TYPES OF NERVES & THEIR VELOCITY CHARACTERISTICS
Prepared by: Muhammad Naveed Iqbal (BEC 01091 056)
With few additions by: Engr. Ijlal Haider
A nerve fiber is a threadlike extension of a nerve cell and consists of an axon and myelin
sheath (if present) in the nervous system. There are nerve fibers in the central nervous
system and peripheral nervous system.
Central Nerve Fiber
In the central nervous system, nerve fibers differ in terms of size, conduction velocity, and
presence or lack of myelin. For example, the olfactory nerve fibers are short and without myelin,
but the optic nerve fibers are myelinated (the olfactory and optic nerves are considered as a parts
of the CNS, while other cranial nerves are a component of the PNS). A bundle of nerve fibers
constitutes a tract in the central nervous system. The pyramidal tract and extrapyramidal tracts
have long nerve fibers that descend from the brain to the spinal cord. These fibers have an
important role in motor control, and are known as descending tracts. There are other bundles of
nerve fibers in the CNS that are called ascending tracts. These carry sensory information from
the periphery to the different areas of the brain (such as the cerebral cortex, cerebellum,
and brain stem).
Peripheral Nerve Fiber
A Nerve may be sensory, motor or sensory-motor (mixed). There are three types of nerve fibers
in a mixed nerve that include:
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Sensory Nerve Fiber
Motor Nerve Fiber
Autonomic Nerve Fibers
Components of Peripheral Nerve Fiber
Each peripheral nerve fiber contains:
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An axon (or a long dendrite of sensory fiber that also is known as an axon)
Axolemma
Myelin sheath (if existence)
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Schwann's sheath (neurolemma)
Endoneurium
Classification of Peripheral nerve fibers
There are three types of peripheral nerve fibers based on their diameter:
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A group
B group
C group
A group
Fibers of the A group have a large diameter and high conduction velocity, and are myelinated
fibers.
The A group consists of four types of nerve fibers:
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A alpha fibers (afferent or efferent fibers)
A beta fibers (afferent or efferent fibers)
A gamma fibers (efferent fibers)
A delta fibers (afferent fibers)
B group
Nerve fibers in this group are myelinated with a small diameter. Generally, they are
the preganglionic fibers of the autonomic nervous system and have a low conduction velocity.
C group
The C group fibers are unmyelinated and as the B group fibers have a small diameter and low
conduction velocity. These fibers include:
Postganglionic fibers in the autonomic nervous system (ANS)
Nerve fibers at the dorsal roots (IV fiber). These fibers carry the following sensory
information:
Pain, Temperature, Touch, Pressure, Itch
Figure: Characteristics of various types of nerves
Figure: Comparison of Speed of difference nerves