A nerve fiber is a threadlike extension of a nerve cell and consists of an axon

Axon

An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....

 and myelin sheath (if present) in the nervous system

Nervous system

The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...

. There are nerve fibers in the central nervous system

Central nervous system

The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...

 and peripheral nervous system

Peripheral nervous system

The peripheral nervous system consists of the nerves and ganglia outside of the brain and spinal cord. The main function of the PNS is to connect the central nervous system to the limbs and organs. Unlike the CNS, the PNS is not protected by the bone of spine and skull, or by the blood–brain...

. A nerve fiber may be myelinated and/or unmyelinated. In the central nervous system (CNS), myelin is produced by oligodendroglia cells. Schwann cells form myelin in the peripheral nervous system (PNS). Schwann cells can also make a thin covering for an axon which does not consist of myelin (in the PNS). A peripheral nerve fiber consists of an axon, myelin sheath, Schwann cells and its endoneurium

Endoneurium

The endoneurium, also referred to as an endoneurial channel, sheath or tube, is a layer of delicate connective tissue made up of endoneurial cells that encloses the myelin sheath of a spinal cord nerve fiber. These are bundled up into groups called nerve fascicles, which have a protective sheath...

. There are no endoneurium and Schwann cells in the central nervous system.

Central nerve fibers

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

Cranial nerves

Cranial nerves are nerves that emerge directly from the brain, in contrast to spinal nerves, which emerge from segments of the spinal cord. In humans, there are traditionally twelve pairs of 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

Brain

The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...

 to the spinal cord

Spinal cord

The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...

. These fibers have an important role in motor control

Motor control

Motor control are information processing related activities carried out by the central nervous system that organize the musculoskeletal system to create coordinated movements and skilled actions...

, 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

Cerebral cortex

The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It is constituted of up to six horizontal layers, each of which has a different...

, cerebellum

Cerebellum

The cerebellum is a region of the brain that plays an important role in motor control. It may also be involved in some cognitive functions such as attention and language, and in regulating fear and pleasure responses, but its movement-related functions are the most solidly established...

, and brain stem

Brain stem

In vertebrate anatomy the brainstem is the posterior part of the brain, adjoining and structurally continuous with the spinal cord. The brain stem provides the main motor and sensory innervation to the face and neck via the cranial nerves...

).

Peripheral nerve fiber types

A peripheral nerve may be sensory, motor or sensory-motor (mixed). There are three types of nerve fibers in a mixed nerve that include:

• Sensory nerve fibers (afferent fibers)
• Motor nerve fibers (efferent fibers)
• Autonomic nerve fibers (autonomic fibers)

Components of peripheral nerve fiber

Each peripheral nerve fiber contains:

• An axon (or a long dendrite of sensory fiber that also is known as an axon)
• Axolemma
  Axolemma
  The axolemma is the cell membrane surrounding an axon. It is responsible for maintaining the membrane potential of the neuron, and it contains ion channels through which ions can flow. When this occurs, the voltage inside the axon changes, and depolarization or hyperpolarization of the membrane can...
  
  
• Myelin sheath (if existence)
• Schwann's sheath (neurolemma
  Neurolemma
  Neurolemma is the outermost nucleated cytoplasmic layer of Schwann cells that surrounds the axon of the neuron. It forms the outermost layer of the nerve fiber in the peripheral nervous system....
  
  )
• Endoneurium

Classification of peripheral nerve fibers

There are three types of peripheral nerve fibers based on their diameter:

• 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:

• 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 these group, are myelinated with a small diameter. they are the preganglionic fibers of the autonomic nervous system. Preganglionic fibers

Preganglionic fibers

In the autonomic nervous system, fibers from the CNS to the ganglion are known as preganglionic fibers.All preganglionic fibers, whether they are in the sympathetic division or in the parasympathetic division, are cholinergic .Sympathetic preganglionic fibers tend to be shorter than parasympathetic...

 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
  Postganglionic fibers
  In the autonomic nervous system, fibers from the ganglion to the effector organ are called postganglionic fibers.-Neurotransmitters:The neurotransmitters used for postganglionic fibers differ:...
  
   in the autonomic nervious system (ANS)
• Nerve fibers at the dorsal roots (IV fiber). These fibers carry the following sensory information:
  • Pain
  • Temperature
  • Touch
  • Pressure
  • Itch

Motor fibers of the A group

A alpha fibers

• Characteristics:
  • High conduction velocity
  • By alpha motor neuron
    Alpha motor neuron
    Alpha motor neurons are large lower motor neurons of the brainstem and spinal cord. They innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their contraction...
    
     for muscle contraction
    Muscle contraction
    Muscle fiber generates tension through the action of actin and myosin cross-bridge cycling. While under tension, the muscle may lengthen, shorten, or remain the same...
    
    .
  • Alpha motor neurons innervate extrafusal muscle fibers (slow and fast fibers)

A beta fibers

• Characteristics:
  • By beta motor neuron
    Beta motor neuron
    A beta motor neuron is a kind of lower motor neuron, along with alpha motor neuron and gamma motor neuron. These motor neurons innervate intrafusal fibers of muscle spindles with collaterals to extrafusal fibers . Axons of beta motor neurons are myelinated...
    
     for muscle spindle fibers contraction.
  • Beta motor neuron
    Beta motor neuron
    A beta motor neuron is a kind of lower motor neuron, along with alpha motor neuron and gamma motor neuron. These motor neurons innervate intrafusal fibers of muscle spindles with collaterals to extrafusal fibers . Axons of beta motor neurons are myelinated...
    
    s innervate intrafusal muscle fibers of muscle spindles (nuclear bag and nuclear chain fibers), with collaterals to extrafusal muscle fibers.

A gamma fibers

• Characteristics:
  • By gamma motor neuron for muscle spindle fibers contraction.
  • Gamma motor neurons innervate intrafusal muscle fibers of muscle spindles (nuclear bag and nuclear chain fibers).

Sensory fibers of the A group

A alpha fibers (Ia fiber or Ib fibers)

• Characteristics:
  • High conduction velocity
  • Ia fibers are related to muscle spindle primary endings (muscle sense)
  • Ib fibers are related to golgi tendon organs (muscle sense)

A beta fibers (II fibers)

• II fibers carry sensory information related to muscle spindle secondary endings, touch, and kinesthesia.

A delta fibers (III fibers)

• III fibers carry sensory information related to pain, temperature, crude touch, and pressure.

Regeneration of peripheral nerve fibers

Peripheral nerves can be easily damaged in a traumatic event due to their sometimes vulnerable positions in the body. However, if a peripheral nerve is damaged, they do have the ability to regenerate if its soma and a small portion of the neurilemma remain. The nerve begins the process by destroying the nerve distal to the site of injury allowing Schwann cells, basal lamina, and the neurilemma near the injury to begin producing a regeneration tube. Nerve growth factors are produced causing many nerve sprouts to bud. When one of the growth processes finds the regeneration tube, it begins to grow rapidly towards its original destination guided the entire time by the regeneration tube. Nerve regeneration is very slow and can take up to several months to complete. While this process does repair some nerves, there will still be some functional deficit as the repairs are not perfect.

See also

• Axon
  Axon
  An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....
  
  
• Peripheral nerve
• Central nervous system
  Central nervous system
  The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...
  
  
• Peripheral nervous system
  Peripheral nervous system
  The peripheral nervous system consists of the nerves and ganglia outside of the brain and spinal cord. The main function of the PNS is to connect the central nervous system to the limbs and organs. Unlike the CNS, the PNS is not protected by the bone of spine and skull, or by the blood–brain...
  
  
• Autonomic nervous system
  Autonomic nervous system
  The autonomic nervous system is the part of the peripheral nervous system that acts as a control system functioning largely below the level of consciousness, and controls visceral functions. The ANS affects heart rate, digestion, respiration rate, salivation, perspiration, diameter of the pupils,...