Arachnoids (brain): anatomy, functions and associated disorders

The meninges are a series of membranes that, along with the skull and spine, protect the central nervous system, So that minor blows or injuries cannot alter their functioning or destroy them completely.

On top of that, they keep the brain in place, and since they contain different types of pain receptors, we can perceive that there is some sort of problem or injury. In this article, we will explore the arachnoid membrane in order to analyze what it is, its peculiarities and its functions.

    Arachnoids: the intermediate meninges

    The arachnoid, along with duramater and piamater, is one of the three meninges. These are membranes that protect our brain and spinal cord from damage from the outside and play an important role in our survival. They are placed one below the other, forming three small protective layers.

    In the case of the arachnoid, we are in front of the intermediate membrane, in contact with the duramater and the piamater. He turns out to be the most fragile and the most prone to come out of the three. Another of its main characteristics is that it is not vascularized.

    Although they are in close contact, the arachnoid separates from the dura through the subdural space, which more than a space is a thin layer of cells through which there is interstitial fluid. As for the piamadre, it separates from it by the subarachnoid space and in turn connects with it by the arachnoid trabeculae.

    One of the main aspects that sets it apart from the other two meninges is the fact that it contains the subarachnoid space, through which cerebrospinal fluid circulates.

    main components

    By observing the arachnoid, we can identify the existence in them of different layers or parts.

    1. Arachnoid or arachnoid barrier layer

    It corresponds to the part of the arachnoid which is in contact with the dura. Their cells are very close together and barely allow the passage of interstitial fluid, being the most resistant part of the arachnoid. This layer prevents ions and molecules from entering or leaving. Nevertheless, in her a series of granulations or arachnoid villi is distinguished by which they are connected to the existing veins to the duramater, something that allows the cerebrospinal fluid to be expelled at the end of its cycle.

    2.arachnoid trabeculae or reticular arachnoid layer

    The cells of the arachnoid barrier layer project towards the piamadre, forming a network that crosses the subarachnoid space which in turn forms a net or mesh which actually gives the meninge its name (due to its resemblance to a spider’s web). In these projections we find mesh fibers, anchor fibers and microfibers. The exact function of trabeculae is not yet fully understood, although it is believed that they are able to sense pressure caused by cerebrospinal fluid.

    3. Subarachnoid space

    Although more than one part of the arachnoid is a space between its laminae, the subarachnoid space is one of the most important parts of the arachnoid. This is so because cerebrospinal fluid passes through it. In this space we can also find a series of important cerebral pits and cisterns in which cerebrospinal fluid accumulates and which allow its distribution.

    In addition to the brain itself, an orbital subarachnoid space can be found around the optic nerve.

    main duties

    The arachnoid is a membrane which, like the other meninges, has several functions that allow and promote our survival.

    1. Protects the nervous system

    Despite being relatively fragile, the arachnoid next to the rest of the meninges helps protect the brain and spinal cord from blows and injuries, As well as pollution and infection by harmful agents.

    2. Distribution of cerebrospinal fluid

    It is in the arachnoid and in the various cisterns of the subarachnoid space that the cerebrospinal fluid circulates., An essential element to keep the neurons of the nervous system alive by nourishing them and thus allowing the elimination of wastes from cerebral functioning.

    3. Vascular-cerebrospinal fluid connection

    Cerebrospinal fluid carries waste from brain activity, but it must be expelled. This is done through the blood in the veins of the dura mater, with which the arachnoid communicates. It also prevents the buildup of excess cerebrospinal fluid, which continues to be secreted.

    4. It allows the brain to float

    The fact that cerebrospinal fluid circulates inside allows the brain to float in a certain way., So that its weight decreases and its morphology is maintained.

    5. Perception of intracranial pressure

    Although this is something that is not fully known, it is believed to be the arachnoid trabeculae. those that allow the body to detect increases in intracranial pressure.

    associated disorders

    There are several conditions associated with alterations of the arachnoid or other meninges. Among these modifications we can find the following.

    1. Arachnoid cyst

    Inside the arachnoid, they can form small cystic structures that fill with cerebrospinal fluid. Although they do not cause problems, they can generate pressure which damages the nervous system. Headaches, sensitivity problems, paresthesias or paralysis are common.

    2. Meningitis

    The arachnoid and the rest of the meninges may succumb to a viral or bacterial infection, inflamed and causing various symptoms such as dizziness, headache, or weakness. It can change the composition of the cerebrospinal fluid flowing through the arachnoid, In addition to causing compression of the brain.

    3. Communicate hydrocephalus

    It is a disorder in which cerebrospinal fluid collects inside the skull, In this case because the parts of the arachnoid that allow communication between it and the blood in the veins are not functioning properly, accumulating too much fluid and not reinserting into the blood.

    4. Subarachnoid hemorrhage

    Occurs due to illness or injury (for example due to head trauma)The blood enters and floods the subarachnoid space. It can be fatal. Headaches, altered consciousness, and gastrointestinal problems such as nausea and vomiting are common.

    Bibliographical references:

    • Kandel, ER; Schwartz, JH; Jessell, TM (2001). Principles of neuroscience. Madrid: McGraw Hill.
    • Martinez, F .; Tomorrow, G .; Panuncio, A. and Laza, S. (2008). Anatomo-clinical examination of the meninges and intracranial spaces with particular reference to chronic subdural hematoma. Mexican Journal of Neuroscience; 9 (1): 47-60

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