous system and for a very good reason. The adrenal medulla, when stimulated, secretes epinephrine and norepinephrine, also known as adrenaline and noradrenaline, which are crucial for initiating the fight or flight response. As you can imagine, these are very important for getting your body responsive and active. Thus, this structure is only innervated by our sympathetic nervous system; the parasympathetic division does not want any extra epinephrine or norepinephrine circulating in your body, as that would not aid the rest and digest process. The adrenal medulla, located in the adrenal gland, sits right on top of the kidney, somewhat like a cap. What's fascinating is that it can be considered a modified sympathetic ganglion. Looking at our image, the adrenal medulla is depicted here, showing that it's innervated by just a preganglionic fiber going all the way to this structure, because the synapsing actually happens within the adrenal medulla itself, acting as a modified ganglion. This singular functionality is unique and vitally important for this division because it will be releasing epinephrine and norepinephrine, essential for the fight or flight response.

That concludes our overview of the sympathetic nervous system. Thanks for sticking around, and I'll see you guys in the next one. Bye bye.