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# 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.

Updated Nov 13, 2026

Also called: Graded potentials, Threshold, Electrotonic spread, Repetitive discharge

## Key points

- Local (graded) potentials: size proportional to stimulus, decremental with distance, can summate in time and space, no refractory period.
- Action potentials: all-or-none, constant amplitude, propagate without decay, followed by a refractory period.
- Threshold is reached when Na⁺ entry exceeds K⁺ exit; it is usually ≈ 15–20 mV above the resting potential in nerve.
- Slow depolarization (e.g. chronic hyperkalemia) inactivates Na⁺ channels: excitability falls even though the cell is closer to threshold (accommodation).
- Rhythmic firing: after each spike K⁺ conductance falls slowly and inward currents bring the membrane back to threshold; the speed of that drift sets the rate.

## 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 mV **Threshold** −55 mV **Peak** +35 mV **Duration** ≈ 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

Neuron action potential: the threshold line marks where local depolarization turns all-or-none. Try high K⁺.

### Action potential

**Maximum diastolic potential** −60 mV **Threshold** −40 mV **Peak** +10 mV **Cycle** ≈ 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

SA node: the slow upward drift between beats is what makes this tissue fire repetitively.

## 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.

## Memory aid

Local potentials whisper and fade; an action potential shouts once and repeats down the line.

## 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.

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## Connections

## Related topics 6

- [The nerve action potential](https://durgical.com/en/physiology/the-nerve-action-potential/)
- [The resting membrane potential](https://durgical.com/en/physiology/the-resting-membrane-potential/)
- [Refractory periods and excitability](https://durgical.com/en/physiology/refractory-periods-and-excitability/)
- [The SA node pacemaker potential](https://durgical.com/en/physiology/the-sa-node-pacemaker-potential/)
- [Synaptic transmission](https://durgical.com/en/physiology/synaptic-transmission/)
- [Sensory receptors, coding and neuronal circuits](https://durgical.com/en/physiology/sensory-receptors-coding-and-neuronal-circuits/)

## Related conditions 2

- [Hyperkalaemia](https://durgical.com/en/conditions/hyperkalaemia/)
- [Hypocalcaemia](https://durgical.com/en/conditions/hypocalcaemia/)

## In Pharmacology 2

- [Local anesthetics](https://durgical.com/en/pharmacology/classes/local-anesthetics/)
- [Lidocaine](https://durgical.com/en/pharmacology/lidocaine/)

[← All of physiology](https://durgical.com/en/physiology/)

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](https://medlineplus.gov/ency/article/001179.htm) (MedlinePlus, US National Library of Medicine, 2025); [Calcium Blood Test: MedlinePlus Medical Test](https://medlineplus.gov/lab-tests/calcium-blood-test/) (MedlinePlus, US National Library of Medicine, 2025); [Epidural Anesthesia](https://www.ncbi.nlm.nih.gov/books/NBK542219/) (StatPearls (NCBI Bookshelf), 2025).

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Representative model, for educational purposes only.

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