These membranes cover the brain and protect it from external agents.
In this article we explain what the meninges are and what functions they perform , and we talk about some of the disorders related to damage to these protective layers.
The meninges: definition and characteristics
The meninges are the membranous connective tissue parts that cover the brain and spinal cord so that they are not in direct contact with the bones of the spine or skull. These membranes help to fix the central nervous system to prevent, for example, the brain from moving inside the skull.
These membranes also contain cerebrospinal fluid , which acts as a shock absorber for the brain protecting it from trauma, while also providing a solution in which the brain is suspended, allowing it to preserve its natural shape.
The meninges cover the structures of the central nervous system so that they are not in direct contact with the bones of the spine or skull, and are made up of three layers of membrane known as the dura mater, arachnoid mater, and pia mater. Each layer plays a vital role in the proper maintenance and function of the central nervous system .
Layers of the meninges
There are three layers of meninges (in order from outermost to innermost): dura mater, arachnoid mater, and pia mater .
Let’s see below in more detail what each of these layers consists of and their functions.
1. Dura mater
The dura mater is the outermost layer of all the meninges . It is located directly below the bones of the skull and spinal column, and its structure is thick, strong and inextensible. Within the cranial cavity, the dura mater contains two sheets of connective tissue: the periosteal layer, which covers the inner surface of the skull bones; and the meningeal layer, the only layer present in the spinal column.
Between these two layers, the venous sinuses are located, responsible for the venous vasculature of the skull, which drains into the internal jugular veins. In some areas within the skull, the meningeal layer of the dura folds inward as dural reflexes. They divide the brain and divide the cranial cavity into several compartments.
2. Arachnoid
The arachnoid is the middle layer of the meninges, lying directly below the dura mater . It is composed of layers of connective tissue, is avascular, and receives no innervation. Beneath the arachnoid is a space known as the subarachnoid space, an area that contains cerebrospinal fluid, which acts to cushion the brain.
In addition, there are small arachnoid projections on the dura (known as arachnoid granulations) that allow cerebrospinal fluid to reenter the circulation through the dural venous sinuses.
3. Our father
The pia mater is one of the meninges that is located below the subarachnoid space . It is very thin and firmly attached to the surface of the brain and spinal cord. It is the only layer that follows the contours of the brain (the gyri and fissures).
Like the dura mater, it is highly vascularized, with blood vessels piercing through the membrane to supply nutrients to the nerve tissue. The pia mater also contains the choroid plexus, a network of capillaries and ependyma (specialized ciliated epithelial tissue) that produces cerebrospinal fluid.
Functions
The meninges function primarily to protect and support the central nervous system . These membranes connect the brain and spinal cord to the skull and spinal canal. The meninges form a protective barrier that protects the sensitive organs of the nervous system against trauma. In addition, they contain an ample supply of blood vessels that carry blood to the CNS tissue.
Another important function of the meninges is that they are responsible for producing cerebrospinal fluid. This clear fluid fills the cavities of the cerebral ventricles and surrounds the brain and spinal cord. Cerebrospinal fluid protects and nourishes the tissue of the central nervous system by acting as a shock absorber, circulating nutrients, and removing waste products.
On the other hand, the dura mater is the brain’s tough but flexible second line of defense after the skull. The flexibility of the dura mater is important because most skull fractures, other than those involving severe penetrating injuries, will not result in cerebrospinal fluid loss through the injury site, which, before the days of antibiotics and emergency medications, you would pose a serious risk of infection and possible death.
The arachnoid provides a stable substrate and space through which cerebrospinal fluid can circulate , and also provides the specialized tissue necessary for it to be absorbed into the bloodstream. Although this fluid is normally sterile and mostly inert (it contains glucose, protein, electrolytes, and very few cells), the brain and spinal neurons need protection from direct contact with the fluid, which is provided by the pia mater.
Disorders related to the meninges
Because of their protective role in the central nervous system, problems involving the meninges can lead to significant disorders and conditions . Let’s see which are the most common below.
1. Meningitis
Meningitis is a dangerous condition that causes inflammation of the meninges. This disease is usually precipitated by an infection of the cerebrospinal fluid. Pathogens such as bacteria, viruses, and fungi can induce meningeal inflammation. Meningitis can also cause brain damage, seizures, and can be fatal if not treated properly.
2. Hematomas
Damage to blood vessels in the brain can cause blood to pool in the brain cavities and brain tissue, forming a hematoma. Hematomas in the brain cause inflammation and swelling that can damage brain tissue. There are two common types of hematomas that affect the meninges: epidural hematomas and subdural hematomas.
Epidural hematoma occurs between the dura mater and the skull. It is usually caused by damage to an artery or sinus venosus as a result of severe head trauma. For its part, the subdural hematoma occurs between the dura mater and the arachnoid, and is usually caused by a head injury that breaks the veins. A subdural hematoma can be acute and develop quickly, or it can develop slowly over a period of time.
3. Meningiomas
Meningiomas are tumors that develop in the meninges. They originate from the arachnoid and put pressure on the brain and spinal cord as they grow. Most meningiomas are benign and grow slowly; however, some can grow rapidly and become cancerous.
Meningiomas can grow to be very large, and treatment often involves surgery, which usually involves removal of the tumor. The most common symptoms include: headaches, vision changes, hearing and smell loss, memory problems, and sometimes seizures.
Bibliographic references
- Cardinali, D. P. (2007). Applied neuroscience: its foundations. Pan American Medical Ed.
- Martinez, F., Tomorrow, G., Panuncio, A., & Laza, S. (2008). Anatomical-clinical review of the meninges and intracranial spaces with special reference to chronic subdural hematoma. Mexican Journal of Neuroscience, 9(1), 47-60.
- Snell, R. S. (2010). Clinical neuroanatomy. Lippincott Williams & Wilkins.
To the classic question “what do you do?” I always answer “basically I am a psychologist”. In fact, my academic training has revolved around the psychology of development, education and community, a field of study influenced my volunteer activities, as well as my first work experiences in personal services.