Myelinated axon

The propagation velocity in a nerve fiber can be increased by myelination of the axon. For unmyelinated fibers, the propagation velocity increases with the square root of the fiber diameter in thin fibers, it can be as low as 0.5 m/s (Table 5.1). The myelin sheath 

C Unmyelinated 0.5-2 0.5-2 Sympathetic, pain, tickle, crude touch and pressure 

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Figure 3. Electron micrographs of myelinated axons of Xenopus laevis. Upper figure Cross section of axon showing microtubules in groups in association with membrane-bound organelles. Lower figure Longitudinal section of axon showing neurofilaments and microtubules in close proximity to membrane-bound organelles. (Courtesy of Dr. R. Smith.)...

Smith, R.S. (1980). The short term accumulation of axonally transported organelles in the region of localized lesions of single myelinated axons. J. Neurocytol. 9, 39-65. 

Lamina II is also known as the substantia gelatinosa (SG) and can be divided into two layers, the outer layer (IIo) and the inner layer (Ili). This layer is densely packed with small neurons and lacks myelinated axons. Neurons with cell bodies in Hi receive inputs from low-threshold mechanoreceptive primary afferents, while those in IIo respond to inputs from high-threshold and thermoreceptive afferents. The intrinsic cells which comprise the SG are predominantly stalk and islet cells. Stalk cells are found located in lamina IIo, particularly on the border of lamina I, and most of their axons have ramifications in lamina I although some also project to deeper layers. These cells are thought to predominantly relay excitatory transmission. Islet cells, on the other hand, are located in Hi and have been demonstrated to contain the inhibitory neurotransmitters, y-aminobutyric acid (GABA), glycine and enkephalins in their dendrites. Hence these cells have been proposed to be inhibitory interneurons. 

Figure 4.4 Saltatory conduction. Transmission of electrical impulses in a myelinated axon occurs by way of saltatory conduction. Composed primarily of lipid, the myelin sheath insulates the axon and prevents generation of membrane potentials. Membrane potentials occur only at gaps in the myelin sheath, referred to as the nodes of Ranvier. Therefore, transmission of the impulse, or generation of action potentials, occurs only at the nodes.

The gray matter, which contains the cell bodies of neurons, is on the outer surface of the cerebrum and forms the cerebral cortex. The white matter, composed of the myelinated axons of neurons, is found underlying the cortex in the core of the cerebrum. These axons are bundled together according to function and organized into units referred to as tracts. The three types of tracts in the cerebrum are ... 

The white matter is composed of myelinated axons of neurons. These axons are grouped together according to function to form tracts. Neurons transmitting impulses toward the brain in the ascending tracts carry sensory information. Those transmitting impulses away from the brain in the descending tracts carry motor information. 

Figure 7.1 Cross-sectional view of the spinal cord. In contrast to the brain, the gray matter of the spinal cord is located internally, surrounded by the white matter. The gray matter consists of nerve cell bodies and unmyelinated intemeuron fibers. This component of the spinal cord is divided into three regions the dorsal, lateral, and ventral horns. The white matter consists of bundles of myelinated axons of neurons, or tracts. Each segment of the spinal cord gives rise to a pair of spinal nerves containing afferent and efferent neurons. Afferent neurons enter the spinal cord through the dorsal root and efferent neurons exit it through the ventral root.

FIGURE 1-14 Transverse sections of a myelinated axon (left) and the process of a fibrous astrocyte (right) in dog spinal cord. The axon contains scattered neurotubules and loosely packed neurofilaments interconnected by side-arm material. The astrocytic process contains a bundle of closely packed filaments with no cross-bridges, flanked by several microtubules. Sometimes, a lumen can be seen within a filament. X60,000. 

Ishibashi, T., Dupree, J. L., Ikenaka, K. et al. A myelin galac-tolipid, sulfatide, is essential for maintenance of channels on myelinated axons but not essential for initial cluster formation. /. Neurosci. 22 6507-6514, 2002. 

It is noteworthy that the axonal degeneration that occurs in the PNS of MAG-null mice is not observed in the CNS, possibly because other CNS myelin proteins enhance axonal stability. These could include PLP and/or CNP, both of which are needed for axonal stability in the CNS where they are present in much higher concentration. In summary, it appears that the most important function of MAG in the PNS is transmitting a signal from Schwann cells to axons that is needed for the stability of myelinated axons, whereas its principal function in the CNS is to transmit a signal in the reverse direction that promotes efficient myelination and oligodendrocyte vitality. 

Scherer, S. S. and Arroyo, E. J. Recent progress on the molecular organization of myelinated axons. /. Peripher. Nerv. Syst. 7,1-12,2002. 

NFH, and to a lesser extent NFM, has a large number of consensus phosphorylation sites for proline-directed kinases in this carboxy-terminal extension (>50 on NFH and >10 on NFM in many species). In large myelinated axons, most, if not all, of these sites are phosphorylated [21, 22]. This phosphorylation of NFH and NFM side-arms alters the charge density on the NF surface, repelling adjacent NFs with similar charge. Such mutual repulsion by the sidearms of NFs is thought to be a major determinant of axonal caliber [24],... 

Schwann cells are responsible for PNS trophic functions that, in CNS, are carried out by both oligodendroglia and astroglia. Schwann cell diseases usually present as disorders of myelination. Axonal degeneration and diminution in axonal diameter may also occur in primary disorders of Schwann cells, as a consequence of loss of Schwann cell trophic support for axons. (See also Chs 4 and 38.)... 

Oligodendrocytes are present in the CNS as well and wrap around axons to form a myelin sheath. Myelin wraps into concentric layers that spiral around the axon. Gaps in the oligodendrocytes are the nodes of Ranvier, where the membrane maintains contact with extracellular fluid. The nodes serve to propagate the action potential in myelinated axons. Schwann cells perform an analogous function, myelinating axons in the peripheral nervous system. Not all neurons are myelinated, but myelination increases the metabolic efficiency of action potentials. Demyelination of neurons produces deficits in neuronal conduction, as is seen in multiple sclerosis. 

Conduction of the action potential in myelinated axons is called saltatory conduction. Because ion flux only occurs at the nodes of Ranvier, the action potential jumps, in effect, from node to node. This provides two advantages, speeding the rate of conduction and reducing the metabolic cost of an action potential, because energy-dependent ion transporters are not needed along myelinated segments. 

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