Synaptic Integration: How Neurons Add Up Their Inputs

EPSPs and IPSPs, temporal and spatial summation, shunting inhibition and dendritic spikes: how a single neuron combines thousands of inputs into one decision.

A typical cortical pyramidal neuron receives several thousand synaptic inputs — some estimates run to around ten thousand. Yet it has only one output: to fire, or not, at each moment. Synaptic integration is how it decides.

Small nudges up and down

An excitatory synapse produces a small, brief depolarisation in the receiving neuron — an excitatory postsynaptic potential (EPSP), often under a millivolt. An inhibitory synapse produces an inhibitory postsynaptic potential (IPSP), pushing the voltage away from threshold. A single EPSP is almost never enough to trigger a spike.

Adding up over time and space

Temporal summation: if the same synapse fires again before its first EPSP has faded, the second adds on top of the first. Spatial summation: EPSPs from different synapses arriving at the same time add together as they spread toward the cell body. IPSPs subtract. The result is sampled at the axon initial segment, where sodium channels are densest and threshold is lowest.

Location matters

Dendrites are leaky cables, so an EPSP far out on a thin branch shrinks as it travels to the cell body. Inputs close to the soma tend to count more. Inhibitory synapses placed right on the cell body or the axon initial segment — such as those of basket and chandelier interneurons — act like vetoes. Shunting inhibition opens channels that don't change the voltage much but "short-circuit" nearby excitatory currents, dividing rather than subtracting.

Dendrites that compute

Dendrites aren't passive. Many contain voltage-gated channels and can produce local dendritic spikes when enough inputs cluster together on one branch, making the branch act like a mini-processor. Some researchers have argued a single cortical neuron is computationally comparable to a small multi-layer artificial network. Action potentials can also travel backwards into dendrites, signalling to synapses that the neuron just fired — useful for learning rules.