Rheobase

Updated 2026-05-14

Rheobase is a quantitative parameter in electrophysiology that denotes the minimum amplitude of an electric current of indefinitely long duration (theoretically infinite) required to elicit an action potential in an excitable cell, such as a neuron or muscle fiber. It is a key point on the strength–duration curve, which characterizes the relationship between the intensity (strength) of an electrical stimulus and the time (duration) for which the stimulus must be applied to reach threshold excitability.

Definition

Rheobase (Ir): The lowest constant current intensity that, when applied for a sufficiently long period, reliably initiates an action potential. In practice, “sufficiently long” is often operationalized as a duration that yields a response equal to that produced by an infinitely long stimulus; typical experimental protocols use durations on the order of several hundred milliseconds to approximate this condition.

Historical Background

The concept was introduced by French physiologist Louis Lapicque in the early 20th century (1907‑1909) as part of his work on the electrical excitability of nerves and muscles. Lapicque formulated the strength–duration relationship mathematically, linking rheobase to another parameter, chronaxie, which describes the minimum stimulus duration required to provoke a response when the current is set at twice the rheobase.

Relationship to the Strength–Duration Curve

The strength–duration curve is commonly expressed by Lapicque’s equation:

$$ I = I_r \left(1 + \frac{c}{t}\right) $$

where:

$I$ = stimulus current amplitude,
$I_r$ = rheobase,
$c$ = chronaxie (time constant),
$t$ = stimulus duration.

According to this model, as the stimulus duration $t$ increases, the required current $I$ asymptotically approaches the rheobase value. Conversely, for very brief pulses, substantially higher currents are needed to achieve threshold excitation.

Measurement Techniques

Threshold Tracking: Incrementally adjust current amplitude while delivering long-duration rectangular pulses until an action potential is consistently evoked.
Stimulus-Response Protocols: Apply a series of pulses with varying durations; extrapolate the asymptotic current value from the plotted strength–duration data, yielding an estimate of rheobase.
Electrophysiological Recording: Use intracellular or extracellular electrodes in isolated nerve or muscle preparations, or in vivo nerve stimulation, to determine the minimal excitatory current.

Physiological and Clinical Significance

Neuronal Excitability: Rheobase reflects the intrinsic excitability of a neuron, influenced by membrane properties, ion channel density, and metabolic state.
Comparative Neurobiology: Differences in rheobase values across cell types aid in classifying neuronal subpopulations (e.g., fast‑spiking interneurons vs. regular‑spiking pyramidal cells).
Diagnostic Neurology: Abnormal rheobase values can indicate pathological alterations in nerve excitability, such as demyelination, channelopathies, or peripheral neuropathies.
Therapeutic Stimulation: In clinical neurostimulation (e.g., transcutaneous electrical nerve stimulation, spinal cord stimulation), knowledge of rheobase assists in setting stimulation parameters that are supra‑threshold yet energy‑efficient.

Related Concepts

Chronaxie: The stimulus duration required to elicit an action potential when the current amplitude is set at twice the rheobase.
Threshold: General term for the minimal stimulus (in terms of current, voltage, or charge) needed to generate a response; rheobase specifically refers to the current component at infinite duration.
Strength–Duration Curve: Graphical representation of the relationship between stimulus intensity and duration for reaching threshold.

Limitations and Considerations

Infinite Duration Approximation: True infinite duration cannot be achieved experimentally; rheobase is therefore an extrapolated value based on long‑duration data.
Temperature and Ionic Environment: Rheobase is temperature‑dependent and varies with extracellular ion concentrations, necessitating controlled experimental conditions.
Cellular Heterogeneity: Within a single tissue, variations in membrane conductance and geometry can produce a distribution of rheobase values rather than a single fixed number.