Resting, Graded & Action Potentials: A Study Guide

Resting membrane potential is the starting electrical state; a graded potential is a variable local change; an action potential is a regenerative electrical signal. Keeping those three ideas separate makes nerve physiology easier to reason through.

This SelfHug study guide uses a simplified neuron model for medical and healthcare learners. Updated 25 October 2026.

1. Resting membrane potential: the starting state

Membrane potential compares the voltage inside a cell with the voltage outside it. A negative value means that the inside is electrically negative relative to the outside. “Resting” describes the baseline state, not an absence of ion movement.

Two ideas work together: ions have different concentrations on the two sides of the membrane, and the membrane is selectively permeable. Potassium permeability has a major influence on the resting voltage in a typical neuron. The sodium–potassium pump helps maintain the concentration gradients on which signalling depends.

Revision check: distinguish a concentration gradient, an electrical gradient and membrane permeability. A channel changes a route for ion movement; a pump uses energy to maintain an ion distribution. They have different roles.

2. Graded potentials: local changes of variable size

A graded potential is a local change from the current membrane voltage. Its size depends on the input. It can make the membrane less negative, called depolarisation, or more negative, called hyperpolarisation.

These signals can combine. Inputs occurring close together in time can produce temporal summation; inputs arising at different locations can contribute to spatial summation. As a local electrical change spreads passively, it decreases with distance.

Ask yourself: is the input bringing the relevant membrane region closer to the conditions needed to fire, or moving it away? A depolarisation does not automatically mean that an action potential has occurred.

3. Action potentials: a regenerative signal

In a typical neuronal axon, sufficient depolarisation can reach threshold and initiate an action potential. Rapid activation of voltage-gated sodium channels increases sodium entry. Sodium-channel inactivation and a delayed increase in potassium conductance help bring the voltage back towards its resting level. Potassium conductance can remain elevated briefly, producing an after-hyperpolarisation.

The signal is regenerated as it travels along the axon. In a given fibre under comparable conditions, crossing threshold does not produce an indefinitely larger spike when the stimulus becomes stronger. Stimulus intensity can instead affect firing frequency and the recruitment of additional fibres.

Refractory behaviour is also important. During the absolute refractory period, many sodium channels remain unavailable for reactivation. Later, during the relative refractory period, a stronger input may be needed to trigger another spike.

Keep the model clear: neurons, skeletal muscle and different cardiac cells have different electrical properties. Do not apply one neuronal waveform or one voltage value to every excitable cell.

4. Resting, graded and action potentials compared

Key distinctions in a simplified neuron model
Question Resting potential Graded potential Action potential
What does it describe? A baseline voltage difference. A local change in voltage. A regenerative electrical event.
Does its size vary with input? Baseline varies with cell conditions. Yes, amplitude is graded. All-or-none initiation in a given fibre.
Can inputs add together? Not an input event itself. Yes: temporal and spatial summation. Individual spikes do not sum in amplitude.
What happens over distance? Describes the resting membrane state. Passive spread decreases in size. Regeneration maintains propagation along the axon.
What should you explain first? Gradients and selective permeability. Input, direction and size of change. Threshold, channel changes and recovery.

5. Work through an example

Imagine a simplified neuron resting at −70 mV, with an illustrative firing threshold of −55 mV. One input changes the voltage at the trigger region to −65 mV. That is a 5 mV depolarisation, but it has not reached the illustrated threshold.

Now imagine several inputs arriving close enough together for their effects to combine. If the resulting depolarisation reaches threshold at the trigger region, an action potential may begin. A hyperpolarising input can oppose that movement towards threshold.

The voltage values are teaching examples, not fixed values for every neuron. In real cells, timing, location, membrane properties and channel availability affect the outcome.

Three questions to answer without looking

  1. Why can a neuron be electrically active even when it is described as resting?
  2. What is the difference between combining graded inputs and increasing the amplitude of an action potential?
  3. Which channel changes explain the rising phase and the return towards resting voltage in the neuronal model?
Check your reasoning

Resting cells maintain ion gradients and allow selective ion movement. Graded inputs can summate; action potentials in a given fibre do not become proportionally taller with each stronger input. Rapid sodium-channel activation supports the rising phase, while sodium-channel inactivation and increased potassium conductance support recovery.

6. Common revision mistakes

  • Memorising voltage numbers without specifying a cell: identify the model before labelling a graph.
  • Calling every depolarisation an action potential: establish whether the event is local and graded or regenerative.
  • Giving the pump and channels the same job: separate gradient maintenance from the rapid conductance changes during a spike.
  • Recognising a diagram but not explaining it: cover the labels and describe each phase in a complete sentence.

7. Choose your next study resource

For a free introduction to a different foundational topic, explore the Homeostasis free sample or browse all free physiology resources.

Books and chapters are separate digital PDF purchases. Read the relevant listing for its scope and current price before choosing.

Sources and further reading

These original study notes draw on the following open educational references. Consult your course materials for the depth and terminology required by your curriculum.