Where is adelta and c fibres present
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A-delta (A-δ) and C Fibres - Location and Distribution
Both A-delta and C fibres are peripheral sensory nerve fibres whose cell bodies sit in the dorsal root ganglia (for the body) or trigeminal ganglia (for the head/face). Their axons extend peripherally to sensory receptors and centrally into the spinal cord.
Where Are They Found (Peripheral Distribution)?
A-delta and C fibres are present in virtually all tissues that can experience pain, including:
| Tissue | A-delta Fibres | C Fibres |
|---|---|---|
| Skin (cutaneous) | Yes - mechanoreceptors, thermoreceptors, polymodal nociceptors (PMNs) | Yes - polymodal nociceptors |
| Muscle (myotomes) | Yes | Yes |
| Bones (sclerotomes) | Yes | Yes |
| Viscera | Limited | Predominant (C fibres dominate visceral pain) |
| Joints | Yes | Yes |
| Cornea | Yes | Yes (one of the densest concentrations) |
| Blood vessel walls | Yes | Yes |
| Dura mater | Yes | Yes |
| Periosteum | Yes | Yes |
The relative concentration of C and A-δ fibres varies by tissue. - Rosen's Emergency Medicine, 9e
Central Termination
Both fibres synapse in the dorsal horn of the spinal cord (posterior grey matter), then ascend via the contralateral spinothalamic tract to the thalamus and cortex.
Key Properties Compared
| Property | A-delta (A-δ) | C Fibres |
|---|---|---|
| Myelin | Lightly myelinated (superficial) | Unmyelinated |
| Diameter | 1-4 mm | 0.5-1.5 mm |
| Conduction velocity | 12-30 m/s | 0.5-2 m/s |
| Pain type | Sharp, fast, "first" pain | Dull, burning, aching, "second" pain |
| Duration | Only while stimulus is active | Persists beyond the stimulus |
| Ascending tract | Contralateral spinothalamic tract | Contralateral spinothalamic tract |
What Stimulates Them?
- A-delta fibres: activated by sharp mechanical stimuli and extreme temperatures (the initial "ouch" response)
- C fibres: activated by mechanical, thermal, and chemical stimuli (polymodal) - they are the dominant pathway for slow, burning visceral and somatic pain
The threshold of activation can be lowered by chemical mediators (prostaglandins, bradykinins, substance P, leukotrienes, serotonin), a process called peripheral sensitization - which explains allodynia and hyperalgesia in inflamed tissues. - Rosen's Emergency Medicine, 9e
In summary: A-delta and C fibres are present throughout the body in the skin, muscles, bones, joints, viscera, and deep tissues - essentially wherever nociception (pain detection) is needed. C fibres dominate in visceral organs, while A-delta fibres are the main carrier of sharp, acute cutaneous pain.
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