Nerve Fibers which carry sensory impulses.
Some sensory nerves are thicker and conduct electrical impulses much faster.
The sensory fibers for joint position sense which help in coordination are much faster.
Those relating touch information are slightly slower.
While those that cause pain sensation are the slowest called the C fiber (small fiber). C fiber disease cannot be detected by EMG/NCV.
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Here is some more information on the different primary afferent axons:
A-alpha nerve fibers carry information related to proprioception (muscle sense).
A-beta nerve fibers carry information related to touch.
A-delta nerve fibers carry information related to pain and temperature.
C-nerve fibers carry information related to pain, temperature and itch.