Simulations Menu

Action Potential Simulations

An Exploration of Neuron Membrane Physiology

Introduction Phases Summation All-or-None Refractory Periods

Summation of Graded Potentials

Introduction

Summation occurs when graded potentials combine at the axon hillock of a postsynaptic neuron. Graded potentials are small, local changes in membrane potential that occur in response to stimuli. In the body, these stimuli are often produced by neurotransmitters released from the synaptic end bulbs of presynaptic neurons. The combined effect of all incoming graded potentials determines whether the membrane reaches threshold and generates an action potential.

Excitatory neurotransmitters produce excitatory graded responses called EPSPs, or excitatory postsynaptic potentials. EPSPs depolarize the membrane, making the neuron’s interior less negative. In the lab, stimulating electrodes are used to produce EPSP-like graded depolarizations.

Most graded depolarizations are too small on their own to reach threshold. However, EPSPs can be summed either over time (temporal summation) or from multiple inputs (spatial summation). Their combined effect may bring the membrane potential to the threshold for an action potential, approximately −55 mV.

Temporal Summation

Temporal summation occurs when repeated excitatory signals from a single location arrive at the axon hillock close together in time. This can happen when a single presynaptic neuron fires rapidly. Each EPSP arrives before the previous EPSP has fully decayed. As a result, the depolarizations accumulate over time, producing a stepwise rise in membrane potential.

In this simulation, repeated pulses from a single stimulating electrode are used to model temporal summation. These pulses produce EPSP-like excitatory potentials that spread toward the axon hillock. Slow, medium, and fast stimulus frequencies allow you to compare how timing affects the amount of summation and whether threshold is reached.

Temporal summation in the human body diagram
Temporal summation in the laboratory diagram

Spatial Summation

Spatial summation occurs when excitatory signals from different locations arrive nearly simultaneously at the axon hillock. This can happen when several presynaptic neurons release neurotransmitter onto different regions of a postsynaptic neuron, including its dendrites and soma.

Each input produces a graded depolarization. These depolarizations spread toward the axon hillock, where they combine. If their summed effect reaches threshold, an action potential is generated.

In this simulation, multiple stimulating electrodes are used to model excitatory inputs arriving from different locations on the neuron. This allows you to observe how combined inputs can increase the membrane potential and determine whether threshold is reached.

Spatial summation in the human body diagram
Spatial summation in the laboratory diagram

Temporal Summation

Use this simulation to create content.

Link to Simulation Terms of Use

Spatial Summation

Use this simulation to create content.

Link to Simulation Terms of Use
Analysis / Quiz
A graded potential is a small (less than threshold), local change in membrane potential that occurs in response to a stimulus.
An EPSP is a small depolarization of the postsynaptic membrane produced by input from a presynaptic neuron. It is generated at the dendrites or soma and spreads toward the axon hillock..
Stimulating electrodes were used to produce EPSP-like depolarizations.
Temporal summation occurs when a single presynaptic neuron or stimulating electrode fires repeatedly and rapidly. Each new signal arrives before the previous EPSP or EPSP-like depolarization has fully decayed. The depolarizations accumulate over time, producing a stepwise rise in membrane potential.
Decreasing the stimulus burst interval allowed less decay of the membrane potential before the next stimulus arrived, so the summed peaks became higher.
The Fast interval delivered pulses close enough together that each new graded potential arrived before the previous one had decayed. The accumulated depolarization reached −55 mV, crossing the threshold needed to open voltage-gated Na⁺ channels and fire an action potential.
Spatial summation occurs when multiple presynaptic neurons or stimulating electrodes fire simultaneously, creating a compounded depolarizing graded potential that travels to the axon hillock of the stimulated neuron. It is called “spatial” because different physical locations (soma and dendrites) of the affected neuron are stimulated.
Each electrode produced its own depolarizing graded potential, raising the summed peak at the axon hillock. With one electrode, the peak was subthreshold. With two electrodes ,the peak was higher but still subthreshold. With three electrodes ,the combined peaks reached threshold, triggering an action potential.

Terms of Use

Human Bio Media materials are open source and can be adapted and shared by anyone under the Creative Commons Attribution 4.0 License.

creative commons

If you are redistributing Human Bio Media materials in print or digital formats, you should include on every page the following attribution:

Access for free at https://humanbiomedia.org/

Reference and Attributions

BioPhysiology – “The Action Potential“.

Cellular Neurophysiology – “Synaptic Transmission in the Central Nervous System“.

Integrated Human Anatomy and Physiology – “The Action Potential“.

Khan Academy – “Neuron action potentials“.

Michigan State U. – “Action Potentials“.

Monash University – “Action Potential Experiments.“

Georgia Tech Biological Sciences – “Neurons.”

OpenStax, Anatomy and Physiology – “The Action Potential“. Access for free at – https://openstax.org/books/anatomy-and-physiology/pages/1-introduction

PhysioWeb – “Neuronal Action Potential“.

ResearchGate – “Mathematical modeling of neurovascular coupling“.

ResearchGate – “Onset Dynamics of Action Potentials in Rat Neocortical Neurons and Identified Snail Neurons: Quantification of the Difference“.

ScienceDirect – “Action Potential“.

ScienceDirect – “Spatial Summation“.

ScienceDirect – “Threshold Potential“.

UEN Integrated Human Anatomy and Physiology – “The Action Potential“

University of Sussex – ” Neuronal Transmission“.

Wikipedia – “Hodgkin–Huxley model“.

Wikipedia – “The Action Potential“.

Wikipedia – “Summation (neurophysiology)“.