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Mesaxon – Definition and Clinical Significance

The mesaxon is a specialized membrane structure found in the myelin sheath of peripheral nerves. It connects the Schwann cell to the axon surface and is essential for proper nerve conduction.

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Things worth knowing about "Mesaxon"

The mesaxon is a specialized membrane structure found in the myelin sheath of peripheral nerves. It connects the Schwann cell to the axon surface and is essential for proper nerve conduction.

What is the Mesaxon?

The mesaxon is a specialized, double-layered membrane structure that forms during the myelination of peripheral nerve fibers. It refers to the region where the plasma membrane of the Schwann cell folds inward around the axon, with the two membrane leaflets apposing each other. There are two distinct types: the inner mesaxon, which is in direct contact with the axon, and the outer mesaxon, located at the outermost layer of the myelin sheath facing the endoneurial fluid.

Anatomy and Formation

During the development of the nervous system, the Schwann cell spirally wraps its plasma membrane around the axon. This process begins with a simple membrane invagination, the mesaxon, which subsequently undergoes multiple turns to form the characteristic, multilamellar myelin sheath. The myelin sheath electrically insulates the axon and enables rapid, saltatory nerve impulse conduction.

Inner Mesaxon

The inner mesaxon lies directly adjacent to the axon and serves as the starting point of the spiral wrapping process. It plays an important role in regulating the exchange of substances between the Schwann cell and the axon.

Outer Mesaxon

The outer mesaxon is located at the outermost layer of the myelin sheath and borders the endoneurium. It is involved in the structural anchoring of the Schwann cell within the surrounding connective tissue.

Clinical Relevance

Disruptions in the structure or function of the mesaxon can lead to demyelinating diseases, in which the myelin sheath of peripheral nerves is damaged, slowing or interrupting nerve conduction. Clinically relevant conditions associated with abnormalities of the mesaxon and myelin sheath include:

Charcot-Marie-Tooth disease: A hereditary peripheral neuropathy in which genetic defects affect myelination and the mesaxon structure.
Guillain-Barre syndrome: An autoimmune condition involving an attack on the peripheral myelin sheath.
Diabetic polyneuropathy: Structural changes to Schwann cells and myelin associated with diabetes mellitus.

Function of the Mesaxon in Nerve Conduction

An intact mesaxon is a prerequisite for the proper formation of the myelin sheath. Only a correctly structured myelin sheath enables saltatory conduction, in which electrical impulses jump from one node of Ranvier to the next. This process greatly increases conduction velocity and reduces the energy demands of the nervous system.

Distinction from the Central Nervous System

In the central nervous system (CNS), myelination is carried out by oligodendrocytes rather than Schwann cells. A classical mesaxon as seen in peripheral nerves is not formed in the CNS, since a single oligodendrocyte myelinates multiple axons simultaneously, and the structural details of the membrane connections differ accordingly.

References

Kandel ER, Schwartz JH, Jessell TM et al. - Principles of Neural Science, 5th edition, McGraw-Hill, 2013.
Bhatt DL et al. - Peripheral Nerve Structure and Function. In: Neurology in Clinical Practice, 6th edition, Elsevier, 2012.
Nave KA, Werner HB - Myelination of the Nervous System: Mechanisms and Functions. Annual Review of Cell and Developmental Biology, 2014; 30: 503-533.

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