There is a curious anomaly hiding in the standard account of myelin.
The received wisdom is straightforward: myelination speeds up signal transmission. The myelin sheath, produced by oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system, enables saltatory conduction, the electrical impulse jumping between Nodes of Ranvier rather than crawling continuously along the axon membrane. The result is dramatically faster signal propagation. A myelinated axon can conduct at up to 120 metres per second. An unmyelinated axon conducts at somewhere between 0.5 and 2 metres per second.
Given this, one might expect that pain signals, among the most urgent communications the nervous system carries, would travel along the fastest, most heavily myelinated pathways available. Evolution, after all, tends to prioritise survival. And what is more survival-critical than the rapid detection of tissue damage?
Yet the opposite is true.
The primary nociceptive fibres, the axons that carry raw pain signals from the periphery, are either thinly myelinated (A-delta fibres) or entirely unmyelinated (C fibres). A-delta fibres conduct at around 5 to 30 metres per second. C fibres conduct at less than 2 metres per second. The sharp, immediate pain you feel when you touch a hot surface is carried by A-delta fibres. The slow, throbbing, aching pain that follows is carried by C fibres, unmyelinated, crawling.
This is not a minor quirk. It is a structural feature of the vertebrate nervous system, conserved across species, that directly contradicts the logic that myelination exists to accelerate urgent signals.
The standard explanation, and its limits
The conventional response to this paradox invokes metabolic economy. Myelination is metabolically expensive. The oligodendrocyte that produces the myelin sheath is itself a remarkable cell, a single oligodendrocyte can myelinate up to around 40-60 axon segments simultaneously, each requiring continuous metabolic investment to maintain. The nervous system, so the argument goes, reserves this investment for pathways where speed is most critical: motor control, proprioception, the rapid reflexes that require precise timing.
Pain, by contrast, does not require millisecond precision. A few hundred milliseconds of delay between tissue damage and conscious perception makes little practical difference to survival.
This explanation is partially convincing. But it leaves something unexplained.
If the function of myelin were purely to accelerate signal transmission, we would expect the most urgent signals to be the most heavily myelinated. Pain signals are urgent. They are not heavily myelinated. The standard account treats this as a trade-off, a compromise between speed and metabolic cost. But trade-offs of this kind usually leave traces: intermediate solutions, partial myelination, variable fibre diameters calibrated to urgency. What we find instead is a clean categorical distinction. Pain travels slowly. Precision movement travels fast.
A different question
The Myelin Mind thesis suggests a different way of framing this anomaly, one that follows from taking seriously what myelin may actually do.
If myelin is not merely insulation that happens to speed up conduction, but rather the biological substrate through which experience becomes structured, patterned, and personal, the accumulated condition of a lived nervous system, then the question changes. It is no longer: why does pain travel slowly when speed would be advantageous? It becomes: what kind of experience does an unmyelinated pathway produce, and what kind does a myelinated pathway produce?
The answer, on this view, is not about velocity at all. It is about the nature of the signal’s relationship to the accumulated history of the organism.
Myelinated pathways carry signals that have been, in some sense, domesticated by experience. The proprioceptive system knows where the body is because it has learned, through years of myelination, the characteristic patterns of bodily position. Motor pathways fire with precision because the oligodendrocyte network has inscribed, at the level of white matter, the habituated sequences of skilled movement. These pathways are fast because they are familiar, not merely in the psychological sense, but in the biological sense. The myelin sheath is the material record of having-done-this-before.
C fibres carry something different. They carry the signal of tissue damage, something that, by definition, should not be familiar. Pain, in its raw nociceptive form, is the signal of an event that the organism has not successfully anticipated and absorbed into its habitual structure. It arrives precisely as something alien, something outside the inscribed patterns of white matter.
On this reading, the slowness of pain is not a metabolic compromise. It is a structural feature of what pain is. Pain travels along unmyelinated pathways because it is, by its nature, the signal that has not yet been integrated into the lived body. The ache that follows injury, the slow, diffuse, C-fibre mediated suffering, is the nervous system encountering something it does not yet know how to domesticate.
The paradox resolved, and deepened
This reframing does not resolve the paradox so much as relocate it. The mystery is no longer why pain travels slowly. The mystery is why, in chronic pain conditions, the experience of pain persists long after the tissue damage has healed.
On the standard account, chronic pain is a malfunction, the pain signal continuing to fire in the absence of its appropriate stimulus. On the Myelin Mind account, chronic pain looks like something more specific: the pathological myelination of what should remain unmyelinated. The inscription, at the level of white matter, of a signal that was never meant to become habitual. PTSD, fibromyalgia, central sensitisation, these conditions share a structural feature: the experience of pain has become, in some sense, familiar. It has been taken up into the organism’s habitual structure. The C-fibre signal has found its way into the myelinated record.
This is a hypothesis, not a conclusion. But it suggests that the distinction between myelinated and unmyelinated pain pathways is not a footnote in nociceptive neuroscience. It is a window into the deeper question of what myelin is for, and what happens when the boundary between the familiar and the alien breaks down.
Jack Parry is a philosopher and biomedical animator at Swinburne University of Technology.
He is the author of The Myelin Mind: The Genesis of Meaning.