The injury is healed. The scar is quiet. The patient is fine with it - doesn't even mention it unless you ask. The imaging is clean. And yet fifteen years later the nervous system is still behaving as though something is wrong. Why?

The answer lives in a single class of spinal neuron. It has a boring name - the wide-dynamic-range neuron, WDR for short - and a job description that explains a lot of chronic, functional pain in one page.

The spinal amplifier

WDR neurons sit in the dorsal horn of the spinal cord. They were described in 1966, and their defining feature is promiscuity: a single WDR neuron receives input from skin, muscle, joint, and viscera - and from across the full afferent spectrum. C fibers carry the slow, diffuse nociceptive signal. Aδ fibers carry sharp, fast pain. Aβ fibers carry ordinary touch. The WDR takes all of them.

That convergence is exactly why old injuries keep mattering. Three properties, working together, let the spinal cord hold a signal long after the tissue has stopped needing defense.

Wide input range

Once a WDR has been driven hard - by a real injury, say - it can be re-driven by Aβ input. Light touch. Shear across a scar. A mechanical load the skin used to ignore. The neuron treats benign input as if it were still the original assault, because it cannot distinguish between reasons for firing, only whether it is firing.

Wind-up

Repeated C-fiber input summates non-linearly. Each pulse leaves the postsynaptic cell a little more excitable than the last one did. This is wind-up. It is mediated by NMDA receptors, and specifically by the removal of the Mg²⁺ block that ordinarily keeps NMDA receptors quiet. Calcium floods in. Gene expression shifts. The neuron's threshold drops. The same input that used to fall below the floor now lands well above it.

Structural plasticity

Over time, the changes stop being metabolic and start being structural. Glial cells release BDNF, which sustains the neuron's excitability. Substance P makes it more responsive still. The dendritic architecture itself rearranges. By the time the patient shows up in your office fifteen years after the original event, the WDR that was once a normal dorsal- horn neuron has become a dedicated amplifier for the old input.

The tissue heals on its own schedule. The spinal cord heals on a different one. Sometimes it doesn't heal at all.

What this looks like in the room

Three tiers of clinical expression, all downstream of the same sensitized neuron.

Locally

The zone around the old injury reads significant - different turgor, different temperature, different palpation quality from the surrounding tissue. Nothing dramatic; just clearly not the same. The patient has stopped noticing.

Regionally

WDR neurons project through segmental pathways that fan out well beyond the original site. A shoulder injury produces a neck that will not release. A pelvic event produces a lumbar that will not stabilize. The CNS holds the guarding; the muscle is not the generator, it is the instrument. You can release it in the room and watch it re-lock by morning.

Systemically

The same dorsal-horn cell projects upward to the hypothalamus, the reticular formation, and the limbic system. The hypothalamus converts chronic afferent drive into HPA-axis output - cortisol, shifted metabolism, drifted sleep architecture. Serotonergic antinociception burns capacity trying to dampen the input; the patient describes the depletion as low mood, flat motivation, thin sleep. Autonomic tone runs warmer than it should.

None of these patients recognize their "old C-section scar" or "that tooth they pulled in college" as the driver. The signal is not routed to awareness. It is routed to the structures that run tone, vigilance, and defense.

How the signal stays loud: a closer look

At the molecular level, three things lock the loop in place:

  • NMDA unblock. High-frequency C input removes the magnesium block on NMDA receptors, letting calcium in. Calcium is the second messenger that drives gene expression.
  • Glial-derived BDNF. Microglia and astrocytes in the dorsal horn release brain-derived neurotrophic factor in response to sustained nociceptive activity. BDNF lowers the firing threshold of the WDR and sustains excitability long after the original input has subsided.
  • Substance P and neurokinins. These neuropeptides amplify the same cell's response to future input, producing a feedback loop where sensitization begets more sensitization.

None of this requires the original tissue to still be damaged. It requires only that the loop was established, and that nothing has intervened to turn it off.

What to do about it

Take the trauma history seriously. Not just recent events - every scar, every extraction, every anesthesia site, every old surgery the patient has stopped thinking about. In a cohort of patients with unexplained chronic complaints, the driver is almost always among them.

Then work at the level of the loop, not the tissue. Local manual therapy - massage, myofascial release, passive stretching - can't reset a sensitized dorsal-horn neuron. The target has to be the afferent input that sustains the loop, and the conditioned defensive reflex the CNS encoded at the moment of the original event.

That is the clinical level P-DTR is designed to work at. The goal is not to mute the signal. The goal is to give the nervous system a reason to turn it off.