Answer: The cerebral córtex.
Explanation: The cerebral córtex is formed by the grey substance, wich contains the neuron brain body. It's a thin layer, the most external part of the brain, having a 1 a 4 milimiters of density.
The córtex is where happens the most sophisticated neural process on the brain, responsible for the reflex of the human body.
The reflex is an automatic body reaction to stimulation. Reflex or responding behaviors are stimulus-response (subject-environment) interactions, unconditioned, constant and imperceptible to human eyes and time, due to how fast they happen.
The reflex is an involuntary fast answer of the brain, provided by an external inducement, which helps us to protect or adapt ourselves for the environment wich we are insert.