Recording membrane potentials, local potentials and repetitive firing
- Mechanism
- Exam priority: medium
Small stimuli make small local voltage changes that fade with distance; only when they reach threshold does an all-or-none action potential fire.
Also called: Graded potentials, Threshold, Electrotonic spread, Repetitive discharge
What it is
Membrane potential is measured with a microelectrode inside the cell against a reference outside, and the waveform is displayed against time. A weak stimulus produces a graded local potential that decays over a few millimetres; when depolarization reaches threshold, enough Na⁺ channels open to start a regenerating, all-or-none action potential. Some cells drift back to threshold on their own and fire repeatedly: this is the basis of pacemakers in heart, gut and some neurons.
How it works, step by step
- A stimulus (current, transmitter or receptor stretch) opens a few channels and depolarizes a small patch of membrane.
- The charge spreads passively along the membrane and decays with distance and time (set by membrane resistance and capacitance).
- If depolarization stays below threshold, K⁺ efflux returns the membrane to rest: a local, subthreshold response.
- Nearby or rapid stimuli add together (spatial and temporal summation) and can reach threshold.
- At threshold, Na⁺ entry depolarizes further, which opens more Na⁺ channels: a positive feedback that fires the action potential.
- In pacemaker cells resting K⁺ conductance is low and inward currents (e.g. HCN or Ca²⁺) slowly depolarize the membrane after each spike.
- When threshold is reached again, another spike fires; hyperpolarizing currents or higher K⁺ conductance slow the rhythm.
Action potential
Rest −70 mVThreshold −55 mVPeak +35 mVDuration ≈ 1–2 ms
Tap a phase (the pills above or its band) or a refractory period (ARP, RRP) at the base of the plot.
- Mainly Na⁺
- Mainly K⁺
- Rest
- Threshold
- ARP: absolute refractory period
- RRP: relative refractory period
Action potential
Maximum diastolic potential −60 mVThreshold −40 mVPeak +10 mVCycle ≈ 700 ms (≈ 85 bpm)
Tap a phase (the pills above or its band) or a refractory period (ARP, RRP) at the base of the plot.
- Mainly Na⁺
- Mainly K⁺
- Mainly Ca²⁺
- Maximum diastolic potential
- Threshold
- ARP: absolute refractory period
- RRP: relative refractory period
Typical values (large myelinated nerve fiber)
| Parameter | Value | Note |
|---|---|---|
| Resting potential | ≈ −70 to −90 mV | |
| Threshold | ≈ −55 to −65 mV | ≈ 15–20 mV above rest |
| Action potential peak | ≈ +30 to +40 mV | |
| Spike duration | ≈ 0.5–1 ms | |
| Maximal firing rate | up to several hundred per second | limited by the refractory period |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
Regulation
- +Low extracellular Ca²⁺: Lowers the threshold of Na⁺ channels: nerves fire spontaneously (tingling, tetany).
- +Mild acute rise in extracellular K⁺: Depolarizes the resting membrane toward threshold.
- −Sustained depolarization (severe hyperkalemia): Na⁺ channels inactivate: weakness, slowed conduction and arrhythmia risk.
- −High extracellular Ca²⁺: Raises threshold: reduced excitability, muscle weakness.
- −Local anesthetics: Block Na⁺ channels from inside, so local potentials cannot reach threshold and conduction stops.
Local potential versus action potential
| Property | Local potential | Action potential |
|---|---|---|
| Amplitude | Graded with stimulus | All-or-none, fixed |
| Spread | Decays with distance (mm) | Regenerates along the axon |
| Summation | Yes | No |
| Refractory period | None | Absolute then relative |
| Examples | EPSP, IPSP, end-plate potential, receptor potential | Nerve impulse, skeletal muscle spike |
Clinical notes
- Hypocalcemia: Chvostek and Trousseau signs and tetany, because low Ca²⁺ makes nerve Na⁺ channels open at less depolarization.
- Local anesthetics block small fibres (pain, autonomic) first; their effect is stronger in rapidly firing nerves (use-dependent block).
- Severe hyperkalemia first raises excitability, then depresses it as Na⁺ channels inactivate: peaked T waves, widened QRS and risk of arrest.
Test yourself
Think of the answer, then tap the question to see it.
Why can two subthreshold stimuli together fire an action potential when neither does alone?
Local potentials are graded and summate; if they arrive close enough in time or space, their depolarizations add and reach threshold.
Why does hypocalcemia cause tetany?
Low extracellular Ca²⁺ lowers the voltage at which Na⁺ channels open, so nerves reach threshold with little or no stimulus and fire repetitively.
Why does a slowly rising depolarization sometimes fail to fire a nerve?
Na⁺ channels inactivate while it rises (accommodation), so not enough remain available to start the regenerative upstroke.
What property lets a cell discharge repeatedly without any outside stimulus?
Its membrane drifts back to threshold after each spike, because of low resting K⁺ conductance and inward pacemaker currents.
Connections
In Pharmacology2
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: High potassium level: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2025); Calcium Blood Test: MedlinePlus Medical Test (MedlinePlus, US National Library of Medicine, 2025); Epidural Anesthesia (StatPearls (NCBI Bookshelf), 2025).
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Representative model, for educational purposes only.