Myelination fibers

Palytoxin (PTX) is one of the most potent marine toxins known and the lethal dose (LD q) of the toxin in mice is 0.5 Mg/kg when injected i.v. The molecular structure of the toxin has been determined fully (1,2). PTX causes contractions in smooth muscle (i) and has a positive inotropic action in cardiac muscle (4-6). PTX also induces membrane depolarization in intestinal smooth (i), skeletal (4), and heart muscles (5-7), myelinated fibers (8), spinal cord (9), and squid axons (10). PTX has been demonstrated to cause NE release from adrenergic neurons (11,12). Biochemical studies have indicated that PTX causes a release of K from erythrocytes, which is followed by hemolysis (13-15). The PTX-induced release of K from erythrocytes is depress by ouabain and that the binding of ouabain to the membrane fragments is inhibited by PTX (15). 

Rat (Sprague- Dawley) 14 d before mating Gd 0-21 -weaning ad libitum (W) 37 M (40% deorease in number of myelinated fibers in the hippooampus) Isaaoson and Taylor 1989... 

Thermal nociceptors and mechanical nociceptors are associated with A-delta fibers. These are small myelinated fibers that transmit impulses at a rate of 5 to 30 m/sec. Polymodal nociceptors are associated with C fibers. These are small unmyelinated fibers that transmit impulses at a rate generally less than 1.0 m/sec (range of 0.5 to 2.0 m/sec). 

FIGURE 4-1 Impulse conduction in unmyelinated (top) and myelinated (bottom) fibers. The arrows show the flow of action currents in local circuits into the active region of the membrane. In unmyelinated fibers the circuits flowthrough the adjacent piece of membrane but in myelinated fibers the circuit flow jumps to the next node. 

FIGURE 4-7 A typical CNS myelinated fiber from the spinal cord of an adult dog. Contrast this figure with the PNS fiber in Figure 4-4. The course of the flattened oligodendrocytic process, beginning at the outer tongue (arrow), can be traced. Note that the fiber lacks investing cell cytoplasm and a basal lamina-as is the case in the PNS. The major dense line and the paler, double intraperiod line of the myelin sheath can be discerned. The axon contains microtubules and neurofilaments. 

In the CNS, myelin is formed by oligodendrocytes. This has many similarities but also points of difference with respect to myelination in the PNS. CNS nerve fibers are not separated by connective tissue nor are they surrounded by cell cytoplasm, and specific glial nuclei are not obviously associated with particular myelinated fibers. CNS myelin is a spiral structure similar to PNS myelin it has an inner mesaxon and an outer mesaxon that ends in a loop, or tongue, of glial cytoplasm (Fig. 4-3). Unlike peripheral nerve, where the sheath is surrounded by Schwann cell cytoplasm on the inside and outside (Fig. 4-10), the cytoplasmic tongue in the CNS is restricted to a small... 

Not all neurons have NFs. Indeed, one entire phylum in the animal kingdom, arthropods, expresses only type V nuclear lamins so arthropod cells have no IF cytoskeletal structures at all. In addition, mature oligodendrocytes lack IFs although their embryonic precursors contain vimentin. Clearly, the IFs are not essential for cell survival. Yet, in large myelinated fibers, NFs make up the bulk of axonal volume and represent a substantial fraction of the total protein in brain. In most organisms, IFs in both glia... 

Dyck, P.J., O Brien, P.C., Ohnishi, A. (1977). Lead neuropathy 2. Random distribution of segmental demyelination among old intemodes of myelinated fibers. J. Neuropathol. Exp. Neurol. 36 570-5. 

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