Biological Chemistry - Berezov T. T., Korovkin B. F. 1998

Nervous Tissue
Neuron Structure
Myelin Structure

Nerve fibers formed by the axons of Nerve Cells can be structurally divided into two types: myelinated and unmyelinated. The conduction pathway of the somatic Nervous system, as well as the Central Nervous System, belongs to the first type, which is functionally more advanced and capable of transmitting nerve impulses at a high speed.

Myelin is a morphological concept. Essentially, myelin is a system formed by multiple layers of neuroglial Cell membranes wrapped around nerve processes (in peripheral nerve trunks, neuroglia is represented by lemmocytes, or Schwann cells, while in the White matter OF the CNS, it is represented by astrocytes).

In terms of chemical composition, myelin is a complex protein-lipid Structure.

Lipids account for up to 80% of the dry weight; 90% of all myelin lipids are represented by Cholesterol, Phospholipids, and cerebrosides. It is believed that molecules of various lipids have a strictly defined arrangement within the lipid layers of myelin sheaths (Fig. 19.3).

* The Fine Structure of neuroglia is discussed in special manuals dedicated to the Histology and Morphology of The Nervous System.

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Fig. 19.2. Schematic representation of the ultrastructure of a nerve cell based on Electron Microscopy data (after A.A. Manina).

ВВ - nuclear membrane invagination; ВН - Nissl substance; Г - Golgi apparatus (Golgi complex); ГГ - Glycogen granules; КГ - tubules of the Golgi complex; КМ - mitochondrial cristae; Л - Lysosomes; ЛГ - lipid granules; М - Mitochondria; ММ - mitochondrial membrane; МЭ - membranes of The Endoplasmic reticulum; Н - neurofibrils; П - Polysomes; ПМ - Plasma Membrane; ПР - presynaptic membrane; ПС - postsynaptic membrane; ПЯ - nuclear pores; Р - Ribosomes; РНП - ribonucleoprotein granules; С - synapse; СП - synaptic vesicles; ЦЭ - cisternae of the endoplasmic reticulum; ЭР - endoplasmic reticulum; Я - Nucleus; ЯМ - nuclear membrane.

Fig. 19.3. Molecular Organization of the myelin sheath (after H. Hydén).

1 - axon; 2 - myelin; 3 - fiber axis; 4 - protein (outer layers); 5 - lipids; 6 - protein (inner layer); 7 - cholesterol; 8 - cerebroside; 9 - sphingomyelin; 10 - phosphatidylserine.