The cerebral meninges are connective tissue membranes that surround the brain and spinal cord. They are located under the skull and have a soft structure whose functions are cerebral and cranial vascularization, and pprotect the central nervous system from possible damage.
The brain and spinal cord are the most protected structures in the entire body, and the meninges do a great deal to accomplish 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 mater, and the pia mater.
These layers have two interspaces called the subarachnoid space, which contains the cerebrospinal fluid , and the subdural space. It is possible to make a distinction between cerebral and spinal meninges, depending on the area they surround. However, here we focus on those that cover the brain.
Layers of the cerebral meninges
Three different layers are distinguished, which are presented here from the outermost (closer to the skull) to the innermost (closer to the brain).
It is located just below the skull, and is a hollow cylinder composed of strong, thick and resistant fibrous tissue. It covers from the brain to the second or third sacral vertebra of the spinal cord. When we refer to the dura that surrounds the spinal cord, it is called the dural sac.
It is born from the mesoderm, one of the layers of cells that make up the embryo, beginning to form at approximately 3 weeks of gestation.
The dura mater 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 bony surface of the skull (called the endocranium), and that contains the venous sinuses. These are venous tubes lined with endothelium, the tissue that surrounds all blood vessels, which are located in the skull and carry blood from the brain to the heart.
The dura mater presents four areas of invagination (internal folding):
– Sickle of the brain or falx cerebri: it is the largest, and consists of a membrane that is located vertically between the left and right hemispheres of the brain. It begins on the frontal ridge of the frontal bone to the internal occipital protuberance (a small depression in the occipital bone)
– store the cerebellum or cerebellar tentorium: this part of the dura is the second largest, it has a crescent shape and separates the cerebellum of the occipital lobe of the brain. More specifically, it supports this lobe and surrounds the surface of the cerebellum. It receives that name because it joins the sickle of the brain in a shape similar to a tent or tent.
– Sickle of the cerebellum or vertical invagination , which is located in the lower part of the cerebellum and separates the two cerebellar hemispheres.
– Diaphragm of the saddle: it is the smallest invagination of all. It is found in the sella turcica, a hole at the base of the skull that houses the pituitary gland (also called the pituitary)
In contact with the dura mater is the arachnoid layer, whose embryonic origin is in the neural crest coming from the ectoderm. It is the middle part of the meninges, and also the most delicate. Its name is due to the fact that its texture resembles a spider’s web.
It consists of a thin, transparent membrane of fibrous tissue, where flat cells that are impervious to liquids abound. The main function of the arachnoid appears to be damping the central nervous system to prevent damage.
It is made up of a homogeneous layer of barrier cells, which is considered the arachnoid itself, and an inner layer of large meshes called the subarachnoid space, which is the one that contains the cerebrospinal fluid (CSF).
Sometimes the arachnoid and the pia mater, the next layer, are considered a single structure called leptomeninges, which comes from the Greek and means “fine meninges.” Embryologically, both layers appear to have a common precursor, and the remains of this precursor appear to be the 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.
The pia mater, or simply pia, 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 a large number of blood and lymphatic vessels. It adapts to the convolutions of the brain, its fissures and furrows. In this way, it allows blood vessels to penetrate throughout this organ, nourishing it.
The pia mater covers practically the entire surface of the brain. It is absent only in the natural openings that exist between the ventricles, the median opening, or foramen of Magendie, and the natural opening (also called the foramen of Luschka).
It binds to the brain through astrocytes, glial or support cells , which perform various tasks such as maintaining optimal extracellular space.
Its fibrous tissue is impermeable to fluids, for this reason it is capable of retaining cerebrospinal fluid.
Thus, it acts as a blood-brain barrier, separating the cerebrospinal fluid from the blood. In this way it allows limiting the amount of sodium, chlorine and potassium; in addition to avoiding the entry of proteins present in the 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 a selective communication with them.
It also has a buffering and protective function for the nervous system. In the spinal cord, it helps prevent deformation of the spinal cord by compression.
Between the pia mater and surrounding the blood vessels is the perivascular space (or Virchow-Robin space) which is less than 2 millimeters in size and is part of the lymphatic system of the brain.
This system is the one that carries lymph to the heart and has several functions, such as the formation and activation of the immune system or absorbing nutrients from the digestive system.
The pia mater joins with the ependyma, a membrane that lines the inside of the cerebral ventricles, cavities that are inside our brain and that are filled with cerebrospinal fluid. In the following video you can see how the pia mater is removed from the surface of the brain:
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:
It is a virtual space that is located between the dura and the arachnoid layer. It is said “virtual” because, in healthy individuals, both layers are held together by capillaries without having spaces.
The subdural space begins to exist once these membranes are detached by some contusion or damage. Normally, due to a subdural hematoma or accumulation of blood between the dura mater and the arachnoid due to the rupture of a blood vessel.
It is also called the leptomeningeal space, and it is a thin space that exists between the arachnoid membrane and the pia mater. It contains arachnoidal cords, as well as nervous 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 median and lateral foramina.
Through the subarachnoid space, its cisternae, and ventricles, 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 a protection for the nervous system like 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 body, renewing itself every 3 or 4 hours. A day about 500 milliliters of this liquid is produced.
It is generated in the choroid plexuses, small vascular structures that are located in the cerebral ventricles. Although the pia mater and other ependymal surfaces also secrete cerebrospinal fluid.
Approximately one third of this comes from the pia mater and arachnoid membranes. Another small amount originates in the brain itself, through the perivascular spaces that surround the blood vessels.
The fluid first circulates through the ventricles of the brain, then passes through one of the cisterns, called the cisterna magna. This empties into the subarachnoid space and from there travels to the brain.
For its removal, it passes through the arachnoid villi, which communicate with the vascular sinuses of the dura mater. These sinuses connect to the bloodstream.
It is also a virtual space, located between the pia mater and the limiting glial membrane. There may be hemorrhage at this level that separates the pia mater from the brain.
Diseases associated with the cerebral meninges
The meninges are associated with a number of brain pathologies, commonly with infections such as meningitis and intracranial hemorrhages. We are going to list the most important diseases that are linked to the cerebral meninges:
Meningitis is an inflammation of the meninges caused by bacteria, viruses, fungi or parasites, although the most common are bacterial.
The majority of 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 it if you have close and long-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 down this condition.
The most common symptoms are fever, headaches, cognitive impairment, nausea, vomiting, muscle weakness, extreme sensitivity to light and sounds, stiff neck, and skin rashes.
There are three types of bleeding that can occur in the meninges:
– Epidural hematoma: occurs when blood accumulates between the skull and the dura mater, although it can also originate in the spine.
It is usually due to physical trauma that causes a rupture of the middle meningeal artery, producing a hemorrhage that increases intracranial pressure.
It is a medical emergency as it can lead to death. In fact, between 15 and 20% of patients with this type of hematoma die.
The main symptoms are coma and drowsiness.
– Subdural hematoma: it is an accumulation of blood between the dura mater and the arachnoid layer. It arises from the breakage of the blood vessels that join both layers, usually due to some serious contusion such as a traffic accident or falls.
This bleeding can increase intracranial pressure, so in severe cases it can be life threatening. Especially in people whose blood does not clot properly.
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 sleepiness.
– Subarachnoid hemorrhage: in this case, the bleeding occurs in the subarachnoid space. This condition is not very frequent, representing between 0.01 and 0.08% of visits to the Emergency Room (Healthline).
Its main cause is congenital saccular aneurysm, which can cause this hemorrhage normally from the age of 40. Other causes are the use of anticoagulants, clotting problems or arteriovenous malformations.
The main symptoms are: sudden and severe headache, as if they felt an explosion, especially in the base of the skull. Numbness, seizures, neck and shoulder pain, confusion, and loss of alertness.
Tumors in the meninges
– Meningiomas: It is a slow-growing brain tumor that is normally benign and usually occupies the arachnoid membrane and the dura mater. It is the most common primary tumor of the nervous system.
If it is small it hardly produces symptoms, the problem arises when it increases its size and, therefore, exerts pressure on the brain 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 brain meninges.
Its initial manifestation is usually a sudden or rapidly progressive loss of hearing or vision.
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