Anatomy and Physiology 2e · The Cardiovascular System: The Heart
Cardiac Muscle and Electrical Activity
On this page 9 sections
In 30 seconds
The heart beats roughly 100,000 times a day without a nerve command, because cardiac muscle is Autorhythmic Able to generate its own electrical impulses Full entry →: specialized cells generate electrical impulses on their own, and a conduction system spreads each impulse so the chambers contract in order. Two ideas anchor the topic. First, cardiac muscle cells are electrically linked by intercalated discs, so an impulse spreads cell-to-cell almost instantly — the atria act as one unit, the ventricles as another. Second, the intrinsic conduction system has a pacemaker hierarchy: the fastest pacemaker sets the rhythm and slower ones act as backups. Understanding this system turns an ECG Surface recording of the heart's summed electrical activity Full entry → from a mysterious squiggle into a readable record of the heart's electrical events.
Why this matters
The ECG is one of medicine's most common diagnostic tools, measuring exactly what this topic describes. Knowing which wave corresponds to atrial depolarization, ventricular depolarization, or ventricular repolarization is the basis for recognizing normal rhythms and, later, arrhythmias and pacemakers. The same concepts explain everyday observations: why your heart races when frightened (sympathetic stimulation of the SA node Primary pacemaker in the right atrial wall Full entry →) and why the heart never "cramps" the way skeletal muscle can (its long Refractory period Time when a cell cannot be re-excited Full entry → makes tetanus impossible).
The college version
Core Concepts
Cardiac muscle cells: built to work together
Cardiac muscle cells are striated with one (occasionally two) central nucleus, but unlike skeletal muscle they are branched and connect end-to-end at intercalated discs. Each disc contains desmosomes, which mechanically rivet cells together so they do not tear apart, and gap junctions, protein channels that let ions flow directly between cells. Because of gap junctions, an impulse spreads electrically from cell to cell with no nerve needed, making the atria and ventricles functional syncytia — stimulate one cell and the whole chamber follows.
Two cell types: pacemakers and workers
About 1% of cardiac muscle cells are autorhythmic (pacemaker) cells, whose membrane potential drifts upward on its own until it reaches threshold and fires — no external stimulus required. The other ~99% are contractile cells, which do the mechanical work but only contract when stimulated. Pacemaker cells form the nodes and conduction fibers; contractile cells form the bulk of the myocardium.
The intrinsic conduction system
The impulse is born in the sinoatrial (SA) node, a patch of autorhythmic cells in the right atrial wall. Firing fastest (commonly taught intrinsic rate ~75–80/min), it sets the pace. The impulse spreads across both atria (the P wave) and reaches the atrioventricular (AV) node in the lower interatrial septum, which delays it ~0.1 s — time for the atria to finish contracting and top off the ventricles. From there it races down the AV bundle (of His), splits into right and left bundle branches, and spreads through the ventricular walls via Purkinje fibers Fast-conducting fibers spreading the impulse through the ventricles Full entry →, contracting the ventricles from the apex upward. The hierarchy is a fail-safe: if the SA node fails, the AV node Relay between atria and ventricles with a built-in delay Full entry → paces at ~50–60/min and Purkinje fibers even slower (~20–40/min, commonly taught); the fastest active pacemaker suppresses the slower ones.
Action potentials: why cardiac muscle beats differently
A contractile cell action potential has rapid depolarization from sodium (Na⁺) entry, a long Plateau Prolonged depolarized phase of a contractile-cell action potential Full entry → with calcium (Ca²⁺) entering through L-type channels while some potassium (K⁺) leaves, then repolarization as potassium exits. The long plateau gives cardiac muscle its extended refractory period — the cell cannot be re-excited until substantially repolarized. Because the refractory period lasts nearly as long as the contraction, the heart cannot undergo tetanus (sustained contraction); it must relax between beats to refill.
A pacemaker cell action potential has no stable resting potential: a slow "funny current" drifts the membrane upward (the prepotential), calcium channels fire the upstroke, and potassium efflux repolarizes. Its slope sets heart rate — and this is the slope the autonomic nervous system speeds up or slows down.
The ECG: listening to the electricity
The electrocardiogram (ECG) records the summed electrical activity of all cardiac cells from body-surface electrodes; it shows depolarization and repolarization waves, not contractions. The P wave is atrial depolarization; the PR segment/interval reflects the AV node delay; the QRS complex is ventricular depolarization (large, because the ventricles have far more muscle; atrial repolarization is hidden inside); the ST segment is the plateau; the T wave is ventricular repolarization.
Common Confusions
| Do not confuse | With | Difference |
|---|---|---|
| ECG waves = contraction | ECG waves = electrical events | Depolarization precedes contraction; the ECG is not mechanical |
| P wave = atrial contraction | P wave = atrial depolarization | Contraction follows the electrical wave |
| All cardiac cells are pacemakers | Only ~1% (autorhythmic cells) are | Contractile cells only respond; they do not fire on their own |
| The heart can tetanize like skeletal muscle | The heart cannot tetanize | The calcium plateau and long refractory period prevent it |
Eli explains
The same idea, in plain words
Explain it like I’m 10
The heart has its own little drummer who never takes a break. The drummer (SA node) sits in the right atrium and taps a steady beat; the beat travels along special wires (the conduction system) so both atria tap together, then both ventricles tap together. A tiny pause button (AV node) between the top and bottom rooms lets the top rooms finish pouring blood down before the bottom rooms squeeze. An ECG is a paper drawing of the drummer's taps.
Worked example
One heartbeat, electrically. The SA node's prepotential drifts to threshold and fires: the impulse spreads across both atria — the P wave appears — and the atria contract, topping off the ventricles. The AV node holds the impulse ~0.1 s (the PR segment), then it jumps down the AV bundle, races along the bundle branches, and erupts through the Purkinje fibers: the QRS complex appears, and the ventricles contract from the apex upward, ejecting blood into the pulmonary trunk and aorta. The cells sit depolarized through the ST segment (the calcium plateau) until potassium efflux repolarizes them — the T wave — and the ventricles relax.
Why doesn't your heart "cramp" during exercise? Skeletal muscle can be re-stimulated into a sustained contraction (tetanus); cardiac muscle cannot, because its long refractory period blocks re-excitation until relaxation has begun — a safety feature, since a tetanized heart would lock up and pump nothing. The trade-off: an excessively fast heart (tachycardia) can pump less blood, because diastole shortens and the ventricles cannot refill.
Key takeaways
- Cardiac muscle is autorhythmic — pacemaker cells generate impulses without nerve input; nerves and hormones only modify the rate.
- Intercalated discs = desmosomes (strength) + gap junctions (coupling); they create functional syncytia.
- Conduction path: SA node → atria → AV node → AV bundle → bundle branches → Purkinje fibers.
- Pacemaker hierarchy (commonly taught): SA ~75–80/min > AV ~50–60/min > Purkinje ~20–40/min; the fastest wins.
- The AV node delays ~0.1 s, so atrial contraction finishes before ventricular contraction begins.
- The contractile-cell Ca²⁺ plateau → long refractory period prevents tetanus, so the heart must relax to refill.
- Pacemaker cells have an unstable prepotential; its slope sets heart rate.
- ECG: P = atrial depolarization, QRS = ventricular depolarization, T = ventricular repolarization.
- Autonomic control targets the SA and AV nodes: sympathetic speeds, parasympathetic (vagus) slows.
Check yourself
5 review questions from the chapter. Try each one, then open the answer.
What are the two components of an Intercalated disc Specialized junction between cardiac muscle cells Full entry →, and what does each do?
Show answer
Desmosomes, which hold adjacent cells together so they do not tear apart, and gap junctions, which let ions pass between cells so the impulse spreads.
List the conduction pathway in order, and state the role of the AV node's delay.
Show answer
SA node → atria → AV node → AV bundle → bundle branches → Purkinje fibers. The ~0.1 s AV delay lets the atrial kick finish before ventricular contraction.
Why can the heart not undergo tetanus?
Show answer
Because the contractile-cell action potential has a long calcium plateau that keeps the cell refractory until relaxation is underway; a new stimulus cannot trigger contraction during the absolute refractory period.
What do the P wave, PR interval, QRS complex, and T wave represent?
Show answer
P wave = atrial depolarization; PR interval = the AV node delay; QRS = ventricular depolarization; T wave = ventricular repolarization.
If the SA node stopped firing, could the heart still beat?
Show answer
Yes. The AV node would take over at ~50–60/min (commonly taught), with Purkinje fibers slower still.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Autorhythmic
- Able to generate its own electrical impulses
- Intercalated disc
- Specialized junction between cardiac muscle cells
- Gap junction
- Protein channel letting ions pass directly between cells
- SA node
- Primary pacemaker in the right atrial wall
- AV node
- Relay between atria and ventricles with a built-in delay
- Purkinje fibers
- Fast-conducting fibers spreading the impulse through the ventricles
- Plateau
- Prolonged depolarized phase of a contractile-cell action potential
- Refractory period
- Time when a cell cannot be re-excited
- ECG
- Surface recording of the heart's summed electrical activity
Sources & references
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
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