Nociception runs on a whole chemical cocktail — substance P is one of the best known players, but it works alongside a network of peptides, lipids, and small molecules that together create, amplify, and transmit pain signals. Here's how they fit together, which connects directly to why some of the "silent" cartilage and meniscus findings from the marathon studies stay silent while bone marrow lesions tend to hurt.
Substance P: The Signal Amplifier
Substance P (SP) is an 11-amino acid neuropeptide released from the peripheral terminals of nociceptive sensory neurons, primarily C-fibers and some A-delta fibers. It acts mainly through the neurokinin-1 receptor (NK1R) and has a distinctive feature: it's only released when nociceptive stimulation is strong enough, meaning it doesn't fire for every minor touch or pressure, only for genuinely noxious input.[pmc.ncbi.nlm.nih][sciencedirect][pmc.ncbi.nlm.nih]
Substance P does double duty. Peripherally, it triggers neurogenic inflammation — it causes local blood vessels to dilate and become leaky (plasma extravasation), and it activates mast cells to degranulate, releasing histamine and other inflammatory mediators that further sensitize the nerve terminal. Centrally, at the spinal cord, it enhances pain transmission by increasing the excitability of second-order neurons, effectively turning up the volume on the pain signal heading to the brain. This dual peripheral-and-central action is part of why substance P is heavily implicated in the transition from acute to chronic pain.[pmc.ncbi.nlm.nih][sciencedirect][sciencedirect][elifesciences]
CGRP: Substance P's Close Partner
Calcitonin gene-related peptide (CGRP) is frequently co-released with substance P from the same sensory neurons and works in a tight feedback loop with it. CGRP is a potent vasodilator and drives much of the neurogenic inflammation at an injury site by activating immune and endothelial cells. It also plays a major role in peripheral sensitization — lowering the activation threshold of nociceptors so they fire more easily and more often. In some models, glutamate release stimulates CGRP release, and CGRP in turn amplifies glutamate signaling, creating a self-reinforcing loop that can sustain pain well beyond the initial stimulus.[elifesciences][pmc.ncbi.nlm.nih][acrjournals.onlinelibrary.wiley][pmc.ncbi.nlm.nih]
The Inflammatory Soup: Prostaglandins, Bradykinin, and Cytokines
Beyond the neuropeptides, a whole set of inflammatory mediators released by damaged tissue and immune cells directly sensitizes nociceptors:
Prostaglandins (especially PGE2) bind G-protein-coupled EP receptors on nerve terminals and change their membrane excitability, lowering the threshold needed to fire. This is the exact pathway NSAIDs target — they block prostaglandin synthesis, which is why they relieve pain[link.springer]
Bradykinin is generated during tissue injury and is one of the most potent direct activators of nociceptors, often triggering immediate, sharp pain[pmc.ncbi.nlm.nih]
Cytokines like IL-6, IL-1β, and TNF-α are released by immune cells during inflammation and sensitize nociceptors, contributing to the more persistent, aching quality of inflammatory pain[pmc.ncbi.nlm.nih][nature]
Nerve growth factor (NGF), released during injury, sensitizes nociceptors and can also promote longer-term changes in nerve excitability[pmc.ncbi.nlm.nih]
Glutamate, the primary excitatory neurotransmitter, contributes to both peripheral sensitization at the injury site and central sensitization in the spinal cord and brain, and it interacts closely with the CGRP feedback loop mentioned above[pmc.ncbi.nlm.nih][jpain]
Why This Matters for the Knee Findings
This biochemistry helps explain the asymptomatic-MRI puzzle from earlier. Nociceptors have to actually be present in a tissue, and inflammatory mediators have to reach a meaningful concentration, before pain gets generated at all. Cartilage has no nerve endings of its own, so it can't participate directly in this cascade until damage progresses far enough to irritate the underlying bone or synovium, which are richly innervated. Bone marrow, by contrast, is well-vascularized and well-innervated, so edema there quickly recruits this whole chemical cascade — prostaglandins, bradykinin, substance P, CGRP — which is exactly why bone marrow lesions tend to produce more immediate, reliable pain than cartilage lesions of similar "severity" on imaging .[pmc.ncbi.nlm.nih]
This is also part of the mechanistic story behind central sensitization: repeated substance P and CGRP release, amplified by the glutamate feedback loop, can lower pain thresholds over time even after the original tissue insult has resolved — which is one reason pain can persist after an MRI finding has normalized, or why some people report ongoing knee complaints without any structural correlate left to explain them.[pmc.ncbi.nlm.nih][pmc.ncbi.nlm.nih]