Cardiac Threshold Potential

Cardiac Threshold Potential is the specific membrane voltage at which the net inward current generated by opening voltage-gated channels first exceeds the net outward current present at that voltage, producing a self-sustaining, regenerative depolarization that proceeds to a full action potential regardless of whether the initiating stimulus continues, and representing the critical dividing line between a subthreshold, decaying local response and a propagated, all-or-none cardiac impulse.

The Biophysical Definition of Threshold

Balance of Inward and Outward Current

At any given membrane potential, both depolarizing (typically sodium or calcium) and repolarizing (typically potassium) currents are simultaneously active to varying degrees; threshold potential is defined as the voltage at which these opposing currents are momentarily equal, such that any further depolarization, however slight, tips the balance toward net inward current and initiates the regenerative upstroke.

Regenerative Feedback Beyond Threshold

Once membrane potential exceeds threshold, opening of additional voltage-gated channels further depolarizes the membrane, which in turn opens still more channels, a positive feedback loop that proceeds to completion (the full action potential upstroke) independent of the original stimulus, distinguishing threshold crossing from the graded, stimulus-proportional responses seen below threshold.

Threshold in Working Myocardium versus Pacemaker Tissue

Sodium Channel-Dominated Threshold

In working atrial and ventricular myocardium, threshold is determined predominantly by the voltage dependence of fast sodium channel activation, occurring at a membrane potential substantially more negative than the resting potential itself, meaning only a modest depolarizing displacement from rest is needed to trigger the regenerative sodium-driven upstroke.

Calcium Channel-Dominated Threshold in Nodal Tissue

In sinoatrial and atrioventricular nodal tissue, which lacks significant functional fast sodium channel expression, threshold is instead determined by the voltage dependence of L-type (and to a lesser extent T-type) calcium channel activation, occurring at a comparatively less negative voltage and contributing to the characteristically slower, calcium-current-driven upstroke of nodal action potentials.

Factors Shifting Threshold

Sodium Channel Availability and Extracellular Ion Concentration

Because threshold reflects the voltage at which a sufficient fraction of available channels open to overcome outward current, anything that reduces channel availability (partial inactivation from a depolarized resting potential, pharmacological channel blockade) effectively shifts the apparent threshold toward less negative (harder to reach) values, since more channels must open to achieve the same net inward current.

Extracellular Potassium

Modest elevations in extracellular potassium can paradoxically bring resting potential closer to threshold (increasing excitability), while more substantial hyperkalemia depolarizes resting potential enough to inactivate a large fraction of sodium channels, effectively raising threshold and reducing excitability, illustrating a biphasic relationship between potassium concentration and the practical distance between resting and threshold potential.

Threshold and the Strength-Duration Relationship

The Rheobase-Chronaxie Framework

The stimulus current required to bring a cell to threshold depends on the duration of the applied stimulus, described by the strength-duration relationship: rheobase is the minimum stimulus current capable of reaching threshold given unlimited duration, while chronaxie is the stimulus duration required at twice rheobase current, together characterizing the practical excitability of cardiac tissue for a given threshold and membrane time constant.

Clinical Application in Pacing

Artificial cardiac pacing systems are calibrated according to the strength-duration relationship for the specific tissue being paced, selecting a pulse amplitude and duration sufficient to reliably exceed threshold with an appropriate safety margin while minimizing unnecessary energy delivery and battery consumption.

Threshold in the Context of Propagation

Liminal Length

For an action potential to propagate successfully from an initiation site into adjacent resting tissue, a sufficient contiguous area of tissue (the liminal length) must be brought to threshold simultaneously, since the depolarizing current sourced from too small an excited region will be dissipated by the surrounding resting tissue's capacitive and resistive load before threshold can be reached in neighboring cells.

Source-Sink Relationships

Successful propagation therefore depends not only on an individual cell reaching threshold but on the relationship between the depolarizing current "source" provided by already-excited tissue and the current "sink" represented by the capacitance and resistance of the adjacent resting tissue that must be brought to threshold, a relationship central to understanding both normal conduction and the conduction block that can occur at abrupt tissue transitions or scar borders.

Pathological Relevance

Threshold Elevation and Conduction Block

Conditions that elevate threshold—hyperkalemia, sodium channel blocking drugs, ischemia—can produce localized failure of propagation (conduction block) even when the initiating impulse itself remains normal, because the elevated threshold prevents the available depolarizing current from bringing downstream tissue to the now-higher voltage required for regenerative activation.

Threshold and Arrhythmia Vulnerability

Regions of abnormally shifted threshold adjacent to regions of normal threshold create the electrophysiological heterogeneity that underlies both unidirectional conduction block and the reentrant circuits it can initiate, linking the cellular-level concept of threshold potential directly to the tissue-level mechanisms of clinically significant arrhythmia.