---
title: "Short note: Resting and action potentials"
subject: "Biomedical Instrumentation"
chapter: 2
key: "semester_7/biomedical_instrumentation:short-note-resting-action-potentials"
source: https://www.bctengineer.com/q/semester_7/biomedical_instrumentation/short-note-resting-action-potentials
site: BCT Engineering
---

# Short note: Resting and action potentials

Biomedical Instrumentation · Chapter 2: Bioelectric Signals and Electrodes

- Asked in: 2075 Chaitra, 4 marks
- Video: [Resting & Action Potentials](https://www.youtube.com/watch?v=IZsqCAVNBWY) — Medical Electronics & Biomedical Instrumentation
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/biomedical_instrumentation/short-note-resting-action-potentials
- Whole chapter: https://www.bctengineer.com/md/semester_7/biomedical_instrumentation/chapter-2

## Question

Write a short note on Resting and Action Potentials.

## Short answer

Resting potential is the membrane potential at rest (−70 mV nerve, −90 mV muscle) caused by K+ leak and Na+/K+ pump maintaining ion gradients. Action potential is an all-or-none response to suprathreshold stimulus: rapid Na+ influx depolarizes to +30 mV, then K+ efflux repolarizes, followed by hyperpolarization. Duration is 1-2 ms in nerves; refractory periods prevent immediate re-excitation. Action potentials propagate without decay and underlie ECG, EEG, EMG signals.

## Answer

#### Resting potential
The electric potential difference across the membrane of a nerve or muscle cell when it is not excited. The inside is negative with respect to the outside: about **−70 mV in nerve** and **−90 mV in skeletal muscle**.

Cause: unequal concentrations of ions. K⁺ is high inside and Na⁺ is high outside; the membrane at rest is more permeable to K⁺ than Na⁺ (about 50 to 100 times). K⁺ leaks out, leaving negative charge inside. The Na⁺–K⁺ pump (3 Na⁺ out, 2 K⁺ in) maintains the gradients. For a single ion the potential is given by the Nernst equation, E = (RT/zF) ln(Co/Ci), e.g. EK ≈ −90 mV; for several ions, by the Goldman equation. The cell is said to be **polarised**.

#### Action potential
A brief, rapid change of membrane potential when a stimulus above the threshold (about −55 mV) is applied. It is an all-or-none response.
```text
 mV
 +40 |        /\
   0 |       /  \
 -55 |------/----\      threshold
 -70 |_____/      \     ___ resting
 -80 |              \__/  after-hyperpolarisation
     +-------------------------> t (ms)
        depolar. repolar.
```
Phases:
1. **Depolarisation:** voltage-gated Na⁺ channels open, Na⁺ rushes in, potential rises to about +30 to +40 mV.
2. **Repolarisation:** Na⁺ channels close, K⁺ channels open, K⁺ leaves, and the potential returns to rest.
3. **Hyperpolarisation:** K⁺ channels close slowly, so the potential dips below rest, and then returns to normal.

The duration is about 1 to 2 ms in nerve (about 200 to 300 ms in cardiac muscle). In the **absolute refractory period** no new action potential can be produced; in the relative refractory period only a stronger stimulus can. The action potential travels along the fibre at 1 to 100 m/s without decay, and is the source of ECG, EEG and EMG signals.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why is the resting membrane potential of a nerve cell negative inside?
   A. Na+ is far more permeable than K+ at rest
   B. K+ leaks out through a membrane much more permeable to K+ than Na+, leaving negative charge inside
   C. The cell has no ion pumps
   D. Cl- is actively pumped in
   Answer: B. At rest the membrane is about 50 to 100 times more permeable to K+; K+ leaks out, so the inside becomes negative. (Key point: Resting potential cause)

2. What is the role of the Na+-K+ pump?
   A. It removes the threshold requirement
   B. It moves 3 Na+ out and 2 K+ in to maintain the ion gradients
   C. It opens voltage-gated Na+ channels
   D. It causes depolarisation directly
   Answer: B. The pump (3 Na+ out, 2 K+ in) maintains the concentration gradients needed for the resting potential. (Key point: Na-K pump)

3. Which sequence of events describes an action potential?
   A. K+ rushes in, then Na+ leaves, then a dip below rest
   B. Na+ channels open and Na+ enters (depolarisation), then K+ leaves (repolarisation), then a dip below rest (hyperpolarisation)
   C. Na+ leaves, then Na+ enters, then K+ enters
   D. Only a slow rise to threshold and no return
   Answer: B. Depolarisation is by Na+ influx, repolarisation by K+ efflux, followed by after-hyperpolarisation. (Key point: Action potential phases)