The meaning of a sentence in McCulloch and Pitts' paper

Hello,
I'm translating a book called Models of mind by Grace Lindasy.
and I want to know What does it mean by its adequate stimulus:

McCulloch and Pitts go on to explain how these biological details are congruent with Boolean logic. The core of their claim is that the activity state of each neuron – either firing or not – is like the truth value of a proposition – either true or false.
In their own words, they ‘conceive of the response of any neuron as factually equivalent to a proposition which proposed its adequate stimulus’.
By ‘its adequate stimulus’ they are referring to something about the world.