Synaptic Summation: EPSPs, IPSPs and Neuronal Firing

A neuron combines incoming influences before it fires. This guide develops the summation idea introduced in our resting, graded and action potentials guide, focusing on how timing, location and inhibition affect the outcome.

For undergraduate revision and postgraduate review. Updated 31 October 2026.

Learning objectives

  • Distinguish an EPSP from an IPSP.
  • Compare temporal and spatial summation.
  • Explain why several excitatory inputs may still fail to trigger firing.

1. EPSPs and IPSPs: effects on firing

Postsynaptic potentials are local, graded voltage changes produced in response to synaptic input. An excitatory postsynaptic potential (EPSP) generally depolarises the receiving membrane and increases the likelihood of an action potential. An inhibitory postsynaptic potential (IPSP) reduces that likelihood; a familiar example is hyperpolarisation.

Keep the incoming presynaptic action potential separate from the postsynaptic response. A spike arriving at a nerve terminal does not guarantee that the receiving neuron will fire.

2. Temporal versus spatial summation

Temporal summation: repeated input arrives before the preceding postsynaptic effect has decayed, so the effects overlap.

Spatial summation: inputs at different synapses contribute overlapping effects. They need not arrive at precisely the same instant, but their effects must overlap sufficiently to interact.

Neither term means that action potentials become taller. It is the graded postsynaptic effects that combine.

3. Work through a simplified comparison

Imagine a neuron with an illustrative resting voltage of −70 mV and a firing threshold of −55 mV. Suppose one excitatory input produces a 6 mV depolarisation at the trigger region: the voltage reaches −64 mV and remains below threshold.

  • Input spaced far apart: if the first effect has faded before the next arrives, repeated input may remain subthreshold.
  • Input close together: overlapping effects may reach threshold. Repeated input from the same synapse illustrates temporal summation.
  • Different active synapses: their combined influence may also reach threshold, illustrating spatial summation.
  • Excitation plus inhibition: concurrent inhibition can prevent firing even when excitation alone would be sufficient.

These voltages are teaching values. Do not assume that three 6 mV inputs always produce exactly 18 mV in a real cell. Timing, synapse location, membrane state and changing conductances influence the result.

4. Beyond simple addition: an advanced revision point

Inhibition need not cause a conspicuous downward voltage deflection. Opening inhibitory channels can increase membrane conductance and reduce the depolarisation produced by excitatory input: this is shunting inhibition.

The response also depends on the synaptic reversal potential and the current membrane voltage. Therefore, “depolarising” and “excitatory” are not interchangeable in every physiological situation. For an advanced answer, explain the conductance change and its effect on firing, rather than classifying a synapse solely by the direction of a voltage trace.

5. Common misconceptions

  • Every EPSP produces a spike: an individual EPSP can be subthreshold.
  • Spatial summation requires perfect simultaneity: overlapping effects are what matter.
  • Inhibition always produces a large hyperpolarisation: shunting inhibition can reduce excitation with little change in resting voltage.
  • More inputs mean a larger action potential: input integration affects whether and when firing occurs, not proportional growth of spike amplitude in a given fibre.

6. Revision questions with answers

  1. Why does a long interval weaken temporal summation? The earlier postsynaptic effect has more time to decay.
  2. Two excitatory synapses are active. Must the neuron fire? No. Their combined effect may remain subthreshold, or inhibition may oppose it.
  3. Which comparison distinguishes temporal from spatial summation? Repeated activity at one synapse versus overlapping activity at different synapses.
  4. Why is simple voltage arithmetic only a teaching model? Synaptic responses depend on membrane voltage, conductance and location, so real integration can be nonlinear.

7. Choose a study resource

Try the free Homeostasis sample to explore SelfHug's teaching style on another foundational topic. It is not a synaptic summation sample.

For related electrical-signalling revision, view the Action Potential and Graded Potential PDF chapter or browse Nerve & Muscle chapters. Paid chapters are separate digital purchases; check the listing for its scope and current price.

Return to the physiology study resource hub for the other free guides.

References and further reading

Use your course textbook and practical teaching to match the depth required by your curriculum.