Page 377 - Atlas of Histology with Functional Correlations

peripheral nerve has been prepared in a longitudinal section (upper figure) and in

a cross section (lower figure).

In the longitudinal section, the myelin sheath (1) is a thick, black band
surrounding a lighter, central axon (2). The length of an axon myelinated by one
Schwann cell is the nodal or internodal segment. Between the internodal
segments, which can be a few millimeters in length, the myelin sheath exhibits

discontinuity that represent the nodes of Ranvier (4), which can span
approximately 1 or 2 micrometers (μm).

A group of nerve fibers or fascicle is surrounded by a light-appearing
connective tissue layer, the perineurium (3, 5, 8). In turn, each individual nerve

fiber or axon is surrounded by a thin layer of connective tissue, the
endoneurium (7, 10). In the transverse plane (lower figure), different diameters
of myelinated axons are seen. The myelin sheath (9) appears as a thick, black
ring around the light, unstained axon (12) that is in the center.

The connective tissue surrounding individual nerve fibers, or the fascicle,

exhibits a rich supply of blood vessels (6, 11) of different sizes.

FIGURE 9.23 ■ Myelinated nerve fibers (longitudinal and transverse sections).
Stain: osmic acid. High magnification.

FUNCTIONAL CORRELATIONS 9.4 ■

Axon Myelination and Supporting Cells in the

PNS

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