Unmyelinated axon bundles

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.)...

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. 

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.

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. 

Because longitudinal resistance is inversely proportional to cross-sectional area, impulses are conduoted faster in large-diameter fibers. The squid axon is unmyelinated and exceptionally large (-800 pp) therefore, impulses are conduoted rapidly along it. Contraction of the mantle of a squid, however, is an uncomplicated procedure that does not require a complex sensorimotor system. Perhaps, during evolution, vertebrates developed a complicated input-output system of many fibers oollected in bundles, as shown in Figure 16.2. 

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