The terminal Schwann cell is a specialized non-myelinating glial cell at the termination sites of myelinated and unmyelinated sensory and motor nerve fibers. Depending on the type of afferent or efferent axons the morphology of the terminal Schwann cell differs [33-42] (fig. 3). Neurons and their terminal Schwann cells are dependent on each other showing complex reciprocal interactions especially during development as it is shown with modern molecular biological and genetic techniques. This functional relationship is time controlled and coordinated and involves the secretion of trophic molecules from both the axon and terminal glial cell [43, 44]. As shown for neuromuscular junctions the terminal Schwann cell plays an important role in nerve terminal growth and maintenance, synaptic modulation, axonal sprouting and regeneration [45-47]. The axon terminal, as well as the nodes of Ranvier, are regions where axonal sprouting may occur. Furthermore, sensory axon and terminal
Fig. 4. Enteric ganglia. a Submucosal enteric plexus of Meissner with ganglionic cells (me), glia cell (gc) and fiber plexus (mep) embedded in the collagen tissue (ct) of the rat jejunum. Semithin section, bar 10 ^m. b Organization of the enteric neuropil resembles the neuropil of the CNS. Preterminal axons with synaptic vesicles (s), ganglionic cell dendrite (dme). Glia cell (gc), axon (ax). Note the lack of connective tissue inside of the ganglionic cell complex. Electron micrograph, bar 0.2 ^m. c Termination of enteric axons close to a capillary (c) in the lamina propria mucosae of the rat jejunum. Note the free axon terminals (ax) only partly covered by their terminal glia cell (ts). Electron micrograph, bar 0.2 ^m.
Schwann cell interactions play a crucial role for the maturation of the mechanoreceptors [48, 49]. Preventing apoptosis of the terminal Schwann cells in transected nerves by in vivo injection of neuregulin - a glial growth factor -will allow axon regeneration and receptor formation . Terminal Schwann cells express the neuregulin receptors ErB2 and ErB3 .
Nociceptors belong to the main group of sensory unmyelinated axons (C-fibers) terminating in all tissue compartments of the body except the brain. Terminal Schwann cells of nociceptors cover the receptive parts of the axons incomplete exposing the axon membrane directly to the tissue microenvironment [33, 35, 52]. This configuration allows controlled interactions between axon terminal and tissue environment. Besides their afferent function nocicep-tors release via the axon reflex various peptides (substance P, calcitonin-gene-related peptides) inducing vasodilatation, plasma extravasation and activation of mast cells - which is called neurogenic inflammation [53, 54].
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