Heart Rate Ranges and Gap Junction Channels – Heart Rate and Electricity

00:01 What’s the normal range of heart rate? Heart rate – SA node heart rate, you want to know, it's between usually, what? 60 and 100 beats.

00:13 So, if you're anywhere between 60 and 100, it’s considered normal.

00:18 If you're outside these normal ranges, if you're above normal, we call that tachycardia.

00:24 If you're below normal, we call it bradycardia.

00:27 So, these are just the terms that are given when you're outside those normal ranges.

00:34 Now, are there times in which a normal healthy person should be above 100 beats per minute? Sure! How about if you're exercising? You normally – there are sometimes when heart rate needs to be that high.

00:47 So, it's not abnormal. It's just called tachycardia.

00:51 If you're below 60 – that's bradycardia – and that is oftentimes too could be normal.

00:58 Someone who is very fit, very athletic, especially from aerobic endurance exercise, oftentimes will have heart rates lower than 60 beats per minute.

01:09 Perfectly normal for them.

01:12 What is your maximal heart rate? That is usually denoted through this, 220 minus age.

01:21 It decreases as you get older.

01:25 So, you will have a lower maximal heart rate at a higher age than you do when you're young.

01:30 Now, this 220 minus age, this is a formula and this is basically an estimate.

01:37 You don't exactly know because the standard deviation is about 15 beats.

01:43 That means, you could be two standard deviations off the mean to be 30 beats different than That means, you could be two standard deviations off the mean to be 30 beats different than what this formula is telling you, but it does give you some insight into what your maximal heart rate might be.

01:53 And why is this sometimes important? Well, if you know what your maximal heart rate is, maybe you want to do something like exercise at a percent of your maximal heart rate, and therefore, you have what that might be.

02:04 So, it gives you your parameters in which your heart would want to operate.

02:09 How do you determine that clinically? What you do is an exercise stress test.

02:13 So, this usually involves either a bicycle or a treadmill in which they keep making it faster and faster and harder and faster and faster and harder and harder and harder until you can’t go anymore. and harder and harder until you can’t go anymore.

02:24 And that will give your maximal heart rate.

02:31 Now, how does this action potential get to the muscle, so the muscle will actually do something like contract because, again, it's only the electrical activities that we've been talking about so far.

02:45 We haven't talked at all about any of the contraction that might happen with the heart.

02:49 We simply have been talking about the electrical propagation across different myocytes.

02:57 So, how this process works is, if you think about, the whole heart is linked together.

03:05 You're going to have an action potential that occurs, it’s going to gradually travel down all the different other myocytes because they all are connected.

03:15 They’re connected by these things, we call, gap junctions.

03:19 So, gap junctions connect all heart cells together.

03:25 This is important.

03:26 This is important.

03:28 Because if they weren't connected, you would not get a beat to occur in a coordinated fashion to do useful work.

03:38 Think about that.

03:40 You wouldn't want one myocyte cell, let’s say, in the left ventricle to contract and one in the top part of the heart, in the atrium, to contract, contract, contract, contract.

03:49 If they're not coordinated, you will not get a nice push out of fluid.

03:54 And so, it's that coordination of all of these myocytes that is important.

03:59 The gap junctions help link all those cells together.

04:06 Gap junctions, here you can see, how they look.

04:10 They are very close proximity to the next membrane that's close to it.

04:16 These particular channels involve usually six different structural proteins that are stuck through both membranes.

04:25 And what happens is, they open up.

04:29 And once they open up and it will allow that information to travel through.

04:33 Now, cells just don't talk to each other.

04:35 It's not like they say hello down a little tube.

04:39 They send in ion signal across those membranes to cause that communication process to happen.

04:49 And that communication is able to happen via these gap junctions.

04:57 So, let's now talk through which particular changes in an ion happen at what time.

05:07 And then, let's link it to its channel.

05:10 Why? Because not only can we, but these are the various things that you might be able to manipulate or block with a drug.

05:18 And/or if a person has pathology, there could be problems with one or more of these channels.

05:25 So, we always start off with the basics, and that is we have to have a sodium potassium pump.

05:30 This is your sodium potassium ATPase.

05:33 It's electrogenic.

05:35 It's creating that membrane potential.

05:37 It's always pumping in potassium, pumping out sodium, but we have now gap junctions in these specific cardiac myocytes.

05:49 So, the neat thing about a gap junction is it allows the transmission of that signal – ionic signal across from one cell to the next cell without leaving the tube.

06:02 That change in ions that enter into the new cell opens up voltage-gated sodium channels.

06:12 So, what happens when you open up voltage-gated sodium channels.

06:15 If you remember from your notion of what should be inside the cell versus outside the cell, what do you know about sodium? It's high on the outside, low on the inside, so it’s going to want to travel in.

06:29 That’s exactly what happens.

06:32 So, you see a nice, large inward flux of sodium as you open up these voltage-gated sodium channels.

06:40 What happens next? This is exciting.

06:43 The next thing that happens is a fast potassium channel opens up.

06:48 These are fast.

06:49 They don’t last very long, so there’s only a little bit of potassium change.

06:55 The next thing that happens, a calcium channel opens.

07:01 This calcium channel has a big effect.

07:04 A large amount of calcium influx into the cell.

07:08 Why does it move into the cell? Because the concentration gradient allows that to occur because it’s higher.

07:14 Calcium is higher on the outside of the cell than the inside.

07:18 The final thing that happens in terms of the current change is that we open up these potassium channels and these will cause repolarization to happen as potassium now leaves the cell.

07:34 So, again, you have the two molecules, sodium and calcium, that enter the cell and potassium leaves the cell through one of two different channels – a fast channel and then a delayed channel that involve repolarization.