Action potential is initiated at the Axon Hillock - Initial segment of the neuron because

The axon hillock contains the highest density of Na+-VGCs.....Hence it is a low threshold trigger zone. Synaptic input located close to the trigger zone is more effective in depolarizing and hyperpolarizing it than synaptic inputs of equal strength located fuher away. Ref guyton and hall textbook of medical physiology 12/e pg 543 and 59