Meninges are the singular meninx. There are three membranous envelopes, which are called pia mater, arachnoid, and dura mater. These surround the spinal cord and brain. The cerebrospinal fluid fills the brain ventricles and the space between the arachnoid and the pia mater. The function of the meninges and of the cerebrospinal fluid is major to protect the central nervous system.
Structure of Meninges
The meningeal envelope that tightly adheres to the surface of the spinal cord and brain is known as the pia mater. It's a thin membrane made up of fibrous tissue that's filled on the outside with a flat cell layer that's considered to be fluid-tight. And, the pia mater is pierced by the blood vessels, which travel to the spinal cord and brain.
Over the pia mater and also separated from it by space known as subarachnoid space is the arachnoid, which is a thin and transparent membrane. It is made up of fibrous tissue and, like the pia mater, which is surrounded by flat cells, is considered to be fluid impervious. The Arachnoid doesn't follow the surface convolutions of the brain, and so it looks like a loosely fitting sac.
A large number of fine filaments known as arachnoid trabeculae move from the arachnoid through the subarachnoid space to blend with pia mater tissue in the brain region, in particular. These arachnoid trabeculae are embryological remnants of the arachnoid and pia mater's common roots, and they have the fragile structure that these two meninges are notorious for. Leptomeninges are the arachnoid and pia mater combined.
The outermost of the three meninges is known as the dura mater (also called pachymeninx), which is a thick, strong, and dense membrane. The dura mater is made up of dense fibrous tissue, with flattened cells similar to those found on the arachnoid and pia mater surfaces covering the inner surface. The dura mater is a sac that surrounds the arachnoid and has been adapted to perform a variety of functions. The dura mater supports and surrounds the large venous channels (also called dural sinuses) carrying blood from the brain towards the heart.
Dura Mater Infolding
The dura mater can be partitioned into many septa that support the meninges of the brain. The falx cerebri is one of these, which is a sickle-shaped partition that lye between the two brain hemispheres. Another one is the tentorium cerebelli that provides a membranous, strong roof over the cerebellum. The falx cerebelli is the third one that projects downward from the tentorium cerebelli between the hemisphere's two cerebellar. The periosteum, or covering, of the inner surfaces of the skull bones is created by the dura mater's outer portion over the brain.
The dura mater, within the vertebral canal, splits into two sheets that are separated by the epidural space, filled with veins. These two sheets' outer constitute the periosteum of the vertebral canal. The inner sheet can be separated from the arachnoid by the narrow subdural space filled with fluid. In some places, the subdural space is absent, and intimately, the arachnoid is fused with the dura mater.
The walls of vast venous channels of the dura mater, where the elongations of the arachnoid, like fingers, penetrate both the dura mater and project into veins, are the most significant region of fusion between these two meninges. The cerebrospinal fluid passes through these fingerlike arachnoid processes, known as arachnoid villi or arachnoid granulations, on its way from the subarachnoid space to the dural sinuses. Often, spinal anaesthetics are introduced into the subarachnoid space.
The meninges' middle element is given as either the arachnoid mater or arachnoid membrane. It is so named due to its resemblance to a spider web. It also cushions the central nervous system meninges. This transparent and thin membrane is composed of fibrous tissue and, like the pia mater, holds an outer layer of tightly packed flat cells by forming the arachnoid barrier.
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The above meninges diagram represents the brain layers.
Since the arachnoid does not obey the brain's surface convolutions, it appears to be a loosely fitting sac. In contrast, in the brain region, a huge count of fine filaments known as arachnoid trabeculae passes from the arachnoid via subarachnoid space to blend with the pia mater tissue. The arachnoid barrier has no extracellular collagen and is thought to be a functional and morphological meningeal barrier between the subarachnoid space and cerebrospinal fluid, as well as blood circulation in the dura.
The pia mater and the arachnoid together are at times called leptomeninges, literally "thin meninges." The acute meningococcal meningitis may lead to an exudate within the leptomeninges along with the meninges of the brain surface. The arachnoid is structurally continuous with the pia since it is bound to it by cobweb-like strands, hence the names leptomeninges and pia-arachnoid. They are also responsible for the beta-trace protein production (prostaglandin D2 synthase), which is a major cerebrospinal fluid protein.