Cable Properties and Dendritic Integration

Synaptic inputs change voltage locally in dendrites. The trigger zone, usually the axon initial segment, is where spikes commonly begin. In a uniform passive-cable model, a graded voltage change gets smaller as it spreads. Current leaks across the membrane and meets axial resistance inside the cell. Equal local voltage changes farther away have a smaller effect at the trigger zone in this model. A regenerated action potential does not fade this way.

A wider cable has lower axial resistance per unit length. With other membrane properties comparable, a fixed local voltage change can spread farther. Higher membrane resistance also reduces leakage. Myelin surrounds axons, raises membrane resistance and lowers capacitance, so less charge is needed for a voltage change. Passive current spreads between nodes, where spikes regenerate. Ordinary dendrites are not myelinated.

Spatial summation combines effects from different synapses. Temporal summation combines repeated inputs from one synapse before earlier effects fade. Overlapping excitatory effects may reach threshold together even when each alone is too small. Inhibitory inputs can oppose depolarisation or reduce excitation by increasing membrane conductance.

Reaching threshold can start a spike if the trigger-zone membrane is able to respond. During the absolute refractory period, sodium-channel inactivation prevents another spike there. Real dendrites can contain active channels that amplify or generate local signals. This can change the effect of distant inputs, so the passive-cable comparison is not a rule for every neuron.