Myelinated axon bundles

In addition to vertical bundles of myelinated axons, the cerebral cortex of monkeys (e.g., DeFelipe et al., 1990) and of humans (e.g., del Rio and DeFelipe, 1995) also contains vertically oriented bundles of unmyelinated axons that are referred to as horsetails. These horsetails are the axonal plexuses of the inhibitory double bouquet cells and can be demonstrated in monkey neocortex by immunolabeling with antibodies to calbindin and tachykinin. As shown by DeFelipe et al. (1990), in the monkey these axonal bundles are widespread and form a regular columnar system descending from layer 2 to layers 3-5. The bundles are most evident in tangential sections taken at the level of layer 3, where they can be seen to have a center-to-center spacing of 15-30 fim. In a later study of the calbindin labeled double bouquet cells in monkey striate cortex, Peters and Sethares (1997) showed that there is one double bouquet cell, and therefore one vertically oriented double bouquet cell axonal plexus, or horsetail, per pyramidal cell module (Fig. 7). Within layer 2/3 the double bouquet axons run alongside the apical dendritic clusters, while in layer 4C they are closely associated with the vertical myelinated axonal bundles. DeFelipe et al. (1989 1990) proposed that the axon terminals of the double bouquet cell synapse with the shafts and spines of basal dendrites and oblique shafts of apical dendrites of pyramidal cells, but the exact role of these vertical bundles of inhibitory axons is not known. It is likely that they constitute a vertical inhibitory system that acts upon pyramidal cells within the minicolumns. 

In a recent review article on the anatomy of autism Amaral et al. (2008) point out that in these studies by Casanova and his colleagues, only 14 cases of autism, 9 of which had seizures and at least 10 with mental retardation, have been examined for minicolumn pathology. Consequently, more studies using a greater number of autistic brains with fewer other complications need to be carried out before any definite conclusions can be reached about changes that can only be attributed to autism. It would also be appropriate to examine brains in which the apical dendritic clusters and myelinated axon bundles have been stained to confirm the sizes of the minicolumns as detected in digitized images from autistic brains. 

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 ... 

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. 

Figure 14.1 A diagram, based on an electron micrograph, of a cross section of a nerve bundle. It shows three axons enclosed in myelin sheaths. The sheaths completely surround the axoplasm. Between the myelinated axons are clusters of nonmyelinated axons. Except for smaller diameter and absence of myelination they are structurally similar to myelinated axons. (Drawn from an electron micrograph of Telford Bridgman, 1995.)...

Woodward, Albert, Joan Epstein, Joseph Gfroerer, Daniel Melnick, Richard Thoreson, and Douglas Wilson. 1997. "The Drug Abuse Treatment Gap Recent Estimates." Health Care Financing Review 18 5-17-Yeomans, John S. 1989. "Two Substrates for Medial Forebrain Bundle Self-Stimulation Myelinated Axons and Dopamine Axons." Neuroscience and Biobehavioral Reviews 13 91-98. 

Fig. 6 Monkey primary visual cortex transverse section taken at the level of layer 5 and stained to show the regularly spaced bundles of myelinated axons (arrows). Scale bar = 25 xm...

Lohmann and Koppen (1995) have also shown that the dendritic clusters and the vertical axonal bundles in rat visual cortex originate from the same neurons, and that the axonal bundles and the dendritic clusters have similar center-to-center spacing. Recently Casanova et al. (2008) have concluded that in the human cortex the pyramidal cell arrays and the vertical bundles of myelinated axons have similar spacing. 

It is evident that the modules of pyramidal cells whose apical dendrites form clusters and the vertical bundles of myelinated axons are facets of the same basic, modular organization of neurons into vertical units that we can refer to as minicolumns. And... 

The central nerv ous system of insects consists essentially of a double nerve cord situated ventrally and punctuated by segmental ganglia from which the peripheral ner es arise. The axon of such a nerve measures up to 10 /xm in diameter and is enclosed in a thin non-myelinated lipoprotein sheath. These axons are bundled into nerves which are surrounded by dove-tailed layers of neuroglial cells, and the whole is enclosed by a protein lamella. The polarization of a resting nerve is very similar to that of vertebrate nerve (see Section 7.5.1). On electrical stimulation, successive spikes can be obtained at intervals of a millisecond but the action potentials are propagated only at about 2 m s , i.e. some 50 times slower than in the larger myelinated axons of vertebrates. 

CARS microscopy was used for the study of axonal myelin under physiological conditions in spinal cord, using white matter strips isolated from the spinal cord of guinea pigs and kept alive in oxygen-bubbled Krebs solution. Both forward-and epi-detected CARS were used to probe the axon bundles and resolve detailed structures such as nodes of Ranvier. In addition, simultaneous CARS microscopy of myelin and TEPF imaging of intra- and extra-axonal calcium was demonstrated [58]. 

Myelination in the PNS is preceded by invasion of the nerve bundle by Schwann cells, rapid multiplication of these cells and segregation of the individual axons by Schwann cell processes. Smaller axons (<1 pm), which will remain unmyelinated, are segregated several may be surrounded by one Schwann cell, each within its own pocket, similarly to the single axon shown in Figure 4-10A. Large axons (>1 pm) destined for myelination are enclosed singly, one cell per axon per internode. These cells line up along the axons with intervals between them the intervals become the nodes of Ranvier. 

The exact reason for the differential susceptibility of nerve fibers based on their axonal diameter is not known. One possible explanation is that the anesthetic is able to affect a critical length of the axon more quickly in unmyelinated fibers, or small myelinated neurons with nodes of Ranvier that are spaced closely together compared to larger fibers where the nodes are farther apart.17 As indicated earlier, a specific length of the axon must be affected by the anesthetic so that action potentials cannot be transmitted past the point of blockade. Other factors such as the firing rate of each axon or the position of the axon in the nerve bundle (e.g., in the outer part of the bundle versus buried toward the center of the nerve) may also affect susceptibility to local anesthesia.62 In any event, from a clinical perspective the smaller-diameter fibers appear to be affected first, although the exact reasons for this phenomenon remain to be determined. 

Figure 30.2. Peripheral nerve. This diagram shows a peripheral nerve in cross section. The nerve contains three bundles (fascicles), with each fascicle containing a mixture of myelinated and unmyelinated axons. (From H. H. Schaumburg et al. Disorders of Peripheral Nerves, F.A. Davis Co., Philadelphia, 1983.)...

A nerve is a collection of nerve fibers (fascicles). Axons must be electrically isolated from each other to maintain channel separation. Each axon, naked or with myelin, is therefore spirally enwrapped by a thin sheath or membrane (neurilemma). Axons are grouped in a bundle (funiculus) surrounded by its own sheath of connective tissue (perineurium). 

In these densely packed synaptic regions - where the terminating fibres branch profusely and lose their myelin cover - there is a sharp increase in the axonal surface area/volume ratio the release of K" and its accumulation extracellularly in association with axonal activity is necessarily likely to be very much greater per unit volume of tissue than in other areas (e g., fibre bundles as in the dorsal columns) ... 

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