Final answer:
Option (C), Substance P reduces the activation threshold of C fibers, which are associated with pain transmission, leading to potentially heightened pain sensitivity.
Step-by-step explanation:
Substance P acts at receptors by reducing the threshold of activation of C fibers. C fibers are involved in the transmission of pain and temperature, and they are unmyelinated, leading to slower signal conduction. When Substance P binds to its receptors on these fibers, it lowers their activation threshold, meaning that less stimulation is required to initiate a pain signal.
This is part of the body's pain response mechanism which can be modulated by various substances. For instance, increased levels of Substance P can lead to heightened pain sensitivity as seen in certain chronic pain conditions.