CLASSIFICATION OF PERIPHERAL NERVE FIBERS BY SIZE AND CONDUCTION VELOCITY
Unmyelinated peripheral nerve fibers (1 to 2 µm in diameter) conduct APs slowly (1 to 2 m/sec) because propagation requires reinitiation of the AP at each adjacent patch of axonal membrane along the entire course of the axon. These peripheral fibers are called group IV fibers. Myelinated peripheral nerve fibers (2 to 20+ µm in diameter) conduct APs rapidly (2 to 120+ m/sec) because propagation is aided by the distant spacing of nodes of Ranvier resulting from the successive internodal myelin sheaths. The larger diameter axons conduct APs the most rapidly.
Clinical conduction-velocity studies can document the conduction velocity of successive classes of myelinated peripheral nerve fibers (group I, II, and III fibers), and they provide evidence of normal or altered nerve conduction and possibly function. Conduction velocity is measured by placing a stimulating electrode at a specific site (in the popliteal fossa) where a current can initiate APs in axons in a specific nerve. Recording electrodes are placed at a distant site, where muscle contractions can be measured and where the time delay of conduction of APs in axons can be measured. The classification system of myelinated nerve fibers in the figure is accompanied by descriptions of the functional types of axons included in each group.
Peripheral axons larger than approximately 2 µm in diameter trigger the process of myelination by adjacent Schwann cells. Peripheral axons of different sizes subserve different functions and are subject to damage by a variety of separate insults. Thus, small-fiber neuropathies, such as leprosy, damage pain, and temperature sensation (via small-diameter axons) and can affect these modalities without concomitant damage to discriminative touch, LMN function, or Ia afferent reflex activity. In contrast, damage to large-diameter axons, as seen in demyelinating neuropathies, can result in flaccid paralysis with loss of tone and reflexes (motor axons) and loss of fine, discriminative sensation (sensory axons) without loss of autonomic functions or loss of pain and temperature sensation, which are carried in part by small unmyelinated axons.