Summary
Postsynaptic potentials add together over space or time to reach the threshold for firing.
In depth
Temporal summation: rapid repeated inputs add; spatial summation: inputs from different synapses add. The axon hillock integrates the total.
Summation lets neurons act as integrators. The time constant and length constant of the membrane affect summation.
Key idea: neurons fire only when total input reaches threshold.
A single EPSP is about 0.1 to 1 mV, much smaller than the 15 to 20 mV needed to reach threshold, so many inputs must be combined. The length constant (how far potentials spread passively) and time constant (how long they persist) determine summation. Dendrites are not passive: active conductances produce dendritic spikes and nonlinear computations. The axon initial segment, with the highest Na+ channel density, is where spikes are triggered.
Neurons act as integrators, weighing inputs (strength, location, timing). Inhibitory synapses near the soma are most effective. Experiments use patch-clamp recording, introduced by Neher and Sakmann (Nobel Prize 1991).
Example: a neuron receiving many weak synchronous inputs fires, but not the same inputs spread over time.
Frequently asked questions
What is Summation?
Postsynaptic potentials add together over space or time to reach the threshold for firing.
Which area of biology covers Summation, and how advanced is it?
Summation is one of the 54 topics in Neuroscience. It is rated Intermediate, so it suits learners who already know the core vocabulary. Related topics in this area include Voltage-Gated Ion Channels, Hodgkin-Huxley Model, Neuron.
Can you give an example of Summation?
A single EPSP is about 0.5 mV, so many are needed to reach threshold.
What should I learn before Summation?
Work through the Beginner topics in Neuroscience first, such as Nervous System, Neuron, Central and Peripheral Nervous Systems, then return to Summation. Intermediate topics assume the Beginner vocabulary.
What topics are related to Summation?
The closest topics to Summation are Voltage-Gated Ion Channels, Hodgkin-Huxley Model, Neuron, Depolarisation and Repolarisation. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Summation?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Summation. To practise the whole of Neuroscience, use the Neuroscience quiz.
How is Summation different from Voltage-Gated Ion Channels?
Summation: Postsynaptic potentials add together over space or time to reach the threshold for firing. Voltage-Gated Ion Channels: Channel proteins with voltage sensors that open or close to shape action potentials. Read both side by side and note what each one explains.
How should I study Summation step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Voltage-Gated Ion Channels and explain the difference aloud. Finally, take the Neuroscience quiz and come back to this page for any question you miss.