What is the name of the part of the neuron that perceives the external stimulus?

Answer: axon
In the classical reflex arc, pseudo-unipolar neurons play an important role. In these neurons, one axon, as it were, in the shape of the letter T is divided into two parts. One part goes to the central nervous system, and the other to the receptor, where the axon branches into free dendrites. In this receptor part, information is perceived, which is then transmitted along the axon of the pseudo-unipolar neuron to the ganglion (cell body), and then further to the central nervous system.
In other cases, the perception of the stimulus is carried out using dendrites.