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An A delta fiber, alpha delta fiber, or Aδ fiber is a type of sensory nerve fiber. Aδ fibers carry cold, pressure and some pain signals. Because Aδ fibers are thinly myelinated, they send impulses faster than unmyelinated C fibers, but more slowly than other, more thickly myelinated "A" class fibers.
The fibers terminate at Rexed laminae I and V.
Structure
They are thin (2 to 5 μm in diameter), myelinated axons with a moderate conduction velocity, or speed of travel of a nerve signal (2 to 30 m/s). These nerve fibers are associated with acute (sharp) pain and therefore constitute the afferent portion of the reflex arc that results in "pulling away" from noxious stimuli (e.g. retracting your hand away from a hot stove). A certain proportion of Aδ fibers are also associated with sensations of temperature (also known as 'cold receptors' in mammals) and pressure. Slowly conducting, unmyelinated C fibers, by contrast, carry slow, burning pain.
Like other sensory fibers, the Aδ fiber is an extension of a pseudounipolar neuron with its cell body located in a dorsal root ganglion or trigeminal ganglion. Within the spinal cord, afferent nociceptor fibers synapse at or near the spinal cord level where they enter.