Cardiac Excitability Control

Cardiac Excitability Control is the regulation of how readily a cardiac cell can generate a new action potential in response to a stimulus, governed principally by the fraction of voltage-gated sodium and calcium channels available for activation at any given moment, and constituting a distinct physiological variable from automaticity or conduction velocity that determines whether, and how strongly, a given depolarizing stimulus succeeds in triggering a propagated response.

Defining Excitability

The Central Determinant: Channel Availability

Excitability is determined predominantly by the proportion of fast sodium channels (in working myocardium) or L-type calcium channels (in nodal tissue) that have recovered from inactivation and are therefore available to open in response to a depolarizing stimulus, since a cell with few available channels requires either a larger stimulus or fails to generate a propagated response altogether.

Excitability versus Related Concepts

Excitability is conceptually distinct from automaticity, which concerns spontaneous depolarization without external stimulus, and from conduction velocity, which concerns the speed of propagation once excitation has occurred; a tissue can be normally automatic and yet transiently inexcitable (as during the absolute refractory period), or excitable but non-automatic (as in ordinary working myocardium at rest).

The Refractory Period as the Central Expression of Excitability Control

Absolute Refractory Period

Immediately following the upstroke of an action potential, essentially all fast sodium channels are inactivated, rendering the cell completely unexcitable regardless of stimulus strength; this absolute refractory period persists through most of the plateau phase and into early repolarization, corresponding to the interval during which channel inactivation dominates channel behavior.

Relative Refractory Period

As repolarization proceeds further, an increasing fraction of sodium channels recover availability, and the cell enters a relatively refractory period during which a sufficiently strong stimulus can trigger a new action potential, though typically one with reduced upstroke velocity, reduced amplitude, and slower subsequent conduction, reflecting the still-incomplete channel recovery.

Supernormal Excitability

In a narrow window late in repolarization, some cardiac tissue exhibits a brief period of supernormal excitability, in which a stimulus weaker than that required at full recovery can nonetheless trigger a response, attributable to the membrane potential at that moment lying closer to threshold despite incomplete channel recovery, a phenomenon of particular relevance to vulnerability windows for arrhythmia induction.

Determinants of Excitability Beyond the Refractory Period

Resting or Diastolic Membrane Potential

Because sodium channel availability is itself a steady-state function of membrane voltage even outside the immediate post-action-potential period, a chronically depolarized resting or diastolic potential (as produced by hyperkalemia or ischemia) reduces baseline channel availability and therefore reduces excitability even in tissue that has had ample time to "recover" from its previous action potential.

Threshold Potential

Excitability is also governed by the threshold potential itself, the membrane voltage at which regenerative channel opening becomes self-sustaining; factors that shift threshold toward the resting potential (making it easier to reach) increase excitability, while factors that shift threshold away from the resting potential decrease it, independent of any change in channel availability.

Autonomic and Pharmacological Modulation

Autonomic Influence

Sympathetic stimulation modestly increases excitability in some cardiac tissues by shifting relevant channel gating parameters, while its more prominent electrophysiological effects operate through automaticity and conduction rather than excitability per se; parasympathetic stimulation in atrial tissue can shorten the action potential and refractory period through specific potassium current activation, altering the temporal window during which excitability is restored.

Antiarrhythmic Drug Action

Class I antiarrhythmic agents, by blocking sodium channels preferentially in their open or inactivated states, reduce the fraction of channels available for a given degree of recovery time, effectively prolonging the functional refractory period and reducing excitability in a manner exploited to suppress certain reentrant and ectopic arrhythmias, though at the cost of also slowing normal conduction.

Pathological Alterations in Excitability

Ischemic Changes

Myocardial ischemia depolarizes the resting membrane potential (through impaired sodium-potassium ATPase function and extracellular potassium accumulation) and directly impairs sodium channel function, both reducing baseline excitability and creating heterogeneous regions of differentially reduced excitability that can support conduction block and reentry.

The Vulnerable Period and Arrhythmia Induction

Because excitability varies continuously and non-uniformly across the repolarization sequence, a premature stimulus arriving during the relative refractory or supernormal period of some, but not all, neighboring tissue can produce non-uniform, fractionated conduction, a mechanism directly linking excitability control to the genesis of reentrant arrhythmias when a premature impulse encounters spatially heterogeneous excitability across the myocardium.

Excitability as a Therapeutic Target

Beyond antiarrhythmic pharmacology, clinical cardiac pacing and defibrillation both depend directly on manipulating excitability: pacing delivers a stimulus timed and sized to exceed threshold in tissue presumed excitable, while defibrillation delivers a large-amplitude shock intended to simultaneously depolarize enough myocardium to render it transiently and uniformly refractory, terminating chaotic reentrant activity by eliminating the excitability heterogeneity that sustains it.