Diffusion properties, brain tissues

Nicholson, C. and L. Tao, Diffusion properties of brain tissue measured with electrode methods and prospects for optical analysis, in J. Dirnagl, Optical Imaging of Brain Function and Metabolism, New York Plenum Press, 1993. 

In other applications of CT, orally administered barium sulfate or a water-soluble iodinated CM is used to opacify the GI tract. Xenon, atomic number 54, exhibits similar x-ray absorption properties to those of iodine. It rapidly diffuses across the blood brain barrier after inhalation to saturate different tissues of brain as a function of its lipid solubility. In preliminary investigations (99), xenon gas inhalation prior to brain CT has provided useful information for evaluations of local cerebral blood flow and cerebral tissue abnormalities. Xenon exhibits an anesthetic effect at high concentrations but otherwise is free of physiological effects because of its nonreactive nature. 

Due to their physicochemical properties trace amines can pass the cell membrane to a limited extent by passive diffusion, with the more lipophilic PEA and TRP crossing membranes more readily than the more polar amines TYR. and OCT. In spite of these features, trace amines show a heterogeneous tissue distribution in the vertebrate brain, and for TYR. and OCT storage in synaptic vesicles as well as activity-dependent release have been demonstrated. So far, trace amines have always been found co-localized with monoamine neurotransmitters, and there is no evidence for neurons or synapses exclusively containing trace amines. 

To avoid the difficulties of crossing the endothelium and nonspecific uptake by the liver, DNA can be delivered directly to the target tissue by injection of polyplexes, lipoplexes, or microspheres with encapsulated DNA. Injection of lipoplexes and polyplexes into the desired tissue can be used to achieve in vivo gene transfer. However, transfection depends on the physico-chemical properties of the complexes and the rate of clearance from the tissue, which varies from tissue to tissue and depends on factors such as the lymph supply. Nevertheless, polyplexes have been found to be sufficiently small and stable so as to diffuse throughout the brain ventricular spaces after a local injection injection [14]. 

The decisive new property responsible for the damage to nerve tissue is, however, the acquisition of partial resistance to proteases, since the refolded protein cannot be completely degraded. This leads to protein deposits in the form of amyloid plaques in the nerve cells and finally to cell death through apoptosis and other processes. The amyloid plaques are also called scrapie-associated fibrils (SAP). As a result, the brains of damaged individuals show focal and diffuse degeneration and vacuole formation, which are responsible for the typical spongiform appearance of the TSE brain. 

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