The process that first detects an external stimulus is

The process that first detects an external stimulus is sensitivity.

Sensitivity is the ability of an organism or organ to respond to external stimuli. Sensitivity means having a strong physical reaction to something.

An external stimuli are reactions to the world around perceived through sensory receptors. The receptors receive information from outside the body and produce systemic responses to that information. For instance, someone throws something on you so you flinch, if the sun blends you, you cover your eyes etc. This is the cerebral response to external stimuli.

External stimuli are related to all senses, the senses activate the cerebral response to them.