Autonomic nervous system effects on the heart

Sometimes in medicine, you'll see that words get used a lot, and sometimes their meaning is pretty simple or straightforward and can be understood. But the words themselves end up being kind of tricky and confusing, and I think some of that has to do with the fact that these words in medicine often come from Latin, and so we haven't really changed the word. And so because Latin is not a language that any of us are really familiar with, it ends up being that the terms become confusing. So what I'm going to do is I'm actually going to lay out four words for you. Going to put out four words, start with "chronotropy." And we're going to go through them one by one, basically kind of describing what they mean. So chronotropy refers to the heart rate. Now I should put in parentheses a simpler way to maybe think about these terms, so chronotropy has to do with heart rate. A second term is "dromotropy." And dromotropy has to do with conduction velocity. How fast a signal is going from one cell to the next, conduction velocity. And then we have "inotropy," and inotropy has to do with contractility, how hard the muscle is contracting, the force of contraction, you can think of it as the force of contraction. And "lusitropy," and lusitropy has to do with the relaxation, how fast it's relaxing. So let's go through these one at a time and talk about the effects of the sympathetics and the parasympathetics on this. So the other thing I'm going to do maybe in white, I'm going to write out where, where these things are primarily being affected, right? Where and then we'll talk about synthetics and parasympathetics. OK. So where is the first one happening? So we talk about chronotropy and, of course, it's not just one cell that's going to be affected, but in general, what cluster of cells are we thinking of here? You're right if you think about the SA node, and this is where the heart rate is usually set, and sympathetics are going to basically make the SA node go and fire faster, because they're going to allow more sodium into those cells. So it's going to get up to the threshold for an action potential more quickly. And parasympathetics basically do the opposite, right, they're going to allow less sodium to get a flow in, and it's going to take longer to get to the threshold potential. So what about dromotropy? This is a word that's maybe less commonly used, and that's why I wanted to throw it up just so that you're at least familiar with the fact that it exists. But dromotropy has to do with conduction velocity. So how fast is the signal being conducted through the heart? You remember for this, the major delay ends up being in the AV node, right, the huge delay-- sometimes we say it's 1/10 of a second-- is in the AV node. So what happens in the AV node with sympathetic stimulation. Well, you remember when you have sympathetics around, they allow the calcium to come into the cell more quickly. So you actually have a faster conduction velocity. So actually there's a less of a delay with sympathetics, and with parasympathetics there's a longer delay, right? Again, it's slower the AV node because you have less calcium coming in during the action potential of the AV node. So the slope of the action potential is slower, is lower, in parasympathetic. Now, what about these last two, inotropy and lusitropy? Well, both have to do with the ventricles, contractility and the force of contraction. And then relaxation is how quickly are the ventricles relaxing. So I'm going to write for both of them "ventricles." And I'm putting both ventricles left and right, because it's not just one or the other, same rules apply in both sides. So ventricles. And already I can tell you that parasympathetics are going to have very little effect here, right, because we said that really sympathetics affect your ventricles, and the parasympathetics simply don't have a similar effect to counterbalance it. So I'm just going to put a little white line there to imply there's no major parasympathetic effect on either inotropy or lusitropy from a ventricle standpoint. All right, so sympathetics. What do they do for inotropy? Or in terms of inotropy? Well, you remember, contractility is related to the amount of calcium that's going to come in during the action potential. So what's going to happen if you have sympathetic stimulation? Well, you're going to get more calcium in there, so you're going to have a harder contraction or a stronger force, I'll just put "harder contraction." Meaning the muscles are going to squeeze down more forcefully than they would otherwise. And what about relaxation? Well, this is the interesting thing we talked about with the sarcoplasmic reticulum, right? That's what's going to mop up all the calcium and allow our heart to get back into a state of relaxing, and it's going to happen much quicker with sympathetics, because they stimulate those ATP channels, or ATP pumps to get calcium pumped back into the sarcoplasmic reticulum, and away or sequestered from the cell itself. So they basically create a faster relaxation. So these are kind of the quick and dirty overview of these four words, and again, you may or may not hear these words, but at least you're familiar with them. Chrono, dromo, ino, and lusi. These four words describe the major functions of the sympathetic and parasympathetic nerves on the heart.