SA node pacemaker potential: phase 4 sets the heart rate
Sinoatrial nodal cells have no stable resting potential. Heart rate is the slope of phase 4, and the vagus and sympathetic nerves simply move that slope.
Chapters (8)
Explain the SA-node pacemaker potential (funny current, T- and L-type Ca²⁺) and how sympathetic and vagal input change its phase 4 slope.
The short version
- Sinoatrial nodal cells have no stable resting potential: phase 4 drifts upward through the funny current, a falling K⁺ current, then T-type and L-type Ca²⁺ channels.
- There are no fast Na⁺ channels in the node, so the upstroke (phase 0) is carried by L-type Ca²⁺ and is slower than in a ventricular myocyte.
- Heart rate is the slope of phase 4: a steeper slope reaches threshold sooner and the heart beats faster.
- Vagal acetylcholine opens K⁺ channels and flattens the slope; sympathetic norepinephrine on β₁ receptors steepens it.
- Resting heart rate is the intrinsic rate of about 100/min held down by vagal tone, typically to 60–80/min.
Why does a 46/min pulse feel normal? The SA node pacemaker potential
Consider an illustrative 34-year-old marathon runner (a made-up example, not patient data) with a resting pulse of 46 beats per minute who feels perfectly well. Yet the sinoatrial node, left alone, fires at about 100 per minute.[1][2]
The SA node pacemaker potential explains the gap. This lesson covers the funny current, the calcium channels that finish phase 4, and how the vagus and sympathetic nerves change the slope of the drift.
What makes a pacemaker cell different
A ventricular myocyte sits at a stable resting potential until something pushes it. A sinoatrial nodal cell has no stable resting potential. It drifts upward on its own, beat after beat, and that drift is called phase 4.[1][2]
The drift starts near −60 mV and reaches threshold at roughly −40 to −30 mV.[1][2] Three things drive it.
Phase 0 and phase 3: no fast sodium channels
Here is the twist: there are no fast Na⁺ channels in this cell. The upstroke, phase 0, is carried by L-type Ca²⁺, so it is slower than in a ventricular myocyte. There is no plateau.[1][2]
Phase 3 is the usual repolarization: K⁺ leaves, Ca²⁺ channels close, and the cell returns to about −60 mV. Then the drift starts again.[1][2]
| Phase | What happens | Main current |
|---|---|---|
| 4 | Slow upward drift from about −60 mV to threshold | Funny current, falling K⁺ outflow, T-type then L-type Ca²⁺ |
| 0 | Upstroke, slower than in a ventricular myocyte | L-type Ca²⁺ (no fast Na⁺ channels) |
| 3 | Return to about −60 mV | K⁺ leaves, Ca²⁺ channels close |
Heart rate is the slope of phase 4
The rate of the heart is simply how fast phase 4 climbs to threshold. A steeper slope reaches threshold sooner, so the heart beats faster. A shallower slope takes longer, so the heart beats slower. The nerves only have to move that slope.[1][2]
| Input | Transmitter and receptor | Effect on phase 4 | Heart rate |
|---|---|---|---|
| Vagus | Acetylcholine; opens K⁺ channels, so K⁺ leaves the cell | Slope flatter; longer to reach threshold | Fewer beats each minute |
| Sympathetic | Norepinephrine on β₁ receptors; opens Na⁺ and Ca²⁺ channels | More positive charge flows in; slope steeper | Threshold sooner; heart speeds up |
In short: the vagus flattens the slope; the sympathetic nerves steepen it.[1][2]
Intrinsic rate, vagal tone and atropine
Without any nervous input, the node fires at about 100 beats per minute. At rest the vagus is always active, a steady brake called vagal tone. That brake holds a typical resting rate to about 60–80.[1][2]
Trained athletes typically have higher vagal tone (and some intrinsic slowing of the node), so a rate of 46/min is no mystery.
To go above 100, you need sympathetic drive and the vagal brake lifted.[1][2]
The brake can be demonstrated. Atropine blocks the muscarinic receptor, and heart rate rises by roughly 20–40 beats per minute.[1]
- Intrinsic rate of the node ≈ 100/min
- Observed resting rate = 46/min
- If the node still fired at 100/min, the brake would be about 100 − 46 = 54/min (illustrative)
Practice question
Original · USMLE styleA 26-year-old man lies resting during an autonomic physiology study. His heart rate is 62/min. He is given intravenous atropine, and his heart rate rises to 96/min.
Common questions
What causes phase 4 of the SA node pacemaker potential?
A slow drift upward from about −60 mV driven by the funny current (mostly inward sodium), a falling outward K⁺ current, then T-type Ca²⁺ channels near −50 mV and L-type Ca²⁺ channels near −40 mV.
Why does the SA node have no fast sodium upstroke?
SA nodal cells have no fast Na⁺ channels. Phase 0 is carried by L-type Ca²⁺ channels, so the upstroke is slower than in a ventricular myocyte.
How do the vagus and sympathetic nerves change heart rate?
Both change the slope of phase 4. Vagal acetylcholine opens K⁺ channels and flattens the slope; sympathetic norepinephrine on β₁ receptors opens Na⁺ and Ca²⁺ channels and steepens it.
What is the intrinsic rate of the SA node?
About 100 beats per minute without nervous input. At rest, vagal tone holds a typical rate to about 60–80.
Transcript
Read the full narration, by chapter709 words
0:00A runner at 46 beats per minute
A 34-year-old marathon runner comes to clinic for a check-up. Her resting pulse is 46 bpm, and she feels perfectly well. Yet the sinoatrial node, left alone, fires at about 100. So what is slowing her down? Today: explain the pacemaker potential, the funny current and the calcium channels, and how the vagus and sympathetic nerves change its slope.
0:25The pacemaker potential: phase 4, phase 0, phase 3
Start with what makes a pacemaker cell different. A ventricular myocyte sits at a stable resting potential until something pushes it. A sinoatrial nodal cell has no stable resting potential. It drifts upward on its own, beat after beat, and that drift is called phase four. The drift starts near minus 60 millivolts. Three things drive it. First, the funny current: a slow inward flow of mostly sodium, switched on by hyperpolarization, which is why it is called funny. Second, a falling outward potassium current. Third, calcium: T-type channels open near minus 50 millivolts, then L-type channels near minus 40. Threshold is reached at roughly minus 40 to minus 30 millivolts. And here is the twist: there are no fast sodium channels in this cell. The upstroke, phase zero, is carried by L-type calcium, so it is slower than in a ventricular myocyte. Phase three is the usual repolarization: potassium leaves, calcium channels close, and the cell returns to about minus 60. And then the drift starts again.
1:40Heart rate is the slope of phase 4
The rate of the heart is simply how fast phase four climbs to threshold. A steeper slope reaches threshold sooner, so the heart beats faster. A shallower slope takes longer, so the heart beats slower. So the nerves only have to move that slope.
1:56The vagus slows the slope
Start with the vagus nerve. It releases acetylcholine onto the sinoatrial node. Acetylcholine opens potassium channels, so potassium leaves the cell. That slows phase four, and the cell takes longer to reach threshold. Fewer beats each minute.
2:14Sympathetic nerves steepen the slope
Now the sympathetic nerves. They release norepinephrine, which binds beta one receptors. That opens sodium and calcium channels, more positive charge flows in, and phase four climbs faster. Threshold arrives sooner, and the heart speeds up. Vagus slows the slope; sympathetic steepens it.
2:35Back to the runner: vagal tone and atropine
Now back to our runner. Without any nervous input, the node fires at about 100 beats per minute. But at rest the vagus is always active, a steady brake called vagal tone. That brake holds a typical resting rate to about 60 to 80. A highly trained athlete has higher vagal tone, and the node itself adapts, so 46 is no mystery. To go above 100, you need sympathetic drive, and the vagal brake lifted. You can prove the brake exists. Give atropine, which blocks the muscarinic receptor, and heart rate rises by roughly 20 to 40 bpm. So resting heart rate is the intrinsic rate of about 100, minus the vagal brake. Your turn.
3:20Practice question
A 26-year-old man lies resting during an autonomic physiology study. His heart rate is 62 bpm. He is given intravenous atropine, and the rate rises to 96. Which change in his sinoatrial nodal cells best explains the faster rate? A, new fast sodium channels steepening the upstroke. B, a shorter plateau between the upstroke and repolarization. C, a steeper phase four slope after loss of vagal potassium conductance. D, deeper hyperpolarization at the end of phase three. E, a threshold shifted to a more positive potential. Pause here and decide. A is out: sinoatrial cells have no fast sodium channels, and phase zero is not what sets the rate. B is out: these cells have no plateau, so there is no plateau to shorten. D is out: deeper hyperpolarization would lengthen the climb and slow the heart. E is out: a more positive threshold also means a longer climb, and a slower rate. So the answer is C: atropine removes the vagal potassium conductance, phase four steepens, and threshold arrives sooner.
4:41Recap and next lesson
Three things to keep. One: sinoatrial cells have no stable resting potential; phase four drifts up through the funny current and then T-type and L-type calcium. Two: the rate is the slope of phase four; vagus acetylcholine flattens it, and sympathetic norepinephrine steepens it. Three: resting rate is the intrinsic rate of about 100, held down by vagal tone. Next, we put these electrical events on paper: ECG basics, waves and intervals.
Sources
Numbers in the text, e.g. [1], point here- Klabunde RE. CV Physiology, Arrhythmias section: A004 (sinoatrial node action potential), A003 (autonomic modulation), e010 (intrinsic rate, vagal tone, atropine). cvphysiology.com/Arrhythmias/A004 · cvphysiology.com/Arrhythmias/A003 · cvphysiology.com/arrhythmias/e010
- OpenStax. Anatomy and Physiology 2e, 19.2 Cardiac Muscle and Electrical Activity; 19.4 Cardiac Physiology. openstax.org/books/anatomy-and-physiology-2e/pages/19-2-cardiac-muscle-and-electrical-activity · openstax.org/books/anatomy-and-physiology-2e/pages/19-4-cardiac-physiology
The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.
Image credits. Heart conduction plate: Servier Medical Art (smart.servier.com), licensed under CC BY 4.0, recoloured and outlined for the page (https://creativecommons.org/licenses/by/4.0/). The membrane-potential traces, rate bars, vitals slip, question card, formulas and all handwriting are drawn in code (original). Previous: EP 04 · The Ventricular Myocyte Action Potential · Next: EP 06 · ECG Basics: Waves and Intervals
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For exam study only, not medical advice. Read the medical disclaimer.
In this series
Playlist on YouTube- EP 01S2 splitting and heart sounds S1–S4, explained5:26
- EP 02Heart murmurs and dynamic manoeuvres, from preload and afterload5:15
- EP 03HOCM vs aortic stenosis murmur: telling them apart at the bedside5:31
- EP 04Ventricular action potential: phases 0 to 4 and their ion currents5:15
- EP 05SA node pacemaker potential: phase 4 sets the heart rate5:17
- EP 06ECG waves and intervals: what P, PR, QRS, ST and QT mean5:29