The sinoatrial (SA) node is the heart's primary pacemaker, capable of spontaneous rhythmic depolarization at 60-100 bpm due to unique ion channel kinetics: funny currents (If) and L-type calcium channels drive diastolic depolarization toward threshold in the absence of external input. SA nodal tissue lacks a stable resting potential; instead, the membrane potential gradually drifts upward until reaching threshold, triggering an action potential and heartbeat. The intrinsic rhythm is modulated by the autonomic nervous system: parasympathetic activation hyperpolarizes and slows the nodal rate, while sympathetic activation depolarizes and accelerates it.
Study isolated SA node preparations to observe spontaneous depolarization and action potential generation. Use pharmacology (acetylcholine, isoproterenol) to observe autonomic effects on pacemaker rate.
The SA node is not the only cardiac pacemaker; other regions (AV node, Purkinje fibers) also generate action potentials and can pace the heart if the SA node fails, though at slower intrinsic rates.
From your study of action potentials, you know that most excitable cells maintain a stable resting membrane potential — they sit quietly at around −70 to −90 mV until an external stimulus pushes them to threshold. The sinoatrial (SA) node breaks this rule. Its cells never truly rest. Instead, after each action potential repolarizes, the membrane potential immediately begins drifting upward again in a phase called pacemaker potential (or Phase 4 depolarization). This spontaneous drift is what makes the heart beat without any external command — no neural input, no hormonal signal, just an intrinsic property of the ion channels in SA nodal cells.
The pacemaker potential is driven by a specific set of ion currents. As the cell repolarizes past about −60 mV, funny channels (If) open. These are unusual because they are activated by hyperpolarization rather than depolarization — hence the name "funny." They conduct a mixed Na+/K+ inward current that slowly depolarizes the membrane. As the membrane potential rises past about −50 mV, T-type calcium channels open and add more inward current. Finally, near threshold (around −40 mV), L-type calcium channels open and drive the rapid upstroke of the SA node action potential. Notice the contrast with ventricular myocytes you studied in the cardiac cycle: ventricular action potentials have a fast sodium-driven upstroke (Phase 0), but SA node action potentials rely on calcium for their upstroke, which is why they rise more slowly and have a rounded shape rather than a sharp spike.
The rate of this pacemaker cycle — and therefore heart rate — depends on three adjustable parameters: the slope of Phase 4 depolarization (steeper slope = faster drift to threshold = faster heart rate), the maximum diastolic potential (more negative starting point = longer time to reach threshold = slower rate), and the threshold voltage itself. The autonomic nervous system modulates all three. Sympathetic stimulation via norepinephrine activates beta-1 adrenergic receptors, which increase funny current and calcium current through cAMP-dependent phosphorylation — the Phase 4 slope steepens, and the cell reaches threshold sooner. Parasympathetic stimulation via acetylcholine activates muscarinic receptors, which open potassium channels (IKACh) that hyperpolarize the cell and also reduce funny current — the starting point becomes more negative and the slope shallows, both slowing the rate.
The SA node normally fires at 60-100 beats per minute, faster than any other cardiac pacemaker tissue. The AV node has an intrinsic rate of 40-60 bpm, and Purkinje fibers fire at 20-40 bpm. This hierarchy of automaticity ensures that the fastest pacemaker always captures the heart. If the SA node fails or its impulses are blocked, the next fastest pacemaker takes over as an escape rhythm — slower but life-sustaining. Understanding this hierarchy explains both normal heart rhythm and the clinical logic of pacemaker implantation: an artificial pacemaker replaces the SA node's function when disease disrupts the natural one.