General structural principles
At the center of any nerve fiber lies the axis cylinder, which is a nerve cell process (axon or dendrite). Externally, it is surrounded by a sheath formed by glial cells: oligodendrocytes in the central nervous system (CNS) or lemmocytes (Schwann cells) in the peripheral nervous system (PNS).
Under light microscopy, especially in teased preparations, layers of loose connective tissue can be observed between individual fibers. This tissue is called the endoneurium.
Unmyelinated nerve fibers
These fibers are found predominantly in the autonomic nervous system and contain the axons of effector neurons.
In structure, they resemble a cable. Several axis cylinders—usually 10 to 20—are embedded into the cytoplasm of a single glial cell (lemmocyte). Upon immersion, the gliocyte plasmalemma invaginates, and its edges close over the cylinder to form a double membrane known as the mesaxon.
In such a "cable" fiber, the lemmocyte nucleus lies strictly in the center, and the axis cylinders are distributed along the periphery. 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 propagate 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, a single fiber here always contains only one axis cylinder, located strictly in the center.
The sheath of such a fiber consists of two layers:
- Inner (myelin) layer. Represents a very long mesaxon wrapped repeatedly and concentrically around the axis cylinder. It consists of 80% lipids, serving as a powerful electrical insulator.
- Outer layer (neurolemma). This is the cytoplasm and nucleus of the lemmocyte, which have been pushed to the extreme periphery due to the thick myelin layer.
A characteristic feature of PNS myelinated fibers is the Schmidt-Lanterman clefts — oblique lighter areas within the myelin thickness where the mesaxon leaves are loosely apposed and retain layers of cytoplasm. Under light microscopy with osmium tetroxide staining (osmium impregnation), myelin stains dark, and the axon appears as a light spot.
Impulse conduction and nodes of Ranvier
The myelin layer is not continuous. At the junctions of adjacent lemmocytes, myelin is interrupted, and the fiber is covered only by a thinned neurolemma. These regions are called nodes of Ranvier.
Sodium channels are concentrated specifically at the nodes of Ranvier. As a result, the impulse does not travel continuously along the fiber, but "jumps" from node to node. This mechanism is called saltatory (discontinuous) conduction. The spread of the electrical field across the myelin-insulated segment occurs almost instantaneously, increasing the conduction velocity to 5–120 m/s.
Differences in myelination between the CNS and PNS
| Sign | PNS (Peripheral Nervous System) | CNS (Central Nervous System) |
|---|---|---|
| Glial cells | Lemmocytes (Schwann cells) | Oligodendrocytes |
| Axon coverage | One cell forms a sheath for a single axon | One cell myelinates several adjacent axons simultaneously |
| Formation mechanism | Rotation of the lemmocyte around the axon | Extension of oligodendrocyte cytoplasmic tongues without cell rotation |
| Soma localization | Part of the neurolemma (at the fiber periphery) | Lies separately, between fibers |
| Basal lamina | Present | Absent |
Degeneration and regeneration
The ability of nerve fibers to regenerate depends on their localization.
In the CNS, regeneration does not occur: microglia remove the dead debris, and astrocytes form a dense glial scar at the site, blocking growth.
In the PNS, regeneration is possible if the neuron cell body is alive and the distance between the severed ends is small. The process is divided into two stages:
- Reactive processes (first 1–1.5 months). In the distal (separated) segment, Wallerian degeneration occurs: the axon and myelin disintegrate. However, lemmocytes survive. The neuron cell body swells, the nucleus shifts to the periphery, and chromatolysis (disappearance of basophilic substance) occurs.
- Regeneration. Surviving lemmocytes actively divide and align into bands known as Bungner bands. They 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 the Bungner bands, it gradually becomes covered with new myelin and reaches the target organ.
Briefly: answers to topical questions
What is the ultrastructure of a myelinated nerve fiber under electron microscopy?
Under electron microscopy, a myelinated nerve fiber features a single axis cylinder (axon or dendrite) located strictly in the center, and a heterogeneous sheath.
Sheath layers:
- Myelin layer — the inner layer: several tightly apposed turns of the lemmocyte/Schwann cell or oligodendrocyte plasmalemma, concentrically wrapped around the axis cylinder; essentially a very elongated mesaxon.
- Neurolemma (neurilemma) — the outer layer in a PNS myelinated fiber; represents the peripheral zone formed by the displaced outward cytoplasm and nucleus of the lemmocyte, located superficial to the myelin layer.
- Basal lamina — in peripheral nerves, covers the fiber externally superficial to the neurolemma.
In the CNS, the myelin sheath is formed by oligodendrocytes; the oligodendrocyte soma lies between fibers, and an external basal lamina is absent.
Additional elements: in the region of the nodes of Ranvier, the myelin layer is interrupted; Schmidt-Lanterman clefts are narrow, obliquely oriented lighter areas within the myelin where layers of lemmocyte cytoplasm persist, characteristic of PNS myelinated fibers.
What types of nerve fibers are distinguished according to the Erlanger-Gasser classification?
According to the Erlanger-Gasser classification, nerve fiber groups are distinguished: Aα, Aβ, Aγ, Aδ, B, and C.
| Fiber type | Sheath characteristics | Diameter | Conduction velocity |
|---|---|---|---|
| Aα | Thickest myelinated fibers | 12–22 µm | 70–120 m/s |
| B | Thin myelinated / lightly myelinated fibers; preganglionic fibers | 1–3.5 µm | 3–18 m/s |
| C | Thinnest unmyelinated fibers; postganglionic fibers | 0.5–2 µm | 0.5–3 m/s |
Fibers of types A and B are myelinated, type C fibers are unmyelinated. Conduction velocity is determined by the presence of a myelin sheath and fiber diameter.
To easily remember the structure of an unmyelinated fiber (the "cable type"), imagine a classic electrical cable: inside a single thick insulation layer (lemmocyte), a bundle of many thin cores (axons) is hidden.
Class and exam questions
What is the essence of saltatory impulse conduction?
This is the saltatory propagation of a nerve signal in myelinated fibers. Due to the myelin insulation, the impulse jumps between the nodes of Ranvier, which significantly increases transmission speed.
Why do nerve pathways in the brain fail to regenerate after injury?
In the central nervous system, glial cells (astrocytes) rapidly form a dense glial scar at the site of injury. It serves as an impassable physical barrier for the growing axon.
What is a mesaxon?
This is a duplication (double fold) of the glial cell plasmalemma formed when the axon is submerged into the cytoplasm. In myelinated fibers, the mesaxon repeatedly coils to form a dense insulating layer.
What else to study on this topic
- Detailed ultrastructure of the nodes of Ranvier
- Chemical and lipoprotein composition of myelin in the CNS and PNS
- The role of macrophages in the clearance of nerve fiber debris
- Morphological changes in the neuron cell body during axonal injury (chromatolysis)
- Clinical significance of Büngner bands in microsurgical nerve repair
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