Axon hillock

The Axon Hillock is the region from which the axon of a neurone orginates. It is located between the axon and the cell body. The axon hillock normally contains the trigger zone of the neurone. This is the area that must be depolarised to above threshold potential before it initiates the all-or-nothing response of an action potential. 

Graded_potentials passing from the dendrites through the cell body must still be suprathreshold when they reach the axon hillock, in order for an action potential to be fired. If the graded potential is subthreshold at the axon hillock, there will not be a resultant action potential.