Rheobase

Key takeaways

Rheobase belongs to biology; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Rheobase to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Rheobase from memory before moving on to harder problems.

Reference excerpt

Rheobase is a measure of membrane potential excitability. In neuroscience, rheobase is the minimal current amplitude of infinite duration that results in the depolarization threshold of the cell membranes being reached, such as an action potential or the contraction of a muscle. In Greek, the root rhe translates to "current or flow", and basi means "bottom or foundation": thus the rheobase is the minimum current that will produce an action potential or muscle contraction. Rheobase can be best understood in the context of the strength-duration relationship (Fig. 1). The ease with which a membrane can be stimulated depends on two variables: the strength of the stimulus, and the duration for which the stimulus is applied. These variables are inversely related: as the strength of the applied current increases, the time required to stimulate the membrane decreases (and vice versa) to maintain a constant effect. Mathematically, rheobase is equivalent to half the current that needs to be applied for the duration of chronaxie, which is a strength-duration time constant that corresponds to the duration of time that elicits a response when the nerve is stimulated at twice rheobasic strength. The strength-duration curve was first discovered by G. Weiss in 1901, but it was not until 1909 that Louis Lapicque coined the term rheobase. Many studies are being conducted in relation to rheobase values and the dynamic changes throughout maturation and between different nerve fibers. In the past strength-duration curves and rheobase determinations were used to assess nerve injury; today, they play a role in clinical identification of many neurological pathologies, including diabetic neuropathy, CIDP, Machado–Joseph disease, and ALS.

Strength-Duration Curve The strength-duration time constant (chronaxie) and rheobase are parameters that describe the strength-duration curve—the curve that relates the intensity of a threshold stimulus to its duration. As the duration of a test stimulus increases, the strength of the current required to activate a single fiber action potential decreases. The strength-duration curve is a plot of the threshold current (I) versus pulse duration (d) required to stimulate excitable tissue. As mentioned, the two important points on the curve are rheobase (b) and chronaxie (c), which correlates to twice the rheobase (2b). Strength-duration curves are useful in studies where the current required is changed when the pulse duration is changed.

Lapicque's Equation In 1907, Louis Lapicque, a French neuroscientist, proposed his exponential equation for the strength-duration curve. His equation for determining current I:

I = b ( 1 + c d ) , {\displaystyle I=b(1+{c \over d}\,),}

where b relates to the rheobase value and c relates to the chronaxie value over duration d. Lapicque's hyperbolic formula combines the threshold amplitude of a stimulus with its duration. This represents the first manageable with physiologically defined parameters that could compare excitability of different tissues, reflecting an urgent need at the turn of the 20th century. Lapicque used constant-current, capacitor-discharge pulses to obtain chronaxie for a wide variety of excitable tissues. Rheobase in the Lapicque equation is the asymptote of the hyperbolic curve at very long durations.

Weiss's Equation In 1901, G. Weiss proposed another linear equation using a charge Q duration curve. The electrical charge Q can be calculated with the following equation:

Q = b ( d + c ) {\displaystyle Q=b(d+c)} or Q = I d , {\displaystyle Q=Id,}

again, where I is the current is measured in amperes multiplied by duration d. b relates to the rheobase value and c relates to the chronaxie value. Rheobase in the Weiss formula is the slope of the graph. The x-intercept of the Weiss equation is equal to b x c, or rheobase times chronaxie. This equation suggests that a graph of threshold stimulus strength versus stimulus duration should show a decay toward zero as stimulus duration is increased, so the stimulus strength required to reach threshold is predicted to increase during more protracted stimulation. The strength-duration curve for a typical nerve membrane is slightly skewed from the predicted graph, in that the curve flattens out in response to repetitive stimulation reaching an asymptote representing rheobase. When the duration of a stimulus is prolonged, charge transfer and membrane potential rise exponentially to a plateau (instead of increasing linearly with time). When rheobase exceeds the strength of the stimulus, stimulation fails to generate action potentials (even with large values of t); thus if the stimulus is too small, the membrane potential never reaches threshold. The disparity between the shape of the strength-duration curve predicted by Weiss's equation and the one actually observed in neural membranes can be attributed to leakage of charge that occurs under physiological conditions, a feature of the electrical resistance of the membrane. Weiss' equation predicts the relationship between stimulus strength and duration for an ideal capacitor with no leakage resistance. Despite this limitation, Weiss’s equation provides the best fit for strength-duration data and indicates that rheobase and time constant (chronaxie) can be measured from the charge duration curve with a very small margin of error. Weiss used rectangular, constant-current pulses and found that threshold charge required for stimulation increased linearly with pulse duration. He also found that stimulus charge, the product of stimulus current and stimulus duration is proportional to rheobase, so that only two stimulus durations are necessary to calculate rheobase.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Rheobase

Start with the simplest possible case. Write down what Rheobase claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Rheobase before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Rheobase ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Rheobase

In research: Rheobase appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Rheobase in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Rheobase is common in secondary-school and first-year university syllabi. It links to neighbouring topics Neurophysiology, so understanding it makes those chapters shorter.
In everyday life: Look for Rheobase outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Rheobase in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Rheobase means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Rheobase out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Rheobase in simple terms?

Rheobase is a measure of membrane potential excitability. In neuroscience, rheobase is the minimal current amplitude of infinite duration that results in the depolarization threshold of the cell membranes being reached, such as an action potential or the contraction of a muscle.

Why does Rheobase matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Rheobase?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Rheobase.