"If that is how you'd define the mechanism whereby an animal senses and can respond to the stimulation, how is it different in a human?"
It isn’t different in a human machine. It is the same. But the human machine is merely what we occupy. You and I are not the body we occupy any more than we are the car we occupy when we drive. Our feelings of pain are merely via our connection to this body, and our interpretation of the stimulus originating at our nerve endings. And this does not even get into our feelings when shunned by a group, etc. when nothing has been physically done to us. Animals do not contain a human spirit. They are basically automatons.
I think of the human machine as a fighter plane with a pilot, and animals as drone aircraft. What makes the fighter more important to protect is not really the fact that it is more expensive to produce. What makes it more valuable is that it contains a human pilot. What makes a human body more valuable is that it contains a human soul. Once the soul leaves, the body is just so much meat, like a car at a junk yard.
Animals are around for various purposes on this world. Meat, companionship as pets, workers as draft animals or protection as guard dogs etc, or just fitting in as parts of God's grand plan.
Animals don't have souls, but they DO experience physical pain and suffering just as we do. To cause an animal to suffer needlessly is cruel. Recognition of that cruelty is what makes us different from animals. A cat will toy with a mouse and torture it without any thought of how the mouse feels about the situation or implications of "right and wrong".
Many animals DO also show some evidence of limited emotions. A dog will show all the signs of being lonely or sad when he's left alone, joy when you return, fearfulness, remorse when he knows he's been misbehaving, and even a sense of humor at times.
If your experience with the animal world is limited to chickens, you just might have an incomplete view of the world from which to base your opinions on what animals are capable or incapable of experiencing.