General Structural Principles
At the center of any nerve fiber lies the axon (axis cylinder), which is a process of a nerve cell. It is surrounded externally by a sheath formed by glial cells: oligodendrocytes in the central nervous system (CNS) and Schwann cells (lemmocytes) in the peripheral nervous system (PNS).
Under light microscopy, especially in teased preparations, layers of loose connective tissue can be seen between individual fibers. This tissue is called the endoneurium.
Unmyelinated Nerve Fibers
These fibers are found predominantly in the autonomic nervous system and contain axons of effector neurons.
Structurally, they resemble a cable. Multiple axis cylinders—usually 10 to 20—are embedded into the cytoplasm of a single glial cell (lemmocyte). As the cylinders indent the glial cell plasmalemma, its edges meet above the cylinder to form a double membrane called the mesaxon.
In such a cable-like fiber, the lemmocyte nucleus lies strictly in the center, while the axis cylinders are distributed peripherally. Externally, the unmyelinated fiber is covered by a basal lamina. Voltage-gated sodium channels are distributed uniformly along the entire length of the cylinder, causing the impulse to travel continuously but slowly (at a speed of only 1–2 m/s).
Myelinated Nerve Fibers
Myelinated fibers form the white matter of the spinal cord and brain, as well as the pathways of the somatic nervous system. Unlike unmyelinated fibers, each myelinated fiber contains only one axis cylinder, located strictly in the center.
The sheath of such a fiber consists of two layers:
- Inner (myelin) layer. This represents a very long mesaxon wrapped tightly and concentrically around the axis cylinder. It consists of 80% lipids, serving as a powerful electrical insulator.
- Outer layer (neurolemma / neurilemma). This consists of the lemmocyte cytoplasm and nucleus, which are displaced to the extreme periphery due to the thick myelin layer.
A characteristic feature of myelinated PNS fibers is the presence of Schmidt-Lanterman clefts—oblique clearings within the myelin thickness where the mesaxon lamellae are less tightly packed and retain cytoplasmic pockets. Under light microscopy with osmium tetroxide staining, myelin stains dark, while the axon appears as a light spot.
Impulse Conduction and Nodes of Ranvier
The myelin layer is not continuous. At the junctions between adjacent lemmocytes, the myelin is interrupted, leaving the fiber covered only by a thinned neurolemma. These regions are called nodes of Ranvier.
Voltage-gated sodium channels are concentrated precisely at the nodes of Ranvier. Because of this, the nerve impulse does not travel continuously along the fiber, but "jumps" from one node to the next. This mechanism is called saltatory conduction. The spread of the electrical field across the myelin-insulated segment is nearly instantaneous, increasing conduction velocity to 5–120 m/s.
Differences in Myelination between CNS and PNS
| Feature | PNS (Peripheral Nervous System) | CNS (Central Nervous System) |
|---|---|---|
| Glial Cells | Schwann cells (lemmocytes) | Oligodendrocytes |
| Axon Coverage | One cell forms the sheath for a single axon | One oligodendrocyte myelinates multiple adjacent axons |
| Formation Mechanism | Rotation of the lemmocyte around the axon | Extension of oligodendrocyte cytoplasmic processes without cell rotation |
| Soma Localization | Part of the neurolemma (at the fiber periphery) | Located separately, between the fibers |
| Basal Lamina | Present | Absent |
Degeneration and Regeneration
The capacity of nerve fibers to regenerate depends on their location.
In the CNS, regeneration does not occur: microglia clear away cellular debris, and astrocytes form a dense glial scar at the site, which blocks axonal growth.
In the PNS, regeneration is possible if the neuron cell body survives and the gap between severed ends is small. The process occurs in two stages:
- Reactive processes (first 1–1.5 months). Wallerian degeneration occurs in the distal (separated) segment: the axon and myelin break down. However, lemmocytes survive. The neuron cell body swells, the nucleus shifts peripherally, and chromatolysis occurs (disappearance of basophilic substance).
- Regeneration. Surviving lemmocytes actively divide and align into cords known as Bungner's bands. These bridge the fiber ends and act as guiding tracks. A new axon begins to sprout from the proximal end at a rate of 3–4 mm per day. Growing through Bungner's bands, it gradually acquires a new myelin sheath and reaches the target organ.