Para- and Sympathetic Control of the Heart Rate – Heart Rate and Electricity

by Thad Wilson, PhD

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    00:02 Now, what controls SA node rate.

    00:05 Because you know that your heart rate can go slower than 60, they can go faster than 100, so what controls that process? Nerves control it to the greatest degree.

    00:17 So, we have things like the parasympathetic nerve system.

    00:22 So, here, you have cranial nerve number IX – sorry, number X which is the vagus nerve.

    00:28 The vagus nerve releases acetylcholine.

    00:30 Acetylcholine then binds to a muscarinic receptor on the SA node.

    00:36 What this happens, at this point, is it slows down heart rate.

    00:41 So, the more vagal the response, the slower the heart rate.

    00:46 We call that negative chronotropy.

    00:50 Where does that come from? Negative just means less than normal.

    00:54 Chronotropy is what we call heart rate.

    00:57 It seems like a fancy word for heart rate, but you have to know it.

    01:01 How do you get a faster heart rate? There, it is a sympathetic nervous system response.

    01:07 This is your classic fight or flight; or like what I like to call it, fight, flight or maybe it’s simply flight and aggressive conflict mediation.

    01:17 Whatever you do, you know that it's a process in which you’re speeding up the heart.

    01:23 This is a beta-adrenergic response where you have norepinephrine being released from presynaptic terminals, binding to beta-1 adrenergic receptors to speed up the heart.

    01:34 We call that positive chronotropy.

    01:38 Positive means higher than normal.

    01:40 Chronotropy again is our name for heart rate.

    01:43 So, you can see here how the nervous system both speeds up and slows down the heart.

    01:48 All through its actions, only on the SA node, in this case.

    01:54 So, now let's take a couple of minutes and talk through exactly how the sympathetic and parasympathetic nervous systems actually control heart rate because this is a complex process.

    02:06 You know that it’s just going to affect Phase 4, you know it might affect threshold, but you're not sure how it's going to affect all these things.

    02:14 Let’s just talk through it one by one.

    02:16 We’ll break it down.

    02:16 We’ll make it really simple.

    02:18 We’ll give it some boxes and we make sure we get this all, so you'll have it down and you won't forget it once you know how this process works.

    02:27 So, let’s just start off with the parasympathetic nervous system.

    02:30 Again, you have cholinergic nerves that release acetylcholine, which is your neurotransmitter that's going to cause the parasympathetic vagal responses.

    02:40 The first thing that happens is this acetylcholine decreases another molecule located within the myocyte called cAMP.

    02:50 This decrease in cAMP does two things.

    02:54 The first thing it does is it decreases that funny current.

    02:58 Remember, a funny current goes through these HCN channels.

    03:03 It decreases the current, meaning that less sodium is going out – into the cell.

    03:11 If less sodium travels, therefore, you’ll have a slower rate of Phase 4.

    03:19 What do I mean by slower rate? Phase 4 will climb less rapidly.

    03:25 It is going to – there is going to be a less rise overrun of the response.

    03:31 It slows it down. It flattens out Phase 4.

    03:37 The other thing that cAMP does is it phosphorolyzes calcium channels.

    03:44 What this does is decrease calcium influx and moves an action potential away from normal membrane potential.

    03:53 This too slows down the heart rate.

    03:57 The final thing that happens is that the muscarinic activity increases potassium.

    04:05 So, you get more potassium that leaves the cell.

    04:09 If you get more potassium that leaves the cell, remember potassium is positive.

    04:13 It's positive.

    04:14 It has a charge.

    04:15 When it leaves the cell, the cell becomes more negative.

    04:20 As a cell becomes more negative, it's going to take it a longer period of time to reach threshold.

    04:28 So, there’s different ways in which the parasympathetic nervous system changes or slows heart rate.

    04:35 It does it through the rate of Phase 4 depolarization.

    04:39 It moves the action potential threshold away from membrane potential.

    04:44 And it has a negative shift in the membrane potential.

    04:48 All three of those decrease heart rate and decrease conduction through the AV node.

    04:55 Great! So, now that you have those three mechanisms down for the parasympathetic nervous system, let's move right into the sympathetic nervous system.

    05:03 Remember here, we want to speed up the heart because either we want to run away or we’re going to have some good conflict mediation.

    05:12 The first thing that happens here is you release norepinephrine.

    05:16 Norepinephrine binds the beta-1 adrenergic receptors and this increases cAMP.

    05:23 So, it does the opposite of what acetylcholine did to those muscarinic receptors.

    05:28 It works via two mechanisms.

    05:31 The first one is that it is going to phosphorylyze calcium channels to change the calcium influx, which moves membrane potential away from the action potential.

    05:47 Widening the distance needed to travel before you reach threshold.

    05:54 The other thing that cAMP does is it allows for more current to travel through those HCN channels.

    06:03 So, now the HCN channels allow more sodium influx, which increases the rate of rise of Phase 4, meaning that it steepens that slope.

    06:16 So, you're going to now reach threshold quicker because the slope of Phase 4 is steeper.

    06:24 So, between moving threshold and between having this Phase 4 depolarization process happen quicker, you increase heart rate.

    06:34 And you increase the conduction through the AV node.

    06:40 These are ways in which the sympathetic and parasympathetic vagal nerve control heart rate.

    About the Lecture

    The lecture Para- and Sympathetic Control of the Heart Rate – Heart Rate and Electricity by Thad Wilson, PhD is from the course Cardiac Physiology.

    Included Quiz Questions

    1. Increase in the rate of phase 4 depolarization
    2. Increase in the threshold potential to a less negative value
    3. Greater absolute amount of repolarization
    4. Lower or more negative shift in resting membrane potential
    1. M2 receptors
    2. M4 receptors
    3. M3 receptors
    4. M1 receptors
    5. M5 receptors
    1. β 1 adrenergic receptors
    2. α2 adrenergic receptors
    3. β3 adrenergic receptors
    4. α1 adrenergic receptors
    5. β2 adrenergic receptors
    1. Rate of phase 4 depolarization decreases
    2. Increased Na influx
    3. Increased HCN activity
    4. Increased phosphorylation of Ca channels
    5. Increased cyclic adenosine monophosphate

    Author of lecture Para- and Sympathetic Control of the Heart Rate – Heart Rate and Electricity

     Thad Wilson, PhD

    Thad Wilson, PhD

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