Small unmyelinated nerve from the uterine myometrium of a sexually mature woman. Among other structures, the neuropil contains only unmyelinated axons of the central nervous system. In the peripheral nervous system, each individual unmyelinated axon is cradled in Schwann cells (peripheral glial cells) (cf. Figs. 270, 271). The axons of peripheral unmyelinated nerve fibers have diameters between 0.2 and 2.5 | m. They are bedded in Schwann cell invaginations, which look like trenches, grooves or channels. These are open grooves, at the open side only the basal membrane covers the axon. The axons are embedded in invaginations, which are completely lined by the Schwann cell plasmalemma. Mesaxons form connections between the plasmalemma of the large bay-like invaginations and the plasmalemma of the Schwann cell body. Between the outer Schwann cell membrane and the axo-lemma is a circa 10-20-nm wide intracellular cleft. One Schwann cell is recruited for a maximum of 1.5 |im of axon, then follows the next Schwann cell. 3 In this figure, about 20 unmyelinated axons [2 of different sizes are embedded in a Schwann cell. They are all located in groove-like invaginations, and at their surfaces they are only covered by the basal membrane [3 .This arrangement precludes mesaxons. At the surface, the tongue-like processes of ju Schwann cells also border on the basal membrane as well. The basal mem-^ brane envelops the entire nerve and separates it from the connective tissue. Axoplasm [2 contains neurofilaments, neurotubules and small, round osmio-philic mitochondria. Schwann cells also contain mitochondria. The nucleus of the Schwann cell is not cut. In the vicinity of the nerve are collagen fibrils H1, which have been mostly cut across their long axis.
1 Cytoplasmic membranes of Schwann cells
2 Axons with axoplasm
3 Basal membrane
4 Collagen fibrils
Electron microscopy; magnification: x 32 000
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