Not everything that affects, modifies or refines a noun is an adjective, as BillJ mentions in the comments. Some nouns can take arguments, as a verb does, and these can be called complements. It's not that they are changing the meaning of the noun, as an adjective does, but they are completing it (hence 'complement').
Decline, on its own, raises the question "decline in what?". In this case, that question is answered with "response to a specific stimulus". Likewise, response prompts the question "response to what?" - and that question is answered with "a specific stimulus". Specific, on the other hand, is simply an adjective modifying stimulus. Thus, stimulus is a noun modified with an adjective to form the noun phrase specific stimulus. Response is a noun, completed by the noun phrase a specific stimulus (linked with to), and decline is a noun, completed by the noun phrase response to a specific stimulus (linked with in).