Axon hillock

The Axon Hillock is the anatomical part of a neuron that connects the cell body called soma (biology) to the axon. It is attributed as the place where Inhibitory Postsynaptic Potentials (IPSPs) and Excitatory Postsynaptic Potentials (EPSPs) from numerous synaptic inputs on the dendrites or cell body accumulate.

It is electrophysiologically equivalent to the 'initial segment where the summated membrane potential reaches the triggering threshold, an action potential propagates through the rest of the axon (and "backwards" towards the dendrites as seen in backpropagation). The triggering is due to positive feedback between highly crowded voltage gated sodium channels, which are present at the critical density at the axon hillock (and nodes of ranvier) but not in the soma.

Cell biologically it is the neuronal equivalent of tight junction as it acts as a barrier for lateral diffusion of transmembrane proteins (and GPI anchored proteins like thy1 across it.)