Intercostal Space

00:01 Now, that we've seen the ribs, we can talk about the space between the ribs, called the intercostal space.

00:07 There are a lot of muscles and neurovascular structures that exist in this intercostal space.

00:12 The first thing we'll see are the muscles that run between the ribs appropriately called that intercostals.

00:18 And there are three sets stacked on top of each other from superficial to deep.

00:24 The first set is the most external set and called the external intercostals.

00:29 Their fibers run in a diagonal orientation, such as, as they're going from one rib to the rib below, they travel somewhat medially, at least from this anterior point of view.

00:41 And what that really means is that when these muscles contract, they have the effect of elevating the ribcage and expanding it and causing the thoracic cavity to expand helps with inhalation.

00:54 If we go a layer deep to that we have the internal intercostals, but their fibers run in the opposite direction.

01:02 And so they're gonna have the opposite effect.

01:04 They're going to depress the ribcage, decreasing the thoracic cavity, and causing exhalation.

01:11 Finally, the deepest layer is the innermost intercostals.

01:15 And their fibers have the same orientation as the internal, so they do the same thing.

01:21 Now, the vasculature of the intercostal space is very interesting.

01:26 And the first thing we have to talk about are these vessels called the internal thoracic or internal mammary arteries and veins that are branches of the subclavian arteries and veins.

01:38 You'll notice that just below each rib, there's a little vein called an intercostal vein, running along anteriorly until it meets the internal thoracic.

01:49 Similarly, at each level, there's a branch coming off of the internal thoracic artery and running just below the intercostal vein called the intercostal artery.

02:01 Finally, below that is a spinal nerve branch called the intercostal nerve.

02:08 And that's our neurovascular bundle.

02:10 Let's take a closer look at this bundle now in cross section.

02:14 So, in cross section, we again see external intercostal is the most external, then we have the internal, and finally the innermost and it's between the internal intercostal and the innermost intercostal, that this neurovascular bundle travels.

02:31 And some people use the mnemonic VAN, V-A-N to remember the order of this bundle from superior to inferior.

02:39 So superior, we have the intercostal vein, then we have the intercostal artery, and then we have the intercostal nerve, So, vein, artery, and nerve.

02:48 So, that's a good start to talking about the vasculature.

02:51 But there's a little bit more to the story.

02:53 So, let's zoom out a little bit so we could see a fuller picture.

02:58 So we have the subclavian artery, as we mentioned, giving rise to the internal thoracic or internal mammary artery.

03:04 And that's where we're going to get those anterior branches of the intercostal arteries.

03:09 But it turns out, they're going to announce the most with some coming from posterior.

03:15 Most of those are going to come directly off of the aorta, except for the first two, which tend to come off something called the cost of cervical trunk of the subclavian.

03:25 But otherwise, these posterior ones arise directly from the aorta and anastomoses with the ones coming from the internal thoracic artery.

03:34 So, if we put it all together, we can see how that fits.

03:38 So again, from superficial to deep, we have the external intercostal, internal intercostals, and the innermost intercostals.

03:47 Posterior leads where we have the aorta, giving rise to the posterior intercostal arteries that are traveling again between internal and innermost intercostals.

03:59 Now, it's also going to give some other branches.

04:01 So there's a dorsal branch that's going to go off and supply the back.

04:06 And there are going to be branches that pierce through the muscle to provide supply to the overlying skin.

04:12 Before the anastomoses with the anterior intercostals coming from the internal thoracic artery, which itself is going to give some perforating branches to supply the overriding skin.

04:23 And if we go back to an anterior view, we can see the intercostal vessels and nerves in their little bundle in the intercostal space.

04:32 And again, we see the internal thoracic artery running along either side of the sternum internally.

04:38 And if we look inferiorly, we see that it's going to end.

04:41 It's going to bifurcate into two arteries at its termination.

04:45 The first is going to head off, sort of laterally, diagonally, and it's going to supply abdominal muscles and the diaphragm and so it's appropriately called the musculophrenic artery.

04:56 Musculo for the abdominal muscles and phrenic because phrenic refers to diaphragm whenever you see that term.

05:02 The other branch basically continues along the vertical path, becoming the superior epigastric artery.

05:10 And if you haven't picked up by now, the thing with anatomy is if you hear superior so and so there's probably an inferior so and so.

05:18 And there is an inferior epigastric artery that will come from pelvic vessels below, anastomoses with the superior epigastric artery, right around the umbilicus, which is our fancy word for belly button.

05:32 Providing an important source of collateral supply in case you ever get an obstruction in your abdominal aorta.

05:38 All right, so let's look at those nerves a little bit greater detail.

05:43 So, we already mentioned the intercostal nerves run in that same groove in between internal and innermost intercostals.

05:51 And they're also going to give off branches along their course too.

05:55 So they're also going to have lateral cutaneous branches and supply the skin that's overlying the ribcage in that area.

06:02 And that's why dermatomes look the way they do.

06:05 Dermatomes look like rectangular strips that are roughly about the size and shape of an intercostal space.

06:12 There's going to be cutaneous branches anteriorly.

06:16 And there's going to be some stuff happening posteriorly that is a little smaller, so we're going to zoom in to look at it.

06:23 So here we see the spinal cord with its anterior and posterior roots.

06:29 And then we have the posterior ramus of this spinal nerve that's coming out at this level, immediately going to the back and supplying structures in the back.

06:38 Whereas, the anterior ramus is essentially what the intercostal nerve is.

06:44 And running parallel to the vertebra on either side our sympathetic trunk with its ganglia, and they're going to communicate with the spinal nerves at each level via the gray and white rami communicantes, which is just a very long word for communicating branches.

07:00 We're going to wrap up with some minor muscles in the intercostal space, starting with subcostales muscles or subcostales, which are variable in tend to only exists really on the lower ribs.

07:15 But they do attach to the internal surface of one rib, and then skip a couple ribs, and they don't attach again until two or three levels below.

07:27 And they have the same fiber orientation as the internal and innermost intercostal.

07:31 So they're going to have that same function of assisting with exhalation.

07:37 And then the last one is the transversus thoracis.

07:41 Also a very variable muscle but generally is going to sit along the internal surface of the say 2 to 6 ribs, whether it's at the bony part, or the cartilaginous part that varies as well.

07:58 And then they'll attach to the lower half of the sternum where the body and the xiphoid process are.

08:06 Same deal. They have a very minimal effect on breathing.

08:12 And it's the same as the other muscles we just mentioned, where they depress the ribs and assist with exhalation.

08:19 Though it's important to keep in mind that all the muscles we mentioned whether they help with inhalation or exhalation are really assistive or sensory.

08:27 They're not really the primary drivers of breathing.

08:30 By far the primary driver of inhalation is the diaphragm.

08:34 And for the most part, passive exhalation is just from elastin fibers that exist in the lung.

08:39 So these are really minor muscles of respiration.