How does a nerves size and structure contribute to its speed of conduction

How does a nerve’s size and structure contribute to its speed of conduction? How are the peripheral fibers classified? 

The larger the fiber, the less the electrical resistance and the faster the speed of conduction.

Myelin increases a nerve’s diameter and also insulates the current between nodes of Ranvier, increasing the overall conduction velocity.

In myelinated nerves, the conduction velocity can be estimated to be 6 m/s/μm (e.g., a nerve that is 10 μm in diameter will conduct at approximately 60 m/s).

In unmyelinated nerves, the velocity is approximately 1.7 m/s/μm.

Peripheral nerve fibers are classified according to diameter and conduction velocity

Peripheral Nerve Fibers

ClassificationAlternate ClassificationMyelinated?TypeConduction Velocity
AYesSomatic nerves
αIYesSubset of afferent nerves supplying the muscle spindle
Sensitive to the rate of change in fiber length
Also efferent motor neurons
80-120 m/s
IaAfferent fibers from the muscle spindle
IbAfferent fibers serving the Golgi tendon organ at the junction between muscle and tendon
βIIYesSubset of afferent nerves supplying the muscle spindle
Respond to the overall length of the muscle spindle fiber
Fastest cutaneous afferent fibers, supplying the hair and skin follicles
35-75 m/s
δIIIYesConvey “fast pain” sensation from skin and muscle5-30 m/s
BYesPreganglionic efferent fibers of the autonomic nervous system3-15 m/s
CIV, afferentNoPostganglionic efferent nerves of the autonomic nervous system
Convey afferent “slow pain” sensation in somatic nerves
1-2 m/s
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