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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
| Classification | Alternate Classification | Myelinated? | Type | Conduction Velocity |
|---|---|---|---|---|
| A | Yes | Somatic nerves | ||
| α | I | Yes | Subset of afferent nerves supplying the muscle spindle Sensitive to the rate of change in fiber length Also efferent motor neurons | 80-120 m/s |
| Ia | Afferent fibers from the muscle spindle | |||
| Ib | Afferent fibers serving the Golgi tendon organ at the junction between muscle and tendon | |||
| β | II | Yes | Subset 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 |
| δ | III | Yes | Convey “fast pain” sensation from skin and muscle | 5-30 m/s |
| B | Yes | Preganglionic efferent fibers of the autonomic nervous system | 3-15 m/s | |
| C | IV, afferent | No | Postganglionic efferent nerves of the autonomic nervous system Convey afferent “slow pain” sensation in somatic nerves | 1-2 m/s |
