Most neurons receive many synaptic inputs, and a single postsynaptic potential is often too small to trigger an action potential by itself. The neuron therefore integrates voltage changes from multiple synapses over space and time.

Temporal summation

If one synapse is activated repeatedly before the previous postsynaptic potential has fully decayed, the voltage changes can overlap.

EPSP 1      EPSP 2      EPSP 3
   \          \          \
    \____      \____      \____
         \______\__________\___ combined depolarization

This is temporal summation: inputs close together in time combine because membrane voltage does not return instantly to baseline.

Spatial summation

Inputs from different synapses can also arrive at overlapping times. Their effects spread through the dendrites and soma and combine. This is spatial summation.

Excitatory and inhibitory inputs can reinforce or oppose one another.

Distance and location matter

Postsynaptic potentials are local, graded voltage changes rather than regenerated action potentials. As they spread passively through dendrites and soma, their amplitude generally decreases.

A synapse close to the axon initial segment can therefore influence spike initiation differently from an otherwise identical synapse farther away. Dendritic geometry and membrane conductances also affect how strongly an input reaches the spike-initiation region.

From graded inputs to an all-or-none output

Synaptic inputs alter the membrane voltage near the axon initial segment. If the combined effect reaches the threshold region, voltage-gated positive feedback initiates an action potential.

Thus neuronal signaling involves two distinct electrical regimes:

graded postsynaptic potentials
          ↓ spatial + temporal integration
threshold at spike-initiation region
          ↓
all-or-none action potential

A neuron therefore does not simply count excitatory and inhibitory synapses. It integrates their strength, timing, location and electrical effects.