Internode

Host computers. These are the most powerful computers in the system, capable of performing func tions not normally available in other units. They act as the arbitrator unit to route internodal communications. An operator interface is supported and various peripheral devices are coordinated. Computationally intensive tasks, such as optimization or advanced control strategies, are processed here. 

Following initiation of the electrical impulse from the SA node, the impulse travels through the internodal pathways of the specialized atrial conduction system and Bachmann s bundle (Fig. 6-1 j.1 The atrial conducting fibers do not traverse the entire breadth of the left and right atria, as impulse conduction occurs across the internodal pathways, and when the impulse reaches the end of Bachmann s bundle, atrial depolarization spreads as a wave similar to that which occurs upon throwing a... 

An aqueous extract of P. hysterophorus (collected in Puerto Rico) was partitioned into methylene chloride at pH 7, pH 10 and pH 2. Bioassays of the methylene chloride soluble fractions, using the bean second internode bioassay (13), showed that the highest activity was concentrated in the methylene chloride extract at pH 7. Extensive chromatographic purification (flash chromatography, medium pressure LC, preparative TLC) monitored by bioassay led to the isolation of the four sesquiter-... 

American pondweed responded to GA3 and the herbicide sulfometuron within 7 days after exposure (Table I). As with terrestial plants, GA3 caused elongation of shoots (internodes) compared to controls, whereas sulfometuron almost completely blocked elongation since no significant lengthening occurred between day 7 and 14. Hie presence of GA3 did not counteract the inhibition caused by sulfometuron. This is reasonable since... 

The UNSAT-H model user must specify an averaging scheme for the internodal hydraulic and vapor conductivity terms used in soil water calculations. The user must also specify the model node spacing within the soil mass, which may require adjustment by iterative solutions to arrive at a satisfactory numerical analysis. In order to find the correct averaging scheme and node spacing, several... 

An internodal conduction pathway also extends from the SA node and transmits the impulse directly to the atrioventricular (AV) node. This node is located at the base of the right atrium near the interventricular septum, which is the wall of myocardium separating the two ventricles. Because the atria and ventricles are separated from each other by fibrous connective tissue, the electrical impulse cannot spread directly to the ventricles. Instead, the AV node serves as the only pathway through which the impulse can be transmitted to the ventricles. The speed of conduction through the AV node is slowed, resulting in a slight delay (0.1 sec). The cause of this AV nodal delay is partly due to the smaller fibers of the AV node. More importantly, however, fewer gap junctions exist between the cells of the node, which... 

Node of Ranvier The small gap between adjacent segments (internodes) of the myelin sheath. 

The axon emerges from a neuron as a slender thread and frequently does not branch until it nears its target. In contrast to the dendrite and the soma, the axon is myelinated frequently, thus increasing its efficiency as a conducting unit. Myelin, a spirally wrapped membrane (see Ch. 4), is laid down in segments, or internodes, by oligodendrocytes in the CNS and by Schwann cells in the PNS. The naked regions of axon between adjacent myelin internodes are known as nodes of Ranvier (see below). 

Myelinating oligodendrocytes have been studied extensively [5,30] (see Ch. 4). Examination of the CNS during myelinogenesis (Fig. 1-15) reveals connections between the cell body and the myelin sheath [31] however, connections between these elements have never been demonstrated in a normal adult animal, unlike the PNS counterpart, the Schwann cell. In contrast to the Schwann cell (see below), the oligodendrocyte is capable of producing many internodes of myelin simultaneously. It has been estimated that oligodendrocytes in the optic nerve produce... 

FIGURE 1-19 A myelinated PNS axon (A) is surrounded by a Schwann cell nucleus (N). Note the fuzzy basal lamina around the cell, the rich cytoplasm, the inner and outer mesaxons (arrows), the close proximity of the cell to its myelin sheath and the 1 1 (celhmyelin internode) relationship. A process of an endoneurial cell is seen (lower left), and unstained collagen (c) lies in the endoneurial space (white dots). X20,000. 

These loop-shaped terminations of the sheath at the node are called lateral loops. The loops form membrane complexes with the axolemma called transverse bands, whereas myelin in the internodal region is separated from the axon by an extracellular gap of periaxonal space. The transverse bands are helical structures that seal the myelin to the axolemma but provide, by spaces between them, a tortuous path from the extracellular space to the periaxonal space. 

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. 

Myelin deposition in the PNS may result in a single axon having up to 100 myelin layers, and it does not appear that myelin is laid down by a simple rotation of the Schwann cell around the axon. In the CNS, such rotation is precluded by the fact that one glial cell can myelinate several axons. During myelination, there are increases in the length of the internode, the diameter of the axon and the number of myelin layers. Myelin is therefore expanding in all planes at once. Any mechanism to account for this growth must assume that the membrane system is able to expand and contract, and that layers slip over each other. 

Internodal pathways, 5 80 Interparticle forces, 77 800-801 Interpellet axial dispersion, 25 281-283, 288... 

Green alder J versus M internodes Snowshoe hare J has more pinosylvin and pinosylvin methyl ether Clausen etfl/., 1986... 

Paper birch resinifera J growth versus winter-dormant internodes Snowshoe hare J has 25 X concentration of papyriferic acid Reichardtetfll., 1985... 

Particularly valuable and/or vulnerable parts are protected best. Because of their size and seasonal changes, trees provide instructive examples. In green alder, Alnus crispa, defense compounds, such as pinosylvin, pinosylvin methyl ether and papyriferic acid, are concentrated in buds (2.6% of dry weight) and catkins (1.7%), and least abundant in the internodes (0.05%). Snowshoe hares, L. americanus, reject buds and catkins, while they eat second-year internodes (Bryant etu/., 1983 Clausen eta/., 1986). 

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