Nerve fiber is a thread like extension of a neuron, which is formed by the axon and its covering. Thus each nerve fiber is an axon with its coverings. Larger axons are covered by a myelin sheath and are termed myelinated or medullated fibers. The fatty nature of myelin is responsible for the glistening whiteness of the peripheral nerve trunks and white matter of the CNS. Thinner axons, of less than one micron diameter, do not have the myelin sheath and are therefore termed non-myelinated or non-medullated.
However, all the fibers whether myelinated or non-myelinated have a neurolemmal sheath, which is uniformly absent in the tracts. In peripheral nerves, both the myelin and neurolemmal sheaths are derived from Schwann cells.
Myelinated fibers form the bulk of the somatic nerves. Structurally, they are made up of following parts from within outwards.
- Axis cylinder forms the central core of the fiber. It consists of axoplasm covered by axolemma.
- Myelin sheath, derived from Schwann cells, surrounds the axis cylinder. It is made up of alternate concentric layers of lipids and proteins formed by spiralization of the mesaxon: the lipids include cholesterol, glycolipids and phospholipids.
Myelin sheath is interrupted at regular intervals called the nodes of Ranvier where: the adjacent Schwann cells meet. Collateral branches of the axon arise at the nodes of Ranvier. Thicker axons possess a thicker coat of myelin and longer internodes.
Each internode is myelinated by one Schwann cell. Oblique clefts in the myelin, called incisures of Schmidt Lantermann, provide conduction channels for metabolites into the depth of the myelin and to the subjacent axon. Myelin sheath acts as an insulator for the nerve fibers.
- Neurolemmal sheath (sheath of Schwann) surrounds the myelin sheath. It represents the plasma membrane (basal lamina) of the Schwann cell. Beneath the membrane there lies a thin layer of cytoplasm with the nucleus of the Schwann cell. The sheaths of two cells interdigitate at the nodes of Ranvier.
- Neurolemmal sheath is necessary for regeneration of a damaged nerve. Tracts do not regenerate because of absence of neurolemmal sheath.
- Endoneurium is a delicate connective tissue sheath which surrounds the neurolemmal sheath.
Non-myelinated fibers comprise the smaller axons of the CNS, in addition to peripheral postganglionic autonomic fibers, several types of fine sensory fibers (C fibers of skin, muscle and viscera), olfactory nerves, etc. Structurally, a non-myelinated fiber consists of a group of small axons that have invaginated separately a single Schwann cell (in series) without any spiraling of the mesaxon.
The endoneurium, instead of ensheathing individual axons, surrounds all the neurolemmal sheath by virtue of which the non-myelinated fibers, like the myelinated fibers, can regenerate after damage.