Sinus node
Also called sinoatrial node, SA node, pacemaker, intrinsic heart rate
The heart's natural pacemaker, a small patch of cells that starts every normal heartbeat. Left alone it fires about 100 times a minute; the vagus nerve holds it lower at rest.
Every normal heartbeat starts in the sinus node, also called the sinoatrial (SA) node, high in the right upper chamber of the heart. Its cells fire on their own, with no signal from the brain. After a heart transplant, which cuts the nerves to the heart, the new heart keeps beating.
On its own the node is fast. In a worked example, Quigley and colleagues (2024) give a typical 40-year-old an intrinsic heart rate of about 100 beats a minute, measured after drugs block both branches of the nervous system. Shaffer, McCraty and Zerr (2014) report that it falls with age, from an average of 107 at 20 to 90 at 50.
Resting heart rate Resting heart rate How many times your heart beats per minute while you are fully at rest. For adults it is usually 60 to 100 beats per minute, and lower in well-trained athletes. More about Resting heart rate is lower than that because the vagus nerve Vagus nerve The main nerve of the parasympathetic nervous system, running from the brainstem to the heart, lungs and gut. Its fast signals to the heart drive most beat-to-beat HRV. More about Vagus nerve keeps a foot on the brake. MacDonald and colleagues (2020) note that in humans, unlike mice, resting heart rate is mostly set by this parasympathetic Parasympathetic nervous system The "rest and digest" branch of the autonomic nervous system. It slows the heart, mainly through the vagus nerve, and drives most of the beat-to-beat variation in HRV. More about Parasympathetic nervous system input. The sympathetic Sympathetic nervous system The "fight or flight" branch of the autonomic nervous system. It speeds up the heart and makes it pump harder, but it acts too slowly to drive beat-to-beat HRV. More about Sympathetic nervous system nerves speed the node up when you need it.
Why it matters for HRV. HRV is the change in timing of the sinus node from beat to beat. The vagus can change the very next beat, so most fast beat-to-beat variation is vagal. A beat that starts outside the node, an ectopic beat Ectopic beat A heartbeat that starts outside the heart's normal pacemaker and usually comes early. Ectopic beats are common, but they distort HRV unless removed or corrected. More about Ectopic beat , is not part of HRV and has to be removed.
The model below starts at the node’s own pace of 100 beats a minute when both sliders are at zero.
Brake and accelerator
A teaching model, not a measurement
- Heart rate
- 61 bpm
- RMSSD
- 55 ms
Set by the sliders below
Rested morning. After a good night, the brake is strong and the accelerator is light. Heart rate is low and the gaps between beats vary a lot, so HRV is high.
The sliders and presets work when JavaScript is on.
A heart beats at the rate set by two sliders, one for the vagal brake (parasympathetic) and one for the accelerator (sympathetic). A strip of heartbeats and a line of the gap before each beat show how even the rhythm is. Raising the brake slows the heart and makes the gaps vary more, so RMSSD rises. Raising the accelerator speeds the heart and makes the gaps more even, so RMSSD falls. Buttons set the sliders for a rested morning, sleep, stress, the hours after a hard workout and the start of an illness, each with a short explanation.
The lesson The nervous system behind HRV explains how the two branches steer the node.
Sources
- A healthy heart is not a metronome: an integrative review of the heart's anatomy and heart rate variability
- Publication guidelines for human heart rate and heart rate variability studies in psychophysiology—Part 1: Physiological underpinnings and foundations of measurement
- Neurohumoral Control of Sinoatrial Node Activity and Heart Rate: Insight From Experimental Models and Findings From Humans