Pain fibers
- Jennifer de la Cruz
- 4 minutes ago
- 2 min read
By: J. de la Cruz
Aim
Understanding pain fibres is more than just knowing anatomy; it helps nurses grasp the “why” behind the pain experience and improves how we assess and respond to patients who are hurting.
This blog explains what pain fibres are, how they work, and why it matters in clinical practice—so you can connect physiology to real‑world patient care.
Background
Pain fibres are bundles of nerve axons that form part of the body’s protective system. They carry signals from nociceptors—specialised sensory receptors that detect harmful stimuli—to the spinal cord and brain.
A nerve is a whole cable made up of many axons; a pain fibre is one of those axons, often called a somatic or nociceptive fibre when it’s involved in pain transmission. These fibres don’t just send “pain” to the brain; they also trigger fast, reflexive protective responses, like pulling your hand away from a hot surface before the brain fully registers the pain.
Two main fibre types transmit pain signals: Aδ (A‑delta) fibres and C fibres.
How pain fibres work
Nociceptors detect mechanical, thermal, or chemical threats (for example, a surgical incision, burn, or inflammation) and fire action potentials along Aδ and C fibres.
The speed of transmission depends on:
Fibre size (larger fibres are faster),
Myelination (myelinated fibres transmit faster), and
Receptive field size (how big an area each fibre “monitors”).
Aδ (A‑delta) fibres
Aδ fibres are thin, myelinated fibres that transmit signals quickly.
They have small receptive fields, so they give a sharp, well‑localised “first pain” that you can easily point to (for example, the initial sting of a needle or cut).
These fibres are partly responsible for the fast withdrawal reflex that moves a limb away from danger before the brain fully processes the pain.
C fibres
C fibres are unmyelinated and smaller, so conduction is slower and transmission lasts longer.
They produce “second pain”: a dull, aching, burning, throbbing sensation that develops gradually and persists after the initial injury.
C fibres have large, overlapping receptive fields, which is why the pain feels poorly localised, diffuse, and harder for patients to describe (for example, a deep ache in the abdomen or widespread muscle soreness).
C fibres and chronic pain
In chronic pain states, C fibres can become hypersensitive or pathologically active. Their activation threshold drops, so even mild stimuli may feel painful (hyperalgesia/ allodynia).
Persistent nociceptive input from injured tissue (for example, after surgery) can drive central sensitisation in the spinal cord, amplifying and prolonging pain beyond the original injury.
C fibres express a range of receptors that contribute to inflammatory and long‑lasting pain signals.
To sum up, when injury occurs, nociceptors activate both Aδ and C fibres; these signals travel to the spinal cord, ascend to the brain, and become the conscious experience of pain. Understanding Aδ versus C fibre pain helps you interpret what patients tell you:
Sharp, sudden, easy‑to‑point‑to pain suggests Aδ‑dominated, fast‑conducting nociception (often acute or procedural pain).
Dull, burning, aching, or diffuse pain usually reflects C fibre involvement and may indicate ongoing inflammation or chronic pain.
REFERENCES:
Mason, P. (2020). Pain. In Foundations of neuroscience. Michigan State University. https://openbooks.lib.msu.edu/neuroscience/chapter/pain/
Merskey, H., & Bogduk, N. (Eds.). (2022). Classification of chronic pain (2nd ed., revised). IASP.