Arachnoid barrier

The exit of drugs from the CNS can involve (1) diffusion across the blood-brain barrier in the reverse direction at rates determined by the lipid solubility and degree of ionization of the drug, (2) drainage from the cerebrospinal fluid (CSP) into the dural blood sinuses by flowing through the wide channels of the arachnoid villi, and (2) active transport of certain organic anions and cations from the CSF to blood across the choroid plexuses... 

A blood-CSF barrier dysfunction (i.e., pathologically reduced CSF flow) can have different causes reduced CSF production rate, restricted flow in the subarachnoid space, or restricted passage through the arachnoid villi (F2, R5). 

Tire brain, which must function in a chemically stable environment, is protected by a tough outer covering, the arachnoid membrane, and by the blood-brain barrier406 407 and the blood-cerebrospinal barrier. Both of these barriers consist of tight junctions similar to those seen in Fig. 1-15A. They are formed between the endothelial cells of the cerebral capillaries and between the epithelial cells that surround the capillaries of the choroid plexus. The choroid plexus consists of capillary beds around portions... 

It seems reasonable to assume that the mechanisms may be similar to that postulated for tetracyclines, which reduce cerebrospinal fluid absorption, possibly by an effect on cyclic adenosine monophosphate at the arachnoid villi (6). Minocycline crosses the blood-brain barrier more effectively than other tetracyclines, because of its greater lipid solubility. Therefore, a physician who prescribes minocycline should keep his eye on the patient s eyes. 

In addition to the blood-brain barrier, two other barrier layers limit and regulate molecular exchange at the interface between the blood and the neural tissue and its fiuid spaces the choroid plexus epithelium between blood and ventricular CSF and the arachnoid epithelium between blood and subarachnoid CSF. These CNS barriers perform a number of functions such as the ionic homeostasis, the restriction of small molecule permeation, the specific transport of small molecules required to enter or leave the brain, the restriction and regulation of large molecule traffic by reducing the fluid-phase endocytosis via pinocytotic vesicles, the separation of peripheral and central neurotransmitter pools, and the immune privilege [16]. 

The extracellular space ofthe brain can be divided into two major compartments, the CSF and the interstitial fluid (ISF). The CSF and the ISF are separated from the blood by the choroid plexus or the BCS FB and the brain capillary or BBB, respectively. No anatomical barrier exists between the CSF and the ISF a functional barrier is built up by the flow of CSF from its formation site (choroid plexus) to its absorption site (arachnoid villi) [15]. In the case of a human brain, 20 ml CSF is produced per hour and the complete turnover ofthe total 100 ml CS F occurs approximately within 4—5 h, whereas only 2 ml ISF is renewed per hour compared to the total amount of 300 ml ISF [17, 18]. Neurons are bathed by the extracellular (or interstitial) fluid of the brain (ECF = ISF) that forms the microenvironment ofthe CNS [19]. ISF and CSF are low-protein fluids (plasma CSF ratio 260) due to the tightness of the CNS barrier layers [20] furthermore, the brain has no true lymph or lymphatics. 

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