The Nervous System · Part Four · How It Fails and Recovers
Lesson 55 / 61
Pain: Chronic and Neuropathic
Pain that outlives its injury, written into the nervous system as a learned state rather than a passing alarm.
Central sensitization is the engine of chronic and neuropathic pain, the process by which posterior horn neurons enlarge their receptive fields and amplify their response until ordinary touch is read as threat. Where acute nociception reports tissue damage, chronic pain reflects a remodeled circuit, a persistent shift in the central integrative state of the cord and cortex. The Gate Control Theory of Melzack and Wall first framed pain as something the nervous system actively constructs rather than passively receives.
Gate Control Theory
Melzack and Wall, 1965
Sensitization signs
Hyperalgesia and allodynia
Central receptor
NMDA glutamate receptor
Gating site
Substantia gelatinosa, lamina II
01THE GATE
A Spinal Negotiation
The Gate Control Theory, proposed by Melzack and Wall in 1965, locates the first checkpoint of pain in the substantia gelatinosa of the spinal dorsal horn. A presynaptic gate sits between primary afferents and projection neurons, set by the balance between large myelinated A fibers and thin unmyelinated C fibers. When C fiber input outweighs the A fibers, the gate opens and nociceptive traffic ascends through the anterolateral system toward the thalamus.
This was a radical reframing. Pain was no longer a passive wire carrying a fixed signal but an interpreted event, gated at the cord and tuned from above. Melzack and Wall further proposed cortical control centers that would later explain how attention, memory, and fear bias the gate. The lesson is that the spinal cord is already a decision, not a relay, and that decision is where chronic pain begins to take hold.
02SENSITIZED
Lowering The Threshold
Sensitization is the lowering of a nociceptor's threshold so that ordinary stimuli now provoke firing. Peripheral sensitization arises at inflamed tissue through TRP channel changes driven by bradykinin, prostaglandins, and capsaicin sensitive mediators. Central sensitization follows, marked by an enlarged receptive field of posterior horn neurons that recruits skin areas never injured, and by an amplified, prolonged response to a steady input. The cord shifts from one functional level to another, from normal to sensitized.
Two clinical signatures announce this state. Hyperalgesia is an overreaction to a stimulus that is already painful, while allodynia is pain produced by a stimulus that should be harmless, a brush of cloth or a cool breeze. Both reflect a nervous system that has rewritten its own gain. This is a problem of the central integrative state, where the resting bias of the circuit, not the periphery, dictates what reaches awareness as suffering.
03PLASTICITY
Memory In The Cord
Long term potentiation, the same synaptic strengthening that underlies learning, is well documented between C fiber terminals and spinal projection neurons. Presynaptically transmitter release climbs, and postsynaptically NMDA receptor activation drives the insertion and modulation of AMPA receptors, raising calcium, triggering second messengers, and regulating genes. The synapse grows structurally stronger. Pain, in other words, can be learned and stored much as a memory is.
Remodeling does not stop at the cord. Activity dependent spine changes appear in cortical neurons of the insular cortex, the primary somatosensory cortex S1, and the anterior cingulate cortex. Most authors attribute this remodeling to a loss of normal input or a gain of ectopic, aberrant input. The result is a chronic pain state that lives in distributed circuitry, a network signature of neuroplasticity gone protective to a fault, where the map of the body has been redrawn around the injury.
04NEUROPATHIC
When The Wire Misfires
Neuropathic pain arises from injury or dysfunction of the nervous system itself rather than from healthy tissue under threat. Its causes include compression and tractioning, diabetes, and inflammatory conditions, and its hallmark is spontaneous pain, burning pain, and hyperalgesia in the distribution of the damaged nerve. The lesion generates pain without a noxious source, a signal manufactured by the conductor.
The mechanism is layered. Nerve injury activates microglia and astrocytes in the dorsal horn, which release inflammatory substances, while NMDA receptor activation and the nuclear factor kappa B cascade reshape transcription. At the injury site sodium channels upregulate and potassium channels fall, generating ectopic discharge, and sympathetic neuronal sprouting at the dorsal root ganglion can produce sympathetically maintained pain. A damaged nerve becomes a generator, firing on its own clock, indifferent to the body it was built to protect.
Pain is not simply the passive registration of injury but an output of the nervous system, constructed and modulated at every level from the dorsal horn to the cortex.
05SYNDROMES
Pain Without A Lesion
Fibromyalgia and complex regional pain syndrome, or CRPS, are the clearest expressions of pain decoupled from ongoing tissue damage. Fibromyalgia presents as widespread pain whose management, under current EULAR informed flowcharts, blends pharmacologic and non pharmacologic approaches from the outset. CRPS pairs pain with autonomic and trophic disturbance, a regional dysregulation that points away from a single peripheral cause toward central amplification.
Against this amplification the nervous system holds its own brakes. Descending fibers from the raphe nuclei and locus ceruleus act on second order posterior horn neurons to block forward transmission, and endogenous opioids, the endorphins, dynorphins, and enkephalins, are released from the periaqueductal grey. Electrotherapy borrows this logic, using interferential current near 100 Hz to recruit large diameter A fibers and close the gate. The aim of education here is to map the system, not to prescribe, because understanding the circuit is the beginning of respecting it.