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.
CLINICAL
POINT
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.
