Arachnoid (brain): anatomy, functions and associated disorders.

This intermediate meninx that is basic to our survival.

The meninges are a series of membranes that, together with the skull and spinal column, protect the central nervous system.They prevent minor blows or injuries from altering its functioning or destroying it completely.

In addition, they allow the brain to be in place and, because there are different types of pain receptors in them, we can perceive that there is some kind of problem or injury. In this article we are going to explore the arachnoid membrane in order to analyze what it is, its particularities and functions.

• Recommended article: "Parts of the human brain (and their functions)".

Arachnoid: the intermediate meninx

The arachnoid, together with the dura mater and the pia mater, is one of the three meninges.. These are membranes that protect our brain and spinal cord from external injury and play an important role in our survival. They are located one below the other, forming three small protective layers.

In the case of the arachnoid, we are dealing with the intermediate membrane, being in contact with the dura mater and pia mater. It is the most fragile and prone to rupture of the three. Another of its main characteristics is that it is not vascularized.

Although they are in close contact, the arachnoid is separated from the dura mater by the subdural space, which is more a thin layer of cells between which there is interstitial fluid than a space. With respect to the pia mater, it is separated from it by the subarachnoid space, and in turn is connected to it by the arachnoid trabeculae.

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

Main components

Observing the arachnoid we can identify the existence in them of different layers or parts.

1. Arachnoid layer or arachnoid barrier layer

It corresponds to the part of the arachnoid that is in contact with the dura mater.. Its cells are tightly bound and hardly allow the passage of interstitial fluid, being the most resistant part of the arachnoid. This layer prevents ions and molecules from entering or leaving. However, a series of arachnoid granulations or villi can be distinguished in it, through which they connect with the existing veins in the dura mater, which allows the cerebrospinal fluid to be expelled at the end of its cycle.

2. Arachnoid trabeculae or arachnoid reticular layer

The cells of the arachnoidal barrier layer project into the pia mater, forming a network that crosses the subarachnoid space, which in turn forms a network or meshwork that in fact gives its name to the meninx (due to its resemblance to a spider's web). which in turn forms a network or meshwork that in fact gives its name to the meninx (due to its resemblance to a spider's web). Within these projections we find network fibers, anchoring fibers and microfibers. The exact function of the trabeculae is not yet fully known, although it is speculated that they are capable of sensing the pressure caused by the cerebrospinal fluid.

3. Subarachnoid space

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

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

Main functions

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

1. Protects the nervous system

In spite of being relatively fragile, the arachnoid together with the rest of the meninges allow the brain and spinal cord to be protected against blows and injuries, as well as contamination and infection.as well as contamination and infection by harmful agents.

2. Distribution of cerebrospinal fluid

The cerebrospinal fluid circulates in the arachnoid and in the different cisterns of the subarachnoid space.The protein is an essential element to keep the neurons of the nervous system alive by nourishing them and at the same time allowing the elimination of the waste products of brain function.

3. Vascular system-cerebrospinal fluid connection

The cerebrospinal fluid transports the waste products of brain activity, but it is necessary to expel them.. This is done through the Blood of the veins of the dura mater, with which the arachnoid communicates. It also prevents excessive accumulation of cerebrospinal fluid, which is constantly being secreted.

4. It allows the brain to float

The fact that the cerebrospinal fluid circulates inside the brain allows the brain to float in a certain way.This reduces its weight and allows it to maintain its morphology.

5. Perception of intracranial pressure

Although this is not completely known, it is suspected to be the arachnoid trabeculae. which allow the body to detect increases in intracranial pressure.

Associated disorders

There are several affectations that are linked to alterations in the arachnoid or other meninges. Among these alterations we can find the following.

1. Arachnoid cyst

Small cystic structures that fill with cerebrospinal fluid may form within the arachnoid. Although they may not cause problems, it is possible that they generate a pressure that damages the nervous system. Headaches, sensory problems, paresthesia or paralysis are common.

2. Meningitis

Both the arachnoid and the rest of the meninges may succumb to a viral or bacterial infection, becoming inflamed and causing different symptoms such as dizziness, headaches or weakness. The composition of the cerebrospinal fluid that circulates through the arachnoid may be altered, as well as causing compression of the brain and spinal cord.as well as causing compression of the encephalon.

3. Communicating hydrocephalus

This is a disorder in which cerebrospinal fluid accumulates inside the skull, in this case due to the fact that the parts of the arachnoid that allow communication between the arachnoid and the blood veins do not work properly.In this case due to the fact that the parts of the arachnoid that allow communication between the arachnoid and the blood from the veins do not work properly, accumulating too much fluid and not reinserting it into the blood.

4. Subarachnoid hemorrhage

It occurs when due to a disease or injury (such as for example due to a cranioencephalic trauma)blood enters and floods the subarachnoid space. It can be fatal. Headache, altered consciousness and gastrointestinal problems such as nausea and vomiting are common.