Arachnoid membrane

The entire CNS is covered by the meninges, which form a protective covering. The outermost is the dura, which is tough and leathery in consistency. It is highly vascularized and innervated, so it is sensitive to pain. The arachnoid membrane is a weblike, spongy layer beneath the dura. Beneath the arachnoid is the subarachnoid space, which is filled with cerebrospinal fluid. Beneath the subarachnoid space is a thin layer of cells called the pia, which covers the brain and spinal cord. Ventricular System... 

The exterior of the brain and spinal cord is cushioned within the skull and backbone by the cerebral-spinal fluid (CSF). The CSF is kept from direct contact with the brain by the arachnoid membrane covering the surface and the choroid plexus, tissue which selectively interacts with the contents of the CSF. An injection into the spinal cord or through the skull can put a drug directly into contact with the choroid plexus and the brain. For more on this subject you can read "The Mammalian Choroid Plexus" by Spector and Johanson Scientific American November 1989. 

Intrathecal A form of intraspinal anaesthesia or analgesia in which the agent is injected through the dura mater and arachnoid membrane into the cerebro-spinal fluid which surrounds the spinal cord. 

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... 

Finally, the avascular arachnoid membrane, underlying the dura and completely enclosing the CNS, completes the seal between the ECF of the CNS and that of the rest of the body. 

Although the cells of the arachnoid membrane have tight junctions (TJ) between them, due to its avascular nature and relatively small surface area, the arachnoid membrane does not represent a significant potential interface for exchange between the blood and the CNS (Figure 27.1) [6],... 

The BBBs to macromolecules and most polar solutes are created by the formation of tight junctions between the cerebral endothelial cells, the choroid plexus epithelial cells, and the cells of the arachnoid membrane. These tight junctions are a key feature of the BBB and effectively abolish any aqueous paracellular diffusional pathways between the endothelial cells from the blood plasma, or somatic ECF, to the brain ECF. This removal of the paracellular pathway efficiently prevents the free diffusion of polar solutes from blood to brain [11],... 

Central neural blockade. The anesthetic is injected within the spaces surrounding the spinal cord10 (Fig. 12-2). Specifically, the term epidural nerve blockade refers to injection of the drug into the epidural space—that is, the space between the bony vertebral column and the dura mater. A variation of epidural administration known as a caudal block is sometimes performed by injecting the local anesthetic into the lumbar epidural space via the sacral hiatus (see Fig. 12-2). Spinal nerve blockade refers to injection within the subarachnoid space— that is, the space between the arachnoid membrane and the pia mater. Spinal blockade is also referred to as intrathecal anesthesia because the drug is injected within the tissue sheaths surrounding the spinal cord (intrathecal means within a sheath see Chapter 2). 

Spinal nerve block Administration of local anesthesia into the spinal canal between the arachnoid membrane and the pia mater (i.e., the subarachnoid space). 

Johanson CE (1988) The choroid plexus-arachnoid membrane-cerebrospinal fluid system, hi Neuromethods The Neuronal Microenvironment (Boulton A A, Baker GB, Walz W, eds), pp 33—104. Clifton, NJ The Humana Press. 

A cyst that is located in the posterior or lateral epidural space in the spinal canal and that is lined only by fibrous tissue resembling dura and lacking arachnoid membrane is a meningeal cyst or diverticulum (see Table 20.12). A subdural or subarachnoid cyst that has a thinner wall than the epidural cyst and that protrudes toward brain or spinal cord is an arachnoid cyst. Reactivity for vimen-tin, progesterone receptors, and EMA is common. This inununoreactivity resembles that of arachnoid granulations and meningiomas. Other cysts have variable thickness and are more difficult to categorize. 

The BCSFB constitutes an additional compartment in the CNS which drugs must traverse. The BCSFB is comprised of the choroid plexus and arachnoid membranes which consist of choroid and arachnoid epithelium, respectively, as their major cell constituents. These structures interface with nearly all major... 

The cerebrospinal fluid (CSF) fills the subarachnoid space between the arachnoid membranes and the pia mater called leptomeninges which protect the Central Nervous System -CNS- (encephalon and spinal cord). Seventy percent of the CSF is formed in secretory structures called choroid plexi and the remaining 30% is produced from the cerebral capillaries (Carpenter, 1985). 

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