Primary afferent fibers secretes which nociceptive substance at dorsal horn?
**Core Concept**
Primary afferent fibers play a crucial role in transmitting pain signals from the periphery to the central nervous system. These fibers are responsible for releasing various nociceptive substances at the dorsal horn of the spinal cord, which facilitate the transmission of pain signals.
**Why the Correct Answer is Right**
The primary afferent fibers release a variety of nociceptive substances, including substance P, calcitonin gene-related peptide (CGRP), and bradykinin. Among these, substance P is a key mediator of pain transmission, acting on the NK1 receptors in the dorsal horn to facilitate the activation of second-order neurons. Substance P is released from the terminals of primary afferent fibers in response to noxious stimuli, leading to the activation of pain pathways.
**Why Each Wrong Option is Incorrect**
**Option A:** This option might be tempting, but it is not the correct answer. While serotonin is involved in pain modulation, it is not the primary nociceptive substance released by primary afferent fibers at the dorsal horn.
**Option B:** This option is incorrect because CGRP is indeed a nociceptive substance, but it is not the primary substance released by primary afferent fibers at the dorsal horn. CGRP is more involved in the transmission of pain at the peripheral level.
**Option D:** This option is incorrect because bradykinin is a nociceptive substance, but it is not the primary substance released by primary afferent fibers at the dorsal horn. Bradykinin is more involved in the transmission of pain at the peripheral level.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the release of substance P from primary afferent fibers at the dorsal horn is a key step in the transmission of pain signals. This process can be modulated by various factors, including opioids, which act on the μ-opioid receptors to inhibit the release of substance P.
**Correct Answer:** C. Substance P.