Nerve regulation of the heart
Getting a new heart
I'm going to draw a person for you, and this is going to be my side profile of this person. Let's say something like this. And this person has-- I'm going to draw it kind of loosely-- let's say, two arms and one shoulder apparently. I need to draw in another shoulder. There we go. So this is my person. And I'm going to draw in their brain. This is their brain. And they have a heart. This is their heart here. And now you know there are nerves that come down from the brain and serve the heart, right? So these are nerves that are basically going to tell the heart to speed up or beat harder, with higher force of contraction, or maybe relax more quickly. So basically what I'm telling you, these are the sympathetic nerves. And of course, I should also draw in the parasympathetic nerves. So these are going to tell the heart to slow down and also going to increase the delay in the AV node. And these are the parasympathetic nerves, primarily the vagus nerve. You might hear that word too, vagus nerves, which is basically the actual name of the nerve itself. But they are parasympathetic nerves. Now, you have information flowing from the brain down to the heart. But you also have information flowing from the heart up to the brain. So, information is going up as well as down. So what would be an example of information going up? Well, you can imagine that there might be volume receptors, right? So we know that the brain needs to know about the volume status. And in the right atrium, there are some volume receptors that basically stretch. And once they stretch, they tell the brain that there's a lot of blood. Or if they're not stretched, they tell the brain that there's not a lot of blood. And this really helps with autoregulation of blood pressure. So if you want to regulate your blood pressure, you've got to have some information. So autoregulation of blood pressure. So basically, the whole point of me saying all this is that there's a lot of nerves going back and forth between the brain and the heart. Now, let's say this person that I'm describing tragically dies. They die. And they're wish was to donate their heart. And so, over here we have a heart recipient, someone that is interested in getting a heart. Let's say that their heart is not working well or whatever the situation may be. They're actually hoping to get a heart. So this person is looking for a heart, and this person died and their wish was to give a heart. So, let's show that actually. Let's show the heart moving over. Let's say that this person's heart gets sent over to the person on the right. So now their heart gets sent over here. So all is well. And this person's actually obviously very happy that they got a heart. This is obviously a big deal. Let's see if we can do this. There we go. This person's going to be smiling, right? And I should probably erase all these little nerves. But this brings up a really good point, because if the nerves are not coming over-- which they're not, it's just the heart-- then can we regulate all the things that we know that nerves help us regulate? Can we, for example, tweak or change the heart rate if needed? Or can we change the force of contraction if needed? Are any of those things going to be able to be changed, or is this heart basically now always going to be in one status, always just beating at one rate, pumping it with one force, is it ever going to change? Is there any way anything can change this now that the nerves are not connected? Well, the first thing is that there is some evidence that nerves can reconnect from the new body. But that would take a long, long time. So in the short term, what is the answer? Well, you have a couple of glands. You have adrenal glands sitting here. And these glands are sitting literally above the kidneys. So these are the adrenal glands. And you also have thyroid gland. So these are the adrenal glands and this is your thyroid up here. This is your thyroid kind of in your neck region. So these are hormone producers. The thyroid gland, the adrenal glands, they're going to be producing hormones. And those hormones can affect the heart. So, one hormone could be, for example, this would be a thyroid hormone. Down below, you might have epinephrine. So another hormone, number two could be epinephrine. This comes from the adrenals. So you can see that basically there are hormones that are going to affect the heart. Over here in the first case, when the heart was over here, you're getting rapid changes, right? Very, very quick. But as compared to what we were getting, this is going to be a little bit slower in terms of the response rate, because now the hormone has to go through the bloodstream and get to the heart. And it takes a little bit of time. Whereas before with the nerves, you're really getting changes as fast as a nerve impulse, which is on the order of less than a second. So definitely the response rate is going to be slower. So your heart can change its force of contraction and heart rate and conduction velocity. It definitely can change in this new person with a heart transplant. But it will take a little bit longer. And another thing that will change it, in addition to just hormones, I want to remind you, is medication. So let's imagine this person is taking medications. And let's say these medications have certain properties. Let's say they're taking a calcium channel blocker. Let's say they're taking a medicine that's a-- I should probably write meds here-- calcium channel blocker. Well, you know that there are calcium channels in the heart. And if you are blocking them, then that's going to have an effect on the force of contraction, let's say. And what if they're taking a beta blocker? And beta refers to the receptor for epinephrine and norepinephrine. So if they're taking a beta blocker, they're really blocking the effects of, in this case, epinephrine, right? They're blocking epinephrine effects. So you can see how if the person starts taking a medication or let's say they take a few pills on a certain day, or their adrenal glands are making more epinephrine, all these kinds of things are going to essentially change how the heart works, how fast it's working, how hard it's working. So they are going to have changes. It's not like they don't have changes in any of those things. But those changes are going to take place on a slower time scale than what would have happened in the person who has all the nerves connected.