28. Nervous fibres. Types, structural, ultrastructural, chemical and functional characteristics.

A nerve fibre is a threadlike extension of a nerve cell and consists of an axon and a myelin sheath. There are nerve fibres in the central nervous system and peripheral nervous system.

There 3 main types of nerve fibres: A, B and C. 

Type A fibres are the thickest and fastest conducting. They are myelinated. They have a diameter of 1.50-20.00 micron. Their speed of conduction is 4-120m/sec. Type A fibres include skeletomotor fibers, fusimotor fibers and afferent fibers to skin.

Type B fibres are medium in size. They are myelinated. They have a diameter of 1.50-3.50 microns. Their speed of conduction is 3-15m/sec, they are slower than type A fibers. Examples of type B fibers include preganglionic autonomic efferents.

Type C fibres are the smallest and thinnest. They are non-myelinated. They have a diameter of 0.1-2.0 microns. Their speed of conduction is 0.5-4.0m/sec, they have the slowest conduction. Examples of type C fibers include postganglionic autonomic efferent and afferent fibers to skin.

Nerve fibers can be myelinated or non-myelinated. All nerve fibres have a neurolemmal sheath. Both myelin and neurolemmal sheaths are derived from Schwann cells in the PNS and in the CNS it is produced by oligodendrocytes.

Myelinated fibers form the bulk of the somatic nerves. It consists of the axoplasm covered by the axolemma. Myelin sheath are derived from Schwann cells.

The myelin sheath is interrupted at intervals by the nodes of Ranvier. This increases the speed of electrical impulses as it “jumps” from node to node. As a result the speed of transmission is increased in myelinated nerve fibres. This is known as saltatory conduction. Myelin sheath acts as an insulator for the nerve fibers. 

Neurolemmal sheath surrounds the myelin sheath. Neurolemmal sheath is necessary for regeneration of a damaged nerve. Endoneurium is a thin layer of loose connective tissue, which surrounds the neurolemmal sheath.

Non-myelinated fibers comprise of smaller axons of the CNS that have invaginated separately. The endoneurium surrounds all of the neurolemmal sheath by virtue. They can regenerate after damage.