Morphological characteristics and nature of nodes
In myelinated nerve fibers, a single axon occupies a central position. Around it, a dense layered sheath is formed by multiple wrappings of the lemmocyte plasmalemma (mesaxon).
Nodes of Ranvier are nodal zones where the myelin layer is interrupted, leaving the axonal membrane exposed and in contact with the extracellular fluid. The myelin-covered regions are virtually devoid of excitability, whereas the free membrane in the node region possesses the highest level of excitability.
Comparison of myelinated and unmyelinated fibers
The structure of a nerve fiber directly determines the presence or absence of nodes:
| Parameter | Myelinated fibers | Unmyelinated fibers |
|---|---|---|
| Axons | Single, located centrally | Multiple, located peripherally |
| Lemmocyte nuclei | Displaced to the periphery | Located in the center of the fiber |
| Mesaxon | Long, repeatedly wrapped | Short (simple invagination) |
| Nodes of Ranvier | Present | Absent |
| Na⁺ channels | Concentrated at the nodes | Distributed evenly |
Functional role and conduction velocity
The action potential in myelinated structures does not propagate continuously, but is generated exclusively at the nodal zones. This is due to the strict concentration of voltage-gated sodium channels (Na⁺) in the region of the nodes of Ranvier, whereas in unmyelinated fibers they are scattered along the entire length of the axon.
The speed of nerve signal transmission obeys strict regularities:
- For myelinated fibers, the velocity formula is V = d (conduction velocity ranges from 5 to 120 m/s).
- For unmyelinated structures, the relationship is expressed as V = √d (velocity does not exceed 1–2 m/s).
Increasing the number of myelin layers enlarges the overall fiber diameter. Each additional layer lengthens the distance between adjacent nodes of Ranvier, which significantly accelerates impulse transmission.
Additional structures: Schmidt-Lanterman clefts
In addition to the nodes, myelin clefts (Schmidt-Lanterman clefts) are detected within the thickness of the myelin layer.
They are narrow oblique clearings where the mesaxon leaflets do not fit tightly against each other, retaining small layers of lemmocyte cytoplasm. These structures are found exclusively in myelinated fibers of the peripheral nervous system and are completely absent in central nervous system fibers.
Velocity formulas are easy to remember by the number of axons: a myelinated fiber has ONE axon and linear velocity (V = d), while an unmyelinated fiber has many, so the velocity is 'hidden' under the root (V = √d).
Class and exam questions
Where are sodium channels localized in a myelinated nerve fiber?
In myelinated fibers, voltage-gated sodium channels are concentrated exclusively in the region of the nodes of Ranvier, whereas the regions beneath the myelin are virtually unexcitable.
How is the diameter of a myelinated fiber related to nerve impulse velocity?
Velocity is directly proportional to the diameter (V = d). Increasing myelin layers increases the distance between nodes of Ranvier, raising velocity to 5–120 m/s.
Are nodes of Ranvier found in unmyelinated fibers?
No, nodes of Ranvier are formed only in myelinated fibers due to the discontinuous arrangement of the myelin sheath along a single axon.
What else to study on this topic
- Structure and topography of unmyelinated fibers of the autonomic nervous system
- Formation of the mesaxon and wrapping of Schwann cell plasmalemma
- Histological differences between myelinated fibers of the peripheral nervous system and the CNS
- Organization of cytoplasmic layers in Schmidt-Lanterman clefts
- Physiology of the neuromuscular synapse and mechanisms of synaptic transmission
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