Meninges Cerebrales: Layers and Spaces (with Images)

The Cerebral meninges Are membranes of connective tissue (the one that serves as support) and that surround the brain and spinal cord.

They are located under the skull, and have a soft structure whose functions are to support For cerebral and cranial vascularization, offering adequate blood supply, and p Protect the Central Nervous System Of possible damages, mainly through the cerebrospinal fluid.

He brain and the spinal cord Are the most protected structures of the whole organism, and meninges contribute in large part to achieving this goal.

The word"meninges"comes from the Greek"mēninx", which means"membrane". In mammals, they are composed of three layers: the dura mater, the arachnoid and the pia mater.

These layers have two interspaces called the subarachnoid space, which contains the cerebrospinal fluid, and subdural space.

It is possible to distinguish between cerebral and spinal meninges, depending on the area involved. However, here we focus on those that cover the brain.

Layers of the cerebral meninges

There are three different layers, which are presented here from the outermost (closer to the skull) to the innermost (closer to the brain).

Dura mater

It is located just below the skull, and is a hollow cylinder composed of strong, thick, strong fibrous tissue.

It covers from the encephalon to the second or third sacral vertebra of the spinal cord. When we refer to the dura that surrounds the spinal cord is called the dural sac.

Born of Mesoderm , One of the layers of cells that constitute the embryo, beginning to form approximately at 3 weeks of gestation.

The dura has two layers or lamellae: the superficial layer or periosteum that is closest to the skull, and the meningeal layer, which is considered the dura mater itself.

The periosteum consists of a connective tissue membrane that is in contact with the internal bone surface of the skull (called the endocranium), and contains the venous sinuses. These are venous ducts coated with Endothelium , The tissue that envelops all the blood vessels, which are fixed in the skull and carry the blood from the brain to the heart.

The dura has four areas of invagination (internal refolding):

- Sickle of the brain or falx cerebri: Is the largest, and consists of a membrane that lies vertically between the left hemisphere And right brain. It starts at the front ridge of the frontal bone To the internal occipital protuberance (a small subsidence in the Occipital bone )

- The store of the cerebellum or cerebellar tentorium: This part of the dura is the second largest, has a crescent shape and separates the cerebellum of the occipital lobe of the brain. More specifically, it supports said lobe and envelopes the surface of the cerebellum.

It receives that name because it joins the sickle of the brain adopting a form similar to a tent or tent.

- Cerebellar sore or vertical invagination , Which is located at the bottom of the cerebellum store and separates the two cerebellar hemispheres.

- Diaphragm of the chair: Is the smallest invagination of all. It is in the Turkish chair , A hollow located at the base of the skull that houses the pituitary gland (also called Pituitary )

Arachnoid

In contact with the dura is the arachnoid layer, whose embryonic origin is in the Neural crest From the Ectoderm .

It is the middle part of the meninges, and also the most delicate. Its name is because its texture resembles the fabric of a spider.

It consists of a transparent and thin membrane of fibrous tissue, where there are abundant flat cells impervious to liquids. Apparently, the main function of the arachnoid is cushioning the central nervous system to prevent damage.

It consists of a homogeneous sheet of barrier cells, which is considered the arachnoid itself, and an internal layer of large meshes called the subarachnoid space, which is the one containing the cerebrospinal fluid (CSF).

Sometimes the arachnoid and pia mater, the next layer, are considered a single structure called Leptomeninges , Which comes from the Greek and means"thin meninges".

Embryologically both layers appear to have a common precursor, and the remains of this precursor appear to be arachnoid trabeculae.

The arachnoid trabeculae are delicate strands of connective tissue that connect the arachnoid layer and the pia mater, and are located within the subarachnoid space.

Pia mater

Or simply sink, is the layer closest to the brain. Its name comes from Latin and translates as"tender mother". Like the arachnoid layer, it is derived from the neural crest in our embryonic development.

It is a very thin layer and has large numbers of blood and lymph vessels. It adapts to the convolutions of the brain, its fissures and grooves. In this way, it allows the blood vessels to penetrate throughout this organ, nourishing it.

The pia mater covers virtually the entire brain surface. It is absent only in the natural openings between the ventricles, the median opening, or Magendie's hole , And the natural opening (also called Luschka's hole).

It joins the brain through the Astrocytes , Glial cells Or support, which perform various tasks such as the optimal maintenance of the extracellular space.

Its fibrous tissue is impervious to fluids, so it is able to retain the cerebrospinal fluid.

Thus, it exerts a blood-brain barrier, separating the cerebrospinal fluid from the blood. In this way it allows to limit the amount of sodium, chlorine and potassium; In addition to preventing the entry of proteins present in blood plasma or other non-organic molecules.

This barrier is a highly specialized tissue, which isolates certain fluids from others and, at the same time, establishes selective communication with them.

It also has a buffering and protective function of the nervous system. In the spinal cord, it helps to avoid deformation of this by compression.

Between the pia mater and surrounding blood vessels is the perivascular space (or Virchow-Robin space) that is less than 2 millimeters in size and part of the lymphatic system of the brain.

This system is what transports the lymph to the heart and has several functions, such as the formation and activation of the immune system or absorb nutrients from the digestive system.

The piamadre joins with the epidermis, a membrane that lines the inside of the cerebral ventricles, cavities that are inside our brain and are filled with cerebrospinal fluid.

Main spaces of the cerebral meninges

The membranes that make up the cerebral meninges have already been described, but between these different layers there are a series of spaces that are detailed below:

Subdural space

It is a virtual space that is located between the dura mater and the arachnoid layer. It is said"virtual"because, in healthy individuals, both layers are held together by capillaries without spaces.

The subdural space begins to exist once these membranes are detached by some contusion or damage. Normally, for a Subdural hematoma Or accumulation of blood between the dura and the arachnoid due to rupture of a blood vessel.

Subarachnoid space

It is also called the leptomeningeal space, and is a fine space that exists between the arachnoid membrane and the pia mater. It contains arachnoid cords, as well as nerve and vascular structures.

There are certain places where these spaces are wider and communicate with each other, called subarachnoid cisterns.

There is a great variety of these, classified in the literature according to their location. For example, Roldán Valadez et al. (2005) identify and describe 15 different cisterns. These also communicate with the cerebral ventricles through the medial and lateral foramina.

Through the subarachnoid space, its cisterns, and the ventricles, the cerebrospinal fluid or cerebrospinal fluid circulates.

It consists of a transparent liquid that is sometimes considered as a fourth layer of the meninges, since it serves as protection for nervous system as other membranes.

However, it is responsible for many other functions such as lubricating, nourishing and contributing to the electrical activity of the nervous system.

Approximately 150 milliliters of this liquid is always circulating in our organism, being renewed every 3 or 4 hours. Up to about 500 milliliters of this liquid is produced.

Small choroidal structures are generated in the choroid plexuses that are located in the cerebral ventricles. Although pia mater and other ependymal surfaces also secrete cerebrospinal fluid.

Approximately one third of this comes from the pia mater and the arachnoid membranes. Another small amount originates in the brain itself, through the perivascular spaces that envelop the blood vessels.

The fluid first flows through the ventricles of the brain, then passes through one of the cisterns, called cisterna magna. This flows into the subarachnoid space and from there travels to the brain.

For its elimination, it passes through the arachnoid villi, which communicate with the vascular sinuses of the dura mater. These breasts connect to the bloodstream.

Subpial space

It is also a virtual space, which lies between the pia mater and the glial limiting membrane. There may be bleeding at this level that separates the pia mater from the brain.

Diseases associated with cerebral meninges

Meninges are associated with a number of brain pathologies, commonly with infections such as meningitis and intracranial haemorrhages.

Let's list the most important diseases that are linked to brain meninges:

Meningitis

The meningitis Is an inflammation of the meninges caused by bacteria, viruses, fungi or parasites, although the most common are bacterial.

Most cases occur in children and adolescents, and cause significant brain damage. It is a contagious disease that is transmitted by respiratory secretions, being more likely to contract if it has a close and lasting contact with an affected person.

If left untreated, it can be fatal in 50% of cases (World Health Organization). Fortunately, early treatment with antibiotics can slow this condition down.

The most frequent symptoms are fever, Headaches , Cognitive impairment , Nausea, vomiting, muscle weakness, extreme sensitivity to light and sounds, stiff neck and skin rashes.

Bleeding

There are three types of bleeding that can occur in the meninges:

- Epidural hematoma: Occurs when blood accumulates between the skull and the dura, but can also originate in the spine.

It is usually due to physical trauma that causes a rupture of the middle meningeal artery, causing bleeding that increases intracranial pressure.

It is a medical emergency because it can lead to death. In fact, between 15 and 20% of patients with this type of bruising die.

The main symptoms are coma and drowsiness.

- Subdural hematoma: It is a blood accumulation between the dura and the arachnoid layer. It arises from the rupture of the blood vessels that unite both layers, usually by some serious contusion like a traffic accident or falls.

This bleeding can increase intracranial pressure, so in severe cases it can be life threatening. Above all, in people whose blood does not coagulate correctly.

Although some patients require surgical drainage; In other cases, the hematoma stops in time and resolves spontaneously.

The most distinctive symptoms are loss of consciousness, coma, confusion, dizziness, and excessive drowsiness.

- Subarachnoid hemorrhage: In this case, bleeding occurs in the subarachnoid space. This condition is not very frequent, representing between 0.01 and 0.08% of visits to the Emergency (Healthline).

Its main cause is congenital saccular aneurysm, which can produce this hemorrhage normally from the age of 40 years. Other causes are the use of anticoagulants, coagulation problems or arteriovenous malformations.

The main symptoms are: sudden and severe headache, as if they felt an explosion, especially at the base of the skull. Feeling of numbness, convulsions, neck and shoulder pain, confusion and loss of mental alertness.

Tumors in the meninges

- Meningiomas: It's about a brain tumor Of slow growth that is normally benign and usually occupy the arachnoid membrane and dura mater. It is the most frequent primary tumor of the nervous system .

If it is small it only produces symptoms, the problem arises when it is increasing its size and, therefore, exerting pressure in the cerebral structures.

On the other hand, Meningeal carcinomatosis Or leptomeningeal carcinoma are complications arising from other tumors, which arise in other areas of the body such as the lung, skin or breasts.

It occurs in advanced stages of cancer and consists of metastases that affect the cerebral meninges.

Its initial manifestation is usually the abrupt, or rapidly progressive, loss of hearing or vision (Baiges-Octavio & Huerta-Villanueva, 2000).

Meninges in other animals

Other living beings also have meninges in their nervous system, although they are not exactly like those of humans.

In fish, instead of three membranes, there is only one, and it is called the primitive meningeal.

In birds, reptiles and amphibians, the meninges have two layers: a thick dura mater and an inner secondary meningeum.

References

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